Radio-Activity
CAMBRIDGE PHYSICAL SERIES.
GENERAL EDITORS:—F. H. NEVILLE, M.A., F.R.S.
AND W. C. D. WHETHAM, M.A., F.R.S.
RADIO-ACTIVITY
CAMBRIDGE UNIVERSITY PRESS WAREHOUSE
C. F. CLAY, MANAGER.
London: FETTER LANE, E.C.
Glasgow: 50, WELLINGTON STREET.
ALSO
London: H. K. LEWIS, 136, GOWER STREET, W.C.
Leipzig: F. A. BROCKHAUS.
New York: THE MACMILLAN COMPANY.
Bombay and Calcutta: MACMILLAN AND CO., LTD.
[_All Rights reserved._]
RADIO-ACTIVITY
BY
E. RUTHERFORD, D.Sc., F.R.S., F.R.S.C.
MACDONALD PROFESSOR OF PHYSICS, McGILL UNIVERSITY, MONTREAL
SECOND EDITION
CAMBRIDGE
AT THE UNIVERSITY PRESS
1905
_First Edition 1904_
_Second Edition 1905_
J. J. THOMSON
A TRIBUTE OF MY RESPECT AND ADMIRATION
PREFACE TO THE FIRST EDITION.
In this work, I have endeavoured to give a complete and connected
account, from a physical standpoint, of the properties possessed by the
naturally radio-active bodies. Although the subject is comparatively a
new one, our knowledge of the properties of the radio-active substances
has advanced with great rapidity, and there is now a very large amount
of information on the subject scattered throughout the various
scientific journals.
The phenomena exhibited by the radio-active bodies are extremely
complicated, and some form of theory is essential in order to connect in
an intelligible manner the mass of experimental facts that have now been
accumulated. I have found the theory that the atoms of the radio-active
bodies are undergoing spontaneous disintegration extremely serviceable,
not only in correlating the known phenomena, but also in suggesting new
lines of research.
The interpretation of the results has, to a large extent, been based on
the disintegration theory, and the logical deductions to be drawn from
the application of the theory to radio-active phenomena have also been
considered.
The rapid advance of our knowledge of radio-activity has been dependent
on the information already gained by research into the electric
properties of gases. The action possessed by the radiations from
radio-active bodies of producing charged carriers or ions in the gas,
has formed the basis of an accurate quantitative method of examination
of the properties of the radiations and of radio-active processes, and
also allows us to determine with considerable certainty the order of
magnitude of the different quantities involved.
For these reasons, it has been thought advisable to give a brief account
of the electric properties of gases, to the extent that is necessary for
the interpretation of the results of measurements in radio-activity by
the electric method. The chapter on the ionization theory of gases was
written before the publication of J. J. Thomson’s recent book on
“Conduction of Electricity through Gases,” in which the whole subject is
treated in a complete and connected manner.
A short chapter has been added, in which an account is given of the
methods of measurement which, in the experience of the writer and
others, are most suitable for accurate work in radio-activity. It is
hoped that such an account may be of some service to those who may wish
to obtain a practical acquaintance with the methods employed in
radio-active measurements.
My thanks are due to Mr W. C. Dampier Whetham, F.R.S., one of the
editors of the Cambridge Physical Series, for many valuable suggestions,
and for the great care and trouble he has taken in revising the proof
sheets. I am also much indebted to my wife and Miss H. Brooks for their
kind assistance in correcting the proofs, and to Mr R. K. McClung for
revising the index.
E. R.
MACDONALD PHYSICS BUILDING,
MONTREAL,
_February, 1904_.
PREFACE TO THE SECOND EDITION.
I feel that some apology is due to my readers for bringing out at such
an early date a new edition which includes so much new material, and in
which the rearrangement is so extensive as to constitute almost a new
work. Though only a year has passed since the book first made its
appearance, the researches that have been carried out in that time have
been too numerous and of too important a character to permit the
publishing of a mere reprint, unless the author were to relinquish his
purpose of presenting the subject as it stands at the present moment.
The three new chapters which have been added possibly constitute the
most important change in the work. These chapters include a detailed
account of the theory of successive changes and of its application to
the analysis of the series of transformations which occur in radium,
thorium, and actinium.
The disintegration theory, which was put forward in the first edition as
an explanation of radio-active phenomena, has in these later researches
proved to be a most powerful and valuable method of analysing the
connection between the series of substances which arise from the
transformation of the radio-elements. It has disclosed the origin of
radium, of polonium and radio-tellurium, and of radio-lead, and now
binds together in one coherent whole the large mass of apparently
heterogeneous experimental facts in radio-activity which have been
accumulating since 1896. The theory has received a remarkable measure of
verification in the past year, and, in many cases, has offered a
quantitative as well as a qualitative explanation of the connection
between the various properties exhibited by the radio-active bodies. In
the light of this evidence, radio-activity may claim to have assumed the
position of an independent subject, though one with close affinities to
physics on the one hand and to chemistry on the other.
The present edition includes a large amount of new material relating to
the nature and properties of the radiations and the emanations. In the
limits of this book, it would have been found impossible, even had it
been thought desirable, to include more than a brief sketch of the
physiological effects of the rays. The literature on this subject is
already large, and is increasing rapidly. For reasons of space, I have
not been able to refer more than briefly to the mass of papers that have
appeared dealing with the examination of various spring and well waters,
sediments, and soils, for the presence of radio-active matter.
In order to make the book more self-contained, a short account has been
given in Chapter II of the magnetic field produced by an ion in motion,
of the action of an external magnetic and electric field upon it, and of
the determination of the velocity and mass of the particles constituting
the cathode stream.
Two appendices have been added, one giving an account of some work upon
the α rays which was completed too late for inclusion in the subject
matter of the book, and the other containing a brief summary of what is
known in regard to the chemical constitution of the various radio-active
minerals, the localities in which they are found, and their probable
geologic age. For the preparation of the latter, I am indebted to my
friend Dr Boltwood of New Haven, who, in the course of his researches,
has had occasion to analyse most of these minerals in order to determine
their content of uranium and radium. I hope that this account of
radio-active minerals will prove of value to those who are endeavouring
to elucidate the connection between the various radio-active substances
and the inactive products which arise from their transformation.
For the convenience of those who have read the first edition, a list of
the sections and chapters which contain the most important additions and
alterations is added below the table of contents.
The writing of a complete account of a subject like radio-activity, in
which so much new work is constantly appearing, has been a matter of no
little difficulty. Among other things it has involved a continuous
revision of the work while the volume was passing through the press.
I wish to express my thanks to my colleague Professor Harkness for the
care and trouble he has taken in revising the proofs and for many useful
suggestions; also to Mr R. K. McClung for his assistance in correcting
some of the proofs and in preparing the index.
E. R.
MCGILL UNIVERSITY,
MONTREAL,
_9 May, 1905_.
Transcriber’s Note:
The plain text version of this ebook includes complex mathematical
formulas in TeX notation. These formulas are enclosed between double
dollar signs. The HTML version includes the same formulas rendered in
their original form.
ERRATA.
Transcriber’s Note:
These corrections have been applied to the text in the book.
page 48, line 24 section 218 should read section 284
„ 77, last line „ 263 „ „ „ 270
„ 123, 5th line from bottom „ 254 „ „ „ 261
„ 124, 10th „ „ „ „ 246 „ „ „ 253
„ 151, line 3 „ 228 „ „ „ 229
„ 156, 13th line from bottom „ 261 „ „ „ 268
„ 200, line 9 „ 246 „ „ „ 253
„ 216, line 3 „ 260 „ „ „ 267
„ 184, at the top of 5th column of table the letter γ should be
inserted.
TABLE OF CONTENTS.
I. Radio-active Substances 1
II. Ionization Theory of Gases 31
III. Methods of Measurement 82
IV. Nature of the Radiations 108
V. Properties of the Radiations 201
VI. Continuous Production of Radio-active Matter 218
VII. Radio-active Emanations 238
VIII. Excited Radio-activity 295
IX. Theory of Successive Changes 325
X. Transformation Products of Uranium, Thorium and Actinium 346
XI. Transformation Products of Radium 371
XII. Rate of Emission of Energy 418
XIII. Radio-active Processes 437
XIV. Radio-activity of the Atmosphere and of Ordinary Materials 501
Appendix A. Properties of the α Rays 543
Appendix B. Radio-active Minerals 554
Index 559
Plate (Fig. 46A: Spectrum of Radium Bromide) _to face p._ 206
* * * * *
For the convenience of the reader, the sections and chapters which
contain mostly new matter, or have been either partly or wholly
rewritten, are appended below.
Chap. I. Sections 18, 20–23.
„ II. „ 48–52.
„ III. „ 69.
„ IV. „ 83–85, 92, 93, 103, 104, 106–108, 111, 112.
„ V. „ 115, 117, 119, 122.
„ VII. „ 171–173.
„ VIII. „ 182–184, 190.
„ IX-XIV. Mostly rewritten.
ABBREVIATIONS OF REFERENCES TO SOME OF THE JOURNALS.
_Ber. d. deutsch. Chem. Ges._ Berichte der deutschen chemischen
Gesellschaft. Berlin.
_C. R._ Comptes Rendus des Séances de l’Académie des Sciences.
Paris.
_Chem. News._ Chemical News. London.
_Drude’s Annal._ Annalen der Physik. Leipzig.
_Phil. Mag._ Philosophical Magazine and Journal of Science. London.
_Phil. Trans._ Philosophical Transactions of the Royal Society of
London.
_Phys. Rev._ Physical Review. New York.
_Phys. Zeit._ Physikalische Zeitschrift.
_Proc. Camb. Phil. Soc._ Proceedings of the Cambridge Philosophical
Society. Cambridge.
_Proc. Roy. Soc._ Proceedings of the Royal Society of London.
_Thèses-Paris._ Thèses présentées à la Faculté des Sciences de
l’Université de Paris.
_Wied. Annal._ Annalen der Physik. Leipzig.
CHAPTER I.
RADIO-ACTIVE SUBSTANCES.
=1. Introduction.= The close of the old and the beginning of the new
century have been marked by a very rapid increase of our knowledge of
that most important but comparatively little known subject—the
connection between electricity and matter. No study has been more
fruitful in surprises to the investigator, both from the remarkable
nature of the phenomena exhibited and from the laws controlling them.
The more the subject is examined, the more complex must we suppose the
constitution of matter in order to explain the remarkable effects
observed. While the experimental results have led to the view that the
constitution of the atom itself is very complex, at the same time they
have confirmed the old theory of the discontinuous or atomic structure
of matter. The study of the radio-active substances and of the discharge
of electricity through gases has supplied very strong experimental
evidence in support of the fundamental ideas of the existing atomic
theory. It has also indicated that the atom itself is not the smallest
unit of matter, but is a complicated structure made up of a number of
smaller bodies.
A great impetus to the study of this subject was initially given by the
experiments of Lenard on the cathode rays, and by Röntgen’s discovery of
the X rays. An examination of the conductivity imparted to a gas by the
X rays led to a clear view of the mechanism of the transport of
electricity through gases by means of charged ions. This ionization
theory of gases has been shown to afford a satisfactory explanation not
only of the passage of electricity through flames and vapours, but also
of the complicated phenomena observed when a discharge of electricity
passes through a vacuum tube. At the same time, a further study of the
cathode rays showed that they consisted of a stream of material
particles, projected with great velocity, and possessing an apparent
mass small compared with that of the hydrogen atom. The connection
between the cathode and Röntgen rays and the nature of the latter were
also elucidated. Much of this admirable experimental work on the nature
of the electric discharge has been done by Professor J. J. Thomson and
his students in the Cavendish Laboratory, Cambridge.
An examination of natural substances, in order to see if they gave out
dark radiations similar to X rays, led to the discovery of the
radio-active bodies which possess the property of spontaneously emitting
radiations, invisible to the eye, but readily detected by their action
on photographic plates and their power of discharging electrified
bodies. A detailed study of the radio-active bodies has revealed many
new and surprising phenomena which have thrown much light, not only on
the nature of the radiations themselves, but also on the processes
occurring in those substances. Notwithstanding the complex nature of the
phenomena, the knowledge of the subject has advanced with great
rapidity, and a large amount of experimental data has now been
accumulated.
In order to explain the phenomena of radio-activity, Rutherford and
Soddy have advanced a theory which regards the atoms of the radio-active
elements as suffering spontaneous disintegration, and giving rise to a
series of radio-active substances which differ in chemical properties
from the parent elements. The radiations accompany the breaking-up of
the atoms, and afford a comparative measure of the rate at which the
disintegration takes place. This theory is found to account in a
satisfactory way for all the known facts of radio-activity, and welds a
mass of disconnected facts into one homogeneous whole. On this view, the
continuous emission of energy from the active bodies is derived from the
internal energy inherent in the atom, and does not in any way contradict
the law of the conservation of energy. At the same time, however, it
indicates that an enormous store of latent energy is resident in the
radio-atoms themselves. This store of energy has not been observed
previously, on account of the impossibility of breaking up into simpler
forms the atoms of the elements by the action of the chemical or
physical forces at our command.
On this theory we are witnessing in the radio-active bodies a veritable
transformation of matter. This process of disintegration was
investigated, not by direct chemical methods, but by means of the
property possessed by the radio-active bodies of giving out specific
types of radiation. Except in the case of a very active element like
radium, the process of disintegration takes place so slowly, that
hundreds if not thousands of years would be required before the amount
transformed would come within the range of detection of the balance or
the spectroscope. In radium, however, the process of disintegration
takes place at such a rate that it should be possible within a limited
space of time to obtain definite chemical evidence on this question. The
recent discovery that helium can be obtained from radium adds strong
confirmation to the theory; for helium was indicated as a probable
disintegration product of the radio-active elements before this
experimental evidence was forthcoming. Several products of the
transformation of the radio-active bodies have already been examined,
and the further study of these substances promises to open up new and
important fields of chemical enquiry.
In this book the experimental facts of radio-activity and the connection
between them are interpreted on the disintegration theory. Many of the
phenomena observed can be investigated in a quantitative manner, and
prominence has been given to work of this character, for the agreement
of any theory with the facts, which it attempts to explain, must
ultimately depend upon the results of accurate measurement.
The value of any working theory depends upon the number of experimental
facts it serves to correlate, and upon its power of suggesting new lines
of work. In these respects the disintegration theory, whether or not it
may ultimately be proved to be correct, has already been justified by
its results.
=2. Radio-active Substances.= The term “radio-active” is now generally
applied to a class of substances, such as uranium, thorium, radium, and
their compounds, which possess the property of _spontaneously_ emitting
radiations capable of passing through plates of metal and other
substances opaque to ordinary light. The characteristic property of
these radiations, besides their penetrating power, is their action on a
photographic plate and their power of discharging electrified bodies. In
addition, a strongly radio-active body like radium is able to cause
marked phosphorescence and fluorescence on some substances placed near
it. In the above respects the radiations possess properties analogous to
Röntgen rays, but it will be shown that, for the major part of the
radiations emitted, the resemblance is only superficial.
The most remarkable property of the radio-active bodies is their power
of radiating energy spontaneously and continuously at a constant rate,
without, as far as is known, the action upon them of any external
exciting cause. The phenomena at first sight appear to be in direct
contradiction to the law of conservation of energy, since no obvious
change with time occurs in the radiating material. The phenomena appear
still more remarkable when it is considered that the radio-active bodies
must have been steadily radiating energy since the time of their
formation in the earth’s crust.
Immediately after Röntgen’s discovery of the production of X rays,
several physicists were led to examine if any natural bodies possessed
the property of giving out radiations which could penetrate metals and
other substances opaque to light. As the production of X rays seemed to
be connected in some way with cathode rays, which cause strong
fluorescent and phosphorescent effects on various bodies, the substances
first examined were those that were phosphorescent when exposed to
light. The first observation in this direction was made by
Niewenglowski[1], who found that sulphide of calcium exposed to the
sun’s rays gave out some rays which were able to pass through black
paper. A little later a similar result was recorded by H. Becquerel[2]
for a special calcium sulphide preparation, and by Troost[3] for a
specimen of hexagonal blend. These results were confirmed and extended
in a later paper by Arnold[4]. No satisfactory explanations of these
somewhat doubtful results have yet been given, except on the view that
the black paper was transparent to some of the light waves. At the same
time Le Bon[5] showed that, by the action of sunlight on certain bodies,
a radiation was given out, invisible to the eye, but active with regard
to a photographic plate. These results have been the subject of much
discussion; but there seems to be little doubt that the effects are due
to short ultra-violet light waves, capable of passing through certain
substances opaque to ordinary light. These effects, while interesting in
themselves, are quite distinct in character from those shown by the
radio-active bodies which will now be considered.
=3. Uranium.= The first important discovery in the subject of
radio-activity was made in February, 1896, by M. Henri Becquerel[6], who
found that a uranium salt, the double sulphate of uranium and potassium,
emitted some rays which gave an impression on a photographic plate
enveloped in black paper. These rays were also able to pass through thin
plates of metals and other substances opaque to light. The impressions
on the plate could not have been due to vapours given off by the
substances, since the same effect was produced whether the salt was
placed directly on the black paper or on a thin plate of glass lying
upon it.
Becquerel found later that all the compounds of uranium as well as the
metal itself possessed the same property, and, although the amount of
action varied slightly for the different compounds, the effects in all
cases were comparable. It was at first natural to suppose that the
emission of these rays was in some way connected with the power of
phosphorescence, but later observations showed that there was no
connection whatever between them. The uranic salts are phosphorescent,
while the uranous salts are not. The uranic salts, when exposed to
ultra-violet light in the phosphoroscope, give a phosphorescent light
lasting about ·01 seconds. When the salts are dissolved in water, the
duration is still less. The amount of action on the photographic plate
does not depend on the particular compound of uranium employed, but only
on the amount of uranium present in the compound. The non-phosphorescent
are equally active with the phosphorescent compounds. The amount of
radiation given out is unaltered if the active body be kept continuously
in darkness. The rays are given out by solutions, and by crystals which
have been deposited from solutions in the dark and never exposed to
light. This shows that the radiation cannot be due in any way to the
gradual emission of energy stored up in the crystal in consequence of
exposure to a source of light.
=4.= The power of giving out penetrating rays thus seems to be a
specific property of the element uranium, since it is exhibited by the
metal as well as by all its compounds. These radiations from uranium are
persistent, and, as far as observations have yet gone, are unchanged,
either in intensity or character, with lapse of time. Observations to
test the constancy of the radiations for long periods of time have been
made by Becquerel. Samples of uranic and uranous salts have been kept in
a double box of thick lead, and the whole has been preserved from
exposure to light. By a simple arrangement, a photographic plate can be
introduced in a definite position above the uranium salts, which are
covered with a layer of black paper. The plate is exposed at intervals
for 48 hours, and the impression on the plate compared. No perceptible
weakening of the radiation has been observed over a period of four
years. Mme Curie[7] has made determinations of the activity of uranium
over a space of five years by an electric method described later, but
found no appreciable variation during that period.
Since the uranium is thus continuously radiating energy from itself,
without any known source of excitation, the question arises whether any
known agent is able to affect the rate of its emission. No alteration
was observed when the body was exposed to ultra-violet light or to
ultra-red light or to X rays. Becquerel states that the double sulphate
of uranium and potassium showed a slight increase of action when exposed
to the arc light and to sparks, but he considers that the feeble effect
observed was another action superimposed on the constant radiation from
uranium. The intensity of the uranium radiation is not affected by a
variation of temperature between 200° C. and the temperature of liquid
air. This question is discussed in more detail later.
=5.= In addition to these actions on a photographic plate, Becquerel
showed that uranium rays, like Röntgen rays, possess the important
property of discharging both positively and negatively electrified
bodies. These results were confirmed and extended by Lord Kelvin, Smolan
and Beattie[8]. The writer made a detailed comparison[9] of the nature
of the discharge produced by uranium with that produced by Röntgen rays,
and showed that the discharging property of uranium is due to the
production of charged ions by the radiation throughout the volume of the
gas. The property has been made the basis of a qualitative and
quantitative examination of the radiations from all radio-active bodies,
and is discussed in detail in chapter II.
The radiations from uranium are thus analogous, as regards their
photographic and electrical actions, to Röntgen rays, but, compared with
the rays from an ordinary X ray tube, these actions are extremely
feeble. While with Röntgen rays a strong impression is produced on a
photographic plate in a few minutes or even seconds, several days’
exposure to the uranium rays is required to produce a well-marked
action, even though the uranium compound, enveloped in black paper, is
placed close to the plate. The discharging action, while very easily
measurable by suitable methods, is also small compared with that
produced by X rays from an ordinary tube.
=6.= The rays from uranium show no evidence of direct reflection,
refraction, or polarization[10]. While there is no direct reflection of
the rays, there is apparently a diffuse reflection produced where the
rays strike a solid obstacle. This is due in reality to a secondary
radiation set up when the primary rays impinge upon matter. The presence
of this secondary radiation at first gave rise to the erroneous view
that the rays could be reflected and refracted like ordinary light. The
absence of reflection, refraction, or polarization in the penetrating
rays from uranium necessarily follows in the light of our present
knowledge of the rays. It is now known that the uranium rays, mainly
responsible for the photographic action, are deviable by a magnetic
field, and are similar in all respects to cathode rays, _i.e._ the rays
are composed of small particles projected at great velocities. The
absence of the ordinary properties of transverse light waves is thus to
be expected.
=7.= The rays from uranium are complex in character, and, in addition to
the penetrating deviable rays, there is also given off a radiation very
readily absorbed by passing through thin layers of metal foil, or by
traversing a few centimetres of air. The photographic action due to
these rays is very feeble in comparison with that of the penetrating
rays, although the discharge of electrified bodies is mainly caused by
them. Besides these two types of rays, some rays are emitted which are
of an extremely penetrating character and are non-deviable by a magnetic
field. These rays are difficult to detect photographically, but can
readily be examined by the electric method.
=8.= The question naturally arose whether the property of spontaneously
giving out penetrating radiations was confined to uranium and its
compounds, or whether it was exhibited to any appreciable extent by
other substances.
By the electrical method, with an electrometer of ordinary
sensitiveness, any body which possesses an activity of the order of
¹⁄₁₀₀ of that of uranium can be detected. With an electroscope of
special construction, such as has been designed by C. T. R. Wilson for
his experiments on the natural ionization of air, a substance of
activity ¹⁄₁₀₀₀₀ and probably ¹⁄₁₀₀₀₀₀ of that of uranium can be
detected.
If an active body like uranium be mixed with an inactive body, the
resulting activity in the mixture is generally considerably less than
that due to the active substance alone. This is due to the absorption of
the radiation by the inactive matter present. The amount of decrease
largely depends on the thickness of the layer from which the activity is
determined.
Mme Curie made a detailed examination by the electrical method of the
great majority of known substances, including the very rare elements, to
see if they possessed any activity. In cases where it was possible,
several compounds of the elements were examined. With the exception of
thorium and phosphorus, none of the other substances possessed an
activity even of the order of ¹⁄₁₀₀ of uranium.
The ionization of the gas by phosphorus does not, however, seem to be
due to a penetrating radiation like that found in the case of uranium,
but rather to a chemical action taking place at its surface. The
compounds of phosphorus do not show any activity, and in this respect
differ from uranium and the other active bodies.
Le Bon[11] has also observed that quinine sulphate, if heated and then
allowed to cool, possesses for a short time the property of discharging
both positively and negatively electrified bodies. It is necessary,
however, to draw a sharp line of distinction between phenomena of this
kind and those exhibited by the naturally radio-active bodies. While
both, under special conditions, possess the property of ionizing the
gas, the laws controlling the phenomena are quite distinct in the two
cases. For example, only one compound of quinine shows the property, and
that compound only when it has been subjected to a preliminary heating.
The action of phosphorus depends on the nature of the gas, and varies
with temperature. On the other hand, the activity of the naturally
radio-active bodies is spontaneous and permanent. It is exhibited by all
compounds, and is not, as far as is yet known, altered by change in the
chemical or physical conditions.
=9.= The discharging and photographic action alone cannot be taken as a
criterion as to whether a substance is radio-active or not. It is
necessary in addition to examine the radiations, and to test whether the
actions take place through appreciable thicknesses of all kinds of
matter opaque to ordinary light. For example, a body giving out short
waves of ultra-violet light can be made to behave in many respects like
a radio-active body. As Lenard[12] has shown, short waves of
ultra-violet light will ionize the gas in their path, and will be
absorbed rapidly in the gas. They will produce strong photographic
action, and may pass through _some_ substances opaque to ordinary light.
The similarity to a radio-active body is thus fairly complete as regards
these properties. On the other hand, the emission of these light waves,
unlike that of the radiations from an active body, will depend largely
on the molecular state of the compound, or on temperature and other
physical conditions. But the great point of distinction lies in the
nature of the radiations from the bodies in question. In one case the
radiations behave as transverse waves, obeying the usual laws of light
waves, while in the case of a naturally active body, they consist for
the most part of a continuous flight of material particles projected
from the substance with great velocity. Before any substance can be
called “radio-active” in the sense in which the term is used to describe
the properties of the natural radio-active elements, it is thus
necessary to make a close examination of its radiation; for it is
unadvisable to extend the use of the term “radio-active” to substances
which do not possess the characteristic radiating properties of the
radio-active elements which we have described, and the active products
which can be obtained from them. Some of the pseudo-active bodies will
however be considered later in chapter IX.
=10. Thorium.= In the course of an examination of a large number of
substances, Schmidt[13] found that thorium, its compounds, and the
minerals containing thorium, possessed properties similar to those of
uranium. The same discovery was made independently by Mme Curie[14]. The
rays from thorium compounds, like those from uranium, possess the
properties of discharging electrified bodies and acting on a
photographic plate. Under the same conditions the discharging action of
the rays is about equal in amount to that of uranium, but the
photographic effect is distinctly weaker.
The radiations from thorium are more complicated than those from
uranium. It was early observed by several experimenters that the
radiation from thorium compounds, especially the oxide, when tested by
the electrified method, was very variable and uncertain. A detailed
investigation of the radiations from thorium under various conditions
was made by Owens[15]. He showed that thorium oxide, especially in thick
layers, was able to produce conductivity in the gas when covered with a
large thickness of paper, and that the amount of this conductivity could
be greatly varied by blowing a current of air over the gas. In the
course of an examination[16] of this action of the air current, the
writer showed that thorium compounds gave out a material emanation made
up of very small particles _themselves radio-active_. The emanation
behaves like a radio-active gas; it diffuses rapidly through porous
substances like paper, and is carried away by a current of air. The
evidence of the existence of the emanation and its properties, is
considered in detail later in chapter VIII. In addition to giving out an
emanation, thorium behaves like uranium in emitting three types of
radiation, each of which is similar in properties to the corresponding
radiation from uranium.
=11. Radio-active minerals.= Mme Curie has examined the radio-activity
of a large number of minerals containing uranium and thorium. The
electrical method was used, and the current measured between two
parallel plates 8 cms. in diameter and 3 cms. apart, when one plate was
covered with a uniform layer of the active matter. The following numbers
give the order of the saturation current obtained in amperes.
Pitchblende from Johanngeorgenstadt 8·3 × 10⁻¹¹
„ Joachimsthal 7·0 „
„ Pzibran 6·5 „
„ Cornwall 1·6 „
Cleveite 1·4 „
Chalcolite 5·2 „
Autunite 2·7 „
Thorite from 0·3 to 1·4 „
Orangite 2·0 „
Monazite 0·5 „
Xenotine 0·03 „
Aeschynite 0·7 „
Fergusonite 0·4 „
Samarskite 1·1 „
Niobite 0·3 „
Carnotite 6·2 „
Some activity is to be expected in these minerals, since they all
contain either thorium or uranium or a mixture of both. An examination
of the action of the uranium compounds with the same apparatus and under
the same conditions led to the following results:
Uranium (containing a little carbon) 2·3 × 10⁻¹¹ amperes
Black oxide of uranium 2·6 „
Green „ „ 1·8 „
Acid uranic hydrate 0·6 „
Uranate of sodium 1·2 „
Uranate of potassium 1·2 „
Uranate of ammonia 1·3 „
Uranous sulphate 0·7 „
Sulphate of uranium and potassium 0·7 „
Acetate 0·7 „
Phosphate of copper and uranium 0·9 „
Oxysulphide of uranium 1·2 „
The interesting point in connection with these results is that some
specimens of pitchblende have four times the activity of the metal
uranium; chalcolite, the crystallized phosphate of copper and uranium,
is twice as active as uranium; and autunite, a phosphate of calcium and
uranium, is as active as uranium. From the previous considerations, none
of the substances should have shown as much activity as uranium or
thorium. In order to be sure that the large activity was not due to the
particular chemical combination, Mme Curie prepared chalcolite
artificially, starting with pure products. This artificial chalcolite
had the activity to be expected from its composition, viz. about 0·4 of
the activity of the uranium. The natural mineral chalcolite is thus five
times as active as the artificial mineral.
It thus seemed probable that the large activity of some of these
minerals, compared with uranium and thorium, was due to the presence of
small quantities of some very active substance, which was different from
the known bodies thorium and uranium.
This supposition was completely verified by the work of M. and Mme
Curie, who were able to separate from pitchblende by purely chemical
methods two active bodies, one of which in the pure state is over a
million times more active than the metal uranium.
This important discovery was due entirely to the property of
radio-activity possessed by the new bodies. The only guide in their
separation was the activity of the products obtained. In this respect
the discovery of these bodies is quite analogous to the discovery of
rare elements by the methods of spectrum analysis. The method employed
in the separation consisted in examining the relative activity of the
products after chemical treatment. In this way it was seen whether the
radio-activity was confined to one or another of the products, or
divided between both, and in what ratio such division occurred.
The activity of the specimens thus served as a basis of rough
qualitative and quantitative analysis, analogous in some respects to the
indication of the spectroscope. To obtain comparative data it was
necessary to test all the products in the dry state. The chief
difficulty lay in the fact that pitchblende is a very complex mineral,
and contains in varying quantities nearly all the known metals.
=12. Radium.= The analysis of pitchblende by chemical methods, using the
procedure sketched above, led to the discovery of two very active
bodies, polonium and radium. The name polonium was given to the first
substance discovered by Mme Curie in honour of the country of her birth.
The name radium was a very happy inspiration of the discoverers, for
this substance in the pure state possesses the property of
radio-activity to an astonishing degree.
Radium is extracted from pitchblende by the process used to separate
barium, to which radium is very closely allied in chemical
properties[17]. After the removal of other substances, the radium
remains behind mixed with barium. It can, however, be partially
separated from the latter by the difference in solubility of the
chlorides in water, alcohol, or hydrochloric acid. The chloride of
radium is less soluble than that of barium, and can be separated from it
by the method of fractional crystallization. After a large number of
precipitations, the radium can be freed almost completely from the
barium.
Both polonium and radium exist in infinitesimal quantities in
pitchblende. In order to obtain a few decigrammes of very active radium,
it is necessary to use several tons of pitchblende, or the residues
obtained from the treatment of uranium minerals. It is thus obvious that
the expense and labour involved in preparation of a minute quantity of
radium are very great.
M. and Mme Curie were indebted for their first working material to the
Austrian government, who generously presented them with a ton of the
treated residue of uranium materials from the State manufactory of
Joachimsthal in Bohemia. With the assistance of the Academy of Science
and other societies in France, funds were given to carry out the
laborious work of separation. Later the Curies were presented with a ton
of residues from the treatment of pitchblende by the Société Centrale de
Produits Chimiques of Paris. The generous assistance afforded in this
important work is a welcome sign of the active interest taken in these
countries in the furthering of purely scientific research.
The rough concentration and separation of the residues was performed in
the chemical works, and there followed a large amount of labour in
purification and concentration. In this manner, the Curies were able to
obtain a small quantity of radium which was enormously active compared
with uranium. No definite results have yet been given on the activity of
pure radium, but the Curies estimate that it is about one million times
that of uranium, and may possibly be still higher. The difficulty of
making a numerical estimate for such an intensely active body is very
great. In the electric method, the activities are compared by noting the
relative strength of the maximum or saturation current between two
parallel plates, on one of which the active substance is spread. On
account of the intense ionization of the gas between the plates, it is
not possible to reach the saturation current unless very high voltages
are applied. Approximate comparisons can be made by the use of metal
screens to cut down the intensity of the radiations, if the proportion
of the radiation transmitted by such a screen has been determined by
direct experiment on impure material of easily measurable activity. The
value of the activity of radium compared with that of uranium will
however vary to some extent according to which of the three types of
rays is taken as a basis of comparison.
It is thus difficult to control the final stages of the purification of
radium by measurements of its activity alone. Moreover the activity of
radium immediately after its preparation is only about one-fourth of its
final value; it gradually rises to a maximum after the radium salt has
been kept in the dry state for about a month. For control experiments in
purification, it is advisable to measure the initial rather than the
final activity.
Mme Curie has utilized the coloration of the crystals of radiferous
barium as a means of controlling the final process of purification. The
crystals of salts of radium and barium deposited from acid solutions are
indistinguishable by the eye. The crystals of radiferous barium are at
first colourless, but, in the course of a few hours, become yellow,
passing to orange and sometimes to a beautiful rose colour. The rapidity
of this coloration depends on the amount of barium present. Pure radium
crystals do not colour, or at any rate not as rapidly as those
containing barium. The coloration is a maximum for a definite proportion
of radium, and this fact can be utilized as a means of testing the
amount of barium present. When the crystals are dissolved in water the
coloration disappears.
Giesel[18] has observed that pure radium bromide gives a beautiful
carmine colour to the Bunsen flame. If barium be present in any
quantity, only the green colour due to barium is observed, and a
spectroscopic examination shows only the barium lines. This carmine
coloration of the Bunsen flame is a good indication of the purity of the
radium.
Since the preliminary announcement of the discovery of radium,
Giesel[19] has devoted a great deal of attention to the separation of
radium, polonium and other active bodies from pitchblende. He was
indebted for his working material to the firm of P. de Haen, of Hanover,
who presented him with a ton of pitchblende residues. Using the method
of fractional crystallization of the bromide instead of the chloride, he
has been able to prepare considerable quantities of pure radium. By this
means the labour of final purification of radium has been much reduced.
He states that six or eight crystallizations with the bromide are
sufficient to free the radium almost completely from the barium.
=13. Spectrum of radium.= It was of great importance to settle as soon
as possible whether radium was in reality modified barium or a new
element with a definite spectrum. For this purpose the Curies prepared
some specimens of radium chloride, and submitted them for examination of
their spectrum to Demarçay, an authority on that subject. The first
specimen of radium chloride examined by Demarçay[20] was not very
active, but showed, besides the lines due to barium, a very strong new
line in the ultra-violet. In another sample of greater activity, the
line was still stronger and others also appeared, while the intensity of
the new lines was comparable with those present due to barium. With a
still more active specimen which was probably nearly pure, only three
strong lines of barium appeared, while the new spectrum was very bright.
The following table shows the wave-length of the new lines observed for
radium. The wave lengths are expressed in Ångström units and the
intensity of each ray is denoted by a number, the ray of maximum
intensity being 16.
Wave length Intensity Wave length Intensity
4826·3 10 4600·3 3
4726·9 5 4533·5 9
4699·6 3 4436·1 6
4692·1 7 4340·6 12
4683·0 14 3814·7 16
4641·9 4 3649·6 12
The lines are all sharply defined, and three or four of them have an
intensity comparable with any known lines of other substances. There are
also present in the spectrum two strong nebulous bands. In the visible
part of the spectrum, which has not been photographed, the only
noticeable ray has a wave length 5665, which is, however, very feeble
compared with that of wave length 4826·3. The general aspect of the
spectrum is similar to that of the alkaline earths; it is known that
these metals have strong lines accompanied by nebulous bands.
The principal line due to radium can be distinguished in impure radium
of activity 50 times that of uranium. By the electrical method it is
easy to distinguish the presence of radium in a body which has an
activity only ¹⁄₁₀₀ of uranium. With a more sensitive electrometer
¹⁄₁₀₀₀₀ of the activity of uranium could be observed. For the detection
of radium, the examination of the radio-activity is thus a process
nearly a million times more sensitive than spectrum analysis.
Later observations on the spectrum of radium have been made by
Runge[21], Exner and Haschek[22], with specimens of radium prepared by
Giesel. Crookes[23] has photographed the spectrum of radium in the
ultra-violet, while Runge and Precht[24], using a highly purified sample
of radium, observed a number of new lines in the spark spectrum. It has
been mentioned already that the bromide of radium gives a characteristic
pure carmine-red coloration to the Bunsen flame. The flame spectrum
shows two broad bright bands in the orange-red, not observed in
Demarçay’s spectrum. In addition there is a line in the blue-green and
two feeble lines in the violet.
=14. Atomic weight of radium.= Mme Curie has made successive
determinations of the atomic weight of the new element with specimens of
steadily increasing purity. In the first observation the radium was
largely mixed with barium, and the atomic weight obtained was the same
as that of barium, 137·5. In successive observations with specimens of
increasing purity the atomic weights of the mixture were 146 and 175.
The final value obtained recently was 225, which may be taken as the
atomic weight of radium on the assumption that it is divalent.
In these experiments about 0·1 gram of pure radium chloride was obtained
by successive fractionations. The difficulty involved in preparing a
quantity of pure radium chloride large enough to test the atomic weight
may be gauged from the fact that only a few centigrams of fairly pure
radium, or a few decigrams of less concentrated material, are obtained
from the treatment of about 2 tons of the mineral from which it is
derived.
Runge and Precht[25] have examined the spectrum of radium in a magnetic
field, and have shown the existence of series analogous to those
observed for calcium, barium, and strontium. These series are connected
with the atomic weights of the elements in question, and Runge and
Precht have calculated by these means that the atomic weight of radium
should be 258—a number considerably greater than the number 225 obtained
by Mme Curie by means of chemical analysis. Marshall Watts[26], on the
other hand, using another relation between the lines of the spectrum,
deduced the value obtained by Mme Curie. Runge[27] has criticised the
method of deduction employed by Marshall Watts on the ground that the
lines used for comparison in the different spectra were not homologous.
Considering that the number found by Mme Curie agrees with that required
by the periodic system, it is advisable in the present state of our
knowledge to accept the experimental number rather than the one deduced
by Runge and Precht from spectroscopic evidence.
There is no doubt that radium is a new element possessing remarkable
physical properties. The detection and separation of this substance,
existing in such minute proportions in pitchblende, has been due
entirely to the characteristic property we are considering, and is the
first notable triumph of the study of radio-activity. As we shall see
later, the property of radio-activity can be used, not only as a means
of chemical research, but also as an extraordinarily delicate method of
detecting chemical changes of a very special kind.
=15. Radiations from radium.= On account of its enormous activity, the
radiations from radium are very intense: a screen of zinc sulphide,
brought near a few centigrams of radium bromide, is lighted up quite
brightly in a dark room, while brilliant fluorescence is produced on a
screen of platino-barium cyanide. An electroscope brought near the
radium salt is discharged almost instantly, while a photographic plate
is immediately affected. At a distance of one metre, a day’s exposure to
the radium rays would produce a strong impression. The radiations from
radium are analogous to those of uranium, and consist of three types of
rays: easily absorbed, penetrating, and very penetrating. Radium also
gives rise to an emanation similar to that of thorium, but with a very
much slower rate of decay. The radium emanation retains its activity for
several weeks, while that of thorium lasts only a few minutes. The
emanation obtained from a few centigrams of radium illuminates a screen
of zinc sulphide with great brilliancy. The very penetrating rays of
radium are able to light up an X ray screen in a dark room, after
passage through several centimetres of lead and several inches of iron.
As in the case of uranium or thorium, the photographic action is mainly
due to the penetrating or cathodic rays. The radiographs obtained with
radium are very similar to those obtained with X rays, but lack the
sharpness and detail of the latter. The rays are unequally absorbed by
different kinds of matter, the absorption varying approximately as the
density. In photographs of the hand the bones do not stand out as in X
ray photographs.
Curie and Laborde have shown that the compounds of radium possess the
remarkable property of always keeping their temperature several degrees
above the temperature of the surrounding air. Each gram of radium
radiates an amount of energy corresponding to 100 gram-calories per
hour. This and other properties of radium are discussed in detail in
chapters V and XII.
=16. Compounds of radium.= When first prepared in the solid state, all
the salts of radium—the chloride, bromide, acetate, sulphate, and
carbonate—are very similar in appearance to the corresponding salts of
barium, but in time they gradually become coloured. In chemical
properties the salts of radium are practically the same as those of
barium, with the exception that the chloride and bromide of radium are
less soluble in water than the corresponding salts of barium. All the
salts of radium are naturally phosphorescent. The phosphorescence of
impure radium preparations is in some cases very marked.
All the radium salts possess the property of causing rapid colorations
of the glass vessel which contains them. For feebly active material the
colour is usually violet, for more active material a yellowish-brown,
and finally black.
=17. Actinium.= The discovery of radium in pitchblende gave a great
impetus to the chemical examination of uranium residues, and a
systematic search early led to the detection of several new radio-active
bodies. Although these show distinctive radio-active properties, so far
none of them have been purified sufficiently to give a definite spectrum
as in the case of radium. One of the most interesting and important of
these substances was discovered by Debierne[28] while working up the
uranium residues, obtained by M. and Mme Curie from the Austrian
government, and was called by him actinium. This active substance is
precipitated with the iron group, and appears to be very closely allied
in chemical properties to thorium, though it is many thousand times more
active. It is very difficult to separate from thorium and the rare
earths. Debierne has made use of the following methods for partial
separation:
(1) Precipitation in hot solutions, slightly acidulated with
hydrochloric acid, by excess of hyposulphite of soda. The active matter
is present almost entirely in the precipitate.
(2) Action of hydrofluoric acid upon the hydrates freshly precipitated,
and held in suspension in water. The portion dissolved is only slightly
active. By this method titanium may be separated.
(3) Precipitation of neutral nitrate solutions by oxygenated water. The
precipitate carries down the active body.
(4) Precipitation of insoluble sulphates. If barium sulphate, for
example, is precipitated in the solution containing the active body, the
barium carries down the active matter. The thorium and actinium are
freed from the barium by conversion of the sulphate into the chloride
and precipitation by ammonia.
In this way Debierne has obtained a substance comparable in activity
with radium. The separation, which is difficult and laborious, has not
yet been carried far enough to bring out any new lines in the spectrum.
=18.= After the initial announcement of the discovery of actinium,
several years elapsed before any definite results upon it were published
by Debierne. In the meantime, Giesel[29] had independently obtained a
radio-active substance from pitchblende which seemed similar in many
respects to the actinium of Debierne. The active substance belongs to
the group of cerium earths and is precipitated with them. By a
succession of chemical operations, the active substance is separated
mixed with lanthanum. While intensely active in comparison with thorium,
the new active substance closely resembles it in radio-active
properties, although, from the method of separation thorium cannot be
present except in minute quantity. Giesel early observed that the
substance gave off a radio-active emanation. On account of the intensity
of the emanation it emits, he termed it the “emanating substance.”
Recently this name has been changed to “emanium,” and under this title
preparations of the active substance prepared by Giesel have been placed
on the market.
Giesel found that the activity of this substance was permanent and
seemed to increase during the six months’ interval after separation. In
this respect it is similar to radium compounds, for the activity of
radium, measured by the electric method, increases in the course of a
month’s interval to four times its initial value at separation.
There can be no doubt that the “actinium” of Debierne and the “emanium”
of Giesel contain the same radio-active constituent, for recent work[30]
has shown that they exhibit identical radio-active properties. Each
gives out easily absorbed and penetrating rays, and emits a
characteristic emanation of which the rate of decay is the same for both
substances. The rate of decay of the emanation is the simplest method of
distinguishing actinium from thorium, which it resembles so closely in
radio-active as well as in chemical properties. The emanation of
actinium loses its radiating power far more rapidly than that of
thorium, the time taken for the activity to fall to half value being in
the two cases 3·7 seconds and 52 seconds respectively.
The rapid and continuous emission of this short-lived emanation is the
most striking radio-active property possessed by actinium. In still air,
the radio-active effects of this emanation are confined to a distance of
a few centimetres from the active material, as it is only able to
diffuse a short distance through the air before losing its radiating
power. With very active preparations of actinium, the material appears
to be surrounded by a luminous haze produced by the emanation. The
radiations produce strong luminosity in some substances, for example,
zinc sulphide, willemite and platinocyanide of barium. The luminosity
is especially marked on screens of zinc sulphide. Much of this effect is
due to the emanation, for, on gently blowing a current of air over the
substance, the luminosity is displaced at once in the direction of the
current. With a zinc sulphide screen, actinium shows the phenomena of
“scintillations” to an even more marked degree than radium itself.
The preparations of emanium are in some cases luminous, and a
spectroscopic examination of this light has shown a number of bright
lines[31].
The distinctive character of the emanation of actinium, as well as of
the other radio-active products to which it gives rise, coupled with the
permanence of its activity, renders it very probable that actinium will
prove to be a new radio-active element of very great activity. Although
very active preparations of actinium have been obtained, it has not yet
been found possible to free it from impurities. Consequently, no
definite observations have been made on its chemical properties, and no
new spectrum lines have been observed.
A more complete discussion of the radio-active and other properties of
actinium is given in later chapters.
=19. Polonium.= Polonium was the first of the active substances obtained
from pitchblende. It has been investigated in detail by its discoverer
Mme Curie[32]. The pitchblende was dissolved in acid and sulphuretted
hydrogen added. The precipitated sulphides contained an active
substance, which, after separation of impurities, was found associated
with bismuth. This active substance, which has been named polonium, is
so closely allied in chemical properties to bismuth that it has so far
been found impossible to effect a complete separation. Partial
separation of polonium can be made by successive fractionations based on
one of the following modes of procedure:
(1) Sublimation in a vacuum. The active sulphide is more volatile than
that of bismuth. It is deposited as a black substance at those parts of
the tube, where the temperature is between 250 and 300° C. In this way
polonium of activity 700 times that of uranium was obtained.
(2) Precipitation of nitric acid solutions by water. The precipitated
sub-nitrate is much more active than the part that remains in solution.
(3) Precipitation by sulphuretted hydrogen in a very acid hydrochloric
acid solution. The precipitated sulphides are much more active than the
salt which remains in solution.
For concentration of the active substance Mme Curie[33] has made use of
method (2). The process is, however, very slow and tedious, and is made
still more complicated by the tendency to form precipitates insoluble
either in strong or weak acids. After a large number of fractionations,
a small quantity of matter was obtained, enormously active compared with
uranium. On examination of the substance spectroscopically, only the
bismuth lines were observed. A spectroscopic examination of the active
bismuth by Demarçay and by Runge and Exner has led to the discovery of
no new lines. On the other hand Sir William Crookes[34] states that he
found one new line in the ultra-violet, while Berndt[35], working with
polonium of activity 300, observed a large number of new lines in the
ultra-violet. These results await further confirmation.
The polonium prepared by Mme Curie differs from the other radio-active
bodies in several particulars. In the first place the radiations include
only very easily absorbable rays. The two penetrating types of radiation
given out by uranium, thorium, and radium are absent. In the second
place the activity does not remain constant, but diminishes continuously
with the time. Mme Curie states that different preparations of polonium
had somewhat different rates of decay. In some cases, the activity fell
to half value in about six months, and in others, about half value in
eleven months.
=20.= The gradual diminution of the activity of polonium with time
seemed at first sight to differentiate it from such substances as
uranium and radium, the activity of which appeared fairly permanent.
This difference in behaviour is, however, one of degree rather than of
kind. We shall show later that there is present in pitchblende a number
of radio-active substances, the activity of which is not permanent. The
time taken for these bodies to lose half of their activity varies in
different cases from a few seconds to several hundreds of years. In
fact, this gradual loss of activity is an essential feature of our
theory of regarding the phenomena of radio-activity. No radio-active
substance, left to itself, can continue to radiate indefinitely; it must
ultimately lose its activity. In the case of bodies like uranium and
radium, the loss of activity is so slow that no sensible alteration has
been observed over a period of several years, but it can be deduced
theoretically that the activity of radium will eventually decrease to
half value in a period of about 1000 years, while in the case of a
feebly radio-active substance like uranium, more than a 100 million
years must elapse before the diminution of the activity becomes
appreciable.
It may be of interest here to consider briefly the suggestions advanced
at various times to account for the temporary character of the activity
of polonium. Its association with bismuth led to the view that polonium
was not a new active substance, but merely radio-active bismuth, that
is, bismuth which in some way had been made active by admixture with
radio-active bodies. It was known that a body placed in the vicinity of
thorium or radium became temporarily active. The same action was
supposed to take place when inactive matter was in solution with active
matter. The non-active matter was supposed to acquire activity by
“induction,” as it was called, in consequence of its intimate contact
with the active material.
There is no proof, however, that such is the case. The evidence points
rather to the conclusion that the activity is due, not to an alteration
of the inactive body itself, but to an admixture with it of a very small
quantity of intensely active matter. This active matter is present in
pitchblende and is separated with the bismuth but differs from it in
chemical properties.
The subject cannot be considered with advantage at this stage, but will
be discussed later in detail in chapter XI. It will there be shown that
polonium, that is, the radio-active constituent mixed with the bismuth,
is a distinct chemical substance, which is allied in chemical properties
to bismuth, but possesses some distinct analytical properties which
allow of a partial separation from it.
The polonium, if obtained in a pure state, should initially be several
hundred times as active as pure radium. This activity, however, is not
permanent; it decays with the time, falling to half value in about six
months.
The absence of any new lines in the spectrum of radio-active bismuth is
to be expected, for, even in the most active bismuth prepared, the
active matter exists in a very small proportion.
=21.= The discussion of the nature of polonium was renewed by the
discovery of Marckwald[36] that a substance similar to polonium can be
separated from pitchblende; the activity of this substance, he stated,
did not decay appreciably with the time. The method of separation from
the bismuth chloride solution, obtained from uranium residues, was very
simple. A rod of bismuth or antimony, dipped in the active solution,
rapidly became coated with a black deposit which was intensely active.
This process was continued until the whole of the activity was removed
from the solution. The active deposit gave out only easily absorbed
rays, and in that respect resembled the polonium of Mme Curie.
The active substance was found to consist mainly of tellurium, and for
this reason Marckwald gave it the name of radio-tellurium. In later
work, however, Marckwald[37] has shown that the active constituent has
no connection with tellurium, but can always be separated completely
from it by a simple chemical process.
In order to obtain a large amount of the active substance, 2000 kilos.
of pitchblende were worked up. This yielded 6 kilos. of bismuth
oxychloride, and from this was separated 1·5 grams of radio-tellurium.
The tellurium present was precipitated from a hydrochloric acid solution
by hydrazine hydrochloride. The precipitated tellurium still showed some
activity, but this was removed by repeating the process. The active
matter then remained in the filtrate, and, after evaporation, the
addition of a few drops of stannous chloride caused a small quantity of
a dark precipitate which was intensely active. This was collected on a
filter and weighed only 4 milligrams.
When plates of copper, tin or bismuth were dipped into an hydrochloric
acid solution of this active substance, the plates were found to be
covered with a very finely divided deposit. These plates were intensely
active, and produced marked photographic and phosphorescent action. As
an illustration of the enormous activity of this deposit, Marckwald
stated that a precipitate of ¹⁄₁₀₀ milligram on a copper plate, 4 square
centimetres in area, illuminated a zinc sulphide screen so brightly that
it could be seen by an audience of several hundred people.
The active substance of Marckwald is very closely allied in chemical and
radio-active properties to the polonium of Mme Curie. Both active
substances are separated with bismuth and both give out only easily
absorbed rays. The penetrating rays, such as are given out by uranium,
radium or thorium, are completely absent.
There has been a considerable amount of discussion as to whether the
active substance obtained by Marckwald is identical with that present in
the polonium of Mme Curie. Marckwald stated that his active substance
did not sensibly diminish in activity in the course of six months, but
it is doubtful whether the method of measurement used was sufficiently
precise.
The writer has found that radio-tellurium of moderate activity, prepared
after Marckwald’s method and sold by Dr Sthamer of Hamburg, undoubtedly
loses its activity with time. The radio-tellurium is obtained in the
form of a thin radio-active deposit on a polished bismuth rod or plate.
A bismuth rod was found to have lost half its activity in about 150
days, and a similar result has been recorded by other observers.
The two substances are thus similar in both radio-active and chemical
properties, and there can be no reasonable doubt that the active
constituent present in each case is the same. The evidence is discussed
in detail in chapter XI and it will there be shown that the active
substance present in the radio-tellurium of Marckwald is a slow
transformation product of radium.
=22. Radio-active lead.= Several observers early noticed that the lead
separated from pitchblende showed strong radio-active properties, but
considerable difference of opinion was expressed in regard to the
permanence of its activity. Elster and Geitel[38] found that lead
sulphate obtained from pitchblende was very active, but they considered
that the activity was probably due to an admixture of radium or polonium
with the lead, and, by suitable chemical treatment, the lead sulphate
was obtained in an inactive state. Giesel[39] also separated some
radio-active lead but found that its activity diminished with the time.
On the other hand, Hofmann and Strauss[40] obtained lead from
pitchblende whose activity seemed fairly permanent. They state that the
radio-active lead resembled ordinary lead in most of its reactions, but
showed differences in the behaviour of the sulphide and sulphate. The
sulphate was found to be strongly phosphorescent. These results of
Hofmann and Strauss were subjected at the time of their publication to
considerable criticism, and there is no doubt that the lead itself is
not radio-active but contains a small quantity of radio-active matter
which is separated with it. In later work[41], it has been shown that
radio-lead contains several radio-active constituents which can be
removed temporarily from it by suitable chemical methods.
There can be no doubt that the lead separated from pitchblende by
certain methods does show considerable activity and that this activity
is fairly permanent. The radio-active changes occurring in radio-lead
are complicated and cannot be discussed with advantage at this stage,
but will be considered in detail in chapter XI. It will there be shown
that the primary constituent present in lead is a slow transformation
product of radium. This substance then slowly changes into the active
constituent present in polonium, which gives out only easily absorbed
rays.
This polonium can be separated temporarily from the lead by suitable
chemical methods, but the radio-lead still continues to produce
polonium, so that a fresh supply may be obtained from it, provided an
interval of several months is allowed to elapse.
It will be calculated later that in all probability the radio-lead would
lose half of its activity in an interval of 40 years.
The constituent present in radio-lead has not yet been separated, but it
will be shown that, in the pure state, it should have an activity
considerably greater than that of radium itself. Sufficient attention
has not yet been paid to this substance, for, separated in a pure state,
it should be as useful scientifically as radium. In addition, since it
is the parent of polonium, it should be possible to obtain from it at
any time a supply of very active polonium, in the same way that a supply
of the radium emanation can be obtained at intervals from radium.
Hofmann and Strauss have observed a peculiar action of the cathode rays
on the active lead sulphate separated by them. They state that the
activity diminishes with time, but is recovered by exposure of the lead
for a short time to the action of cathode rays. No such action is shown
by the active lead sulphide. This effect is due most probably to the
action of the cathode rays in causing a strong phosphorescence of the
lead sulphate and has nothing to do with the radio-activity proper of
the substance.
=23. Is thorium a radio-active element?= The similarity of the chemical
properties of actinium and thorium has led to the suggestion at
different times that the activity of thorium is not due to thorium
itself, but to the presence of a slight trace of actinium. In view of
the difference in the rate of decay of the emanations of thorium and
actinium, this position is not tenable. If the activity of thorium were
due to actinium, the two emanations, as well as the other products
obtained from these substances, should have identical rates of decay.
Since there is not the slightest evidence that the rate of decay of
activity of the various products can be altered by chemical or physical
agencies, we may conclude with confidence that whatever radio-active
substance is responsible for the activity of thorium, it certainly is
not actinium. This difference in the rate of decay of the active
products is of far more weight in deciding the question whether two
bodies contain the same radio-active constituent than differences in
chemical behaviour, for it is quite probable that the active material in
each case may exist only in minute quantity in the matter under
examination, and, under such conditions, a direct chemical examination
in the first place is of little value.
Recent work of Hofmann and Zerban and of Baskerville, however, certainly
tends to show that the element thorium is itself non-radio-active, and
that the radio-activity observed in ordinary thorium compounds is due to
the admixture with it of an unknown radio-active element. Hofmann and
Zerban[42] made a systematic examination of the radio-activity of
thorium obtained from different mineral sources. They found generally
that thorium, obtained from minerals containing a large percentage of
uranium, were more active than those obtained from minerals nearly free
from uranium. This indicates that the radio-activity observed in thorium
may possibly be due to a transformation product of uranium which is
closely allied chemically to thorium and is always separated with it. A
small quantity of thorium obtained from the mineral gadolinite was found
by Hofmann to be almost inactive, whether tested by the electric or by
the photographic method. Later Baskerville and Zerban[43] found that
thorium obtained from a Brazilian mineral was practically devoid of
activity.
In this connection the recent work of Baskerville on the complexity of
ordinary thorium is of interest. By special chemical methods, he
succeeded in separating two new and distinct substances from thorium,
which he has named carolinium and berzelium. Both of these substances
are strongly radio-active, and it thus seems probable that the active
constituent observed in ordinary thorium may be due to one of these
elements.
If, as we have suggested, thorium itself is not active, it is certainly
a matter of surprise that ordinary commercial thorium and the purest
chemical preparations show about the same activity. Such a result
indicates that the methods of purification have not removed any of the
radio-active constituent originally present.
Whatever the radio-active constituent in thorium may ultimately prove to
be, it is undoubtedly not radium nor actinium nor any of the known
radio-active substances.
In later chapters, the radio-activity of thorium will, for simplicity,
be discussed on the assumption that thorium is itself a radio-active
element. The analysis of the changes which occur will thus not refer to
thorium itself but to the primary radio-active substance usually found
associated with it. The conclusions to be drawn from an examination of
the radio-active processes are for the most part independent of whether
thorium is itself radio-active or whether the radio-activity is due to
an unknown element. If thorium is not radio-active itself, it is not
possible to draw any conclusions upon the question of the duration of
the primary radio-activity associated with it. Such a deduction cannot
be made until the quantity of the radio-active element present in
thorium has been definitely determined.
=24.= If elements heavier than uranium exist, it is probable that they
will be radio-active. The extreme delicacy of radio-activity as a means
of chemical analysis would enable such elements to be recognized even if
present in infinitesimal quantities. It is probable that considerably
more than the three or four radio-elements at present recognized exist
in minute quantity, and that the number at present known will be
augmented in the future. In the first stage of the search, a purely
chemical examination is of little value, for it is not probable that the
new element should exist in sufficient quantity to be detected by
chemical or spectroscopic analysis. The main criteria of importance are
the existence or absence of distinctive radiations or emanations, and
the permanence of the radio-activity. The discovery of a radio-active
emanation with a rate of decay different from those already known would
afford strong evidence that a new radio-active body was present. The
presence of either thorium or radium in matter can very readily be
detected by observing the rate of decay of the emanations given out by
them. When once the existence of a new radio-element has been inferred
by an examination of its radio-active properties, chemical methods of
separation can be devised, the radiating or emanating property being
used as a guide in qualitative and quantitative analysis.
Footnote 1:
Niewenglowski, _C. R._ 122, p. 385, 1896.
Footnote 2:
Becquerel, _C. R._ 122, p. 559, 1896.
Footnote 3:
Troost, _C. R._ 122, p. 564, 1896.
Footnote 4:
Arnold, _Annal. d. Phys._ 61, p. 316, 1897.
Footnote 5:
Le Bon, _C. R._ 122, pp. 188, 233, 386, 462, 1896.
Footnote 6:
Becquerel, _C. R._ 122, pp. 420, 501, 559, 689, 762, 1086, 1896.
Footnote 7:
Mme Curie, _Thèse présentée à la Faculté des Sciences de Paris_, 1903.
Footnote 8:
_Nature_, 56, 1897; _Phil. Mag._ 43, p. 418, 1897; 45, p. 277, 1898.
Footnote 9:
Rutherford, _Phil. Mag._ Jan. 1899.
Footnote 10:
_Ibid._
Footnote 11:
Le Bon, _C. R._ 130, p. 891, 1900.
Footnote 12:
Lenard, _Annal. d. Phys._ 1, p. 498; 3, p. 298, 1900.
Footnote 13:
Schmidt, _Annal. d. Phys._ 65, p. 141, 1898.
Footnote 14:
Mme Curie, _C. R._ 126, p. 1101, 1898.
Footnote 15:
Owens, _Phil. Mag._ Oct. 1899.
Footnote 16:
Rutherford, _Phil. Mag._ Jan. 1900.
Footnote 17:
M. and Mme Curie and G. Bemont, _C. R._ 127, p. 1215, 1898.
Footnote 18:
Giesel, _Phys. Zeit._ 3, No. 24, p. 578, 1902.
Footnote 19:
Giesel, _Annal. d. Phys._ 69, p. 91, 1890. _Ber. d. D. Chem. Ges._ p.
3608, 1902.
Footnote 20:
Demarçay, _C. R._ 127, p. 1218, 1898; 129, p. 716, 1899; 131, p. 258,
1900.
Footnote 21:
Runge, _Astrophys. Journal_, p. 1, 1900. _Annal. d. Phys._ No. 10, p.
407, 1903.
Footnote 22:
Exner and Haschek, _Wien. Ber._ July 4, 1901.
Footnote 23:
Crookes, _Proc. Roy. Soc._ 72, p. 295, 1904.
Footnote 24:
Runge and Precht, _Annal. d. Phys._ XIV. 2, p. 418, 1904.
Footnote 25:
Runge and Precht, _Phil. Mag._ April, 1903.
Footnote 26:
Watts, _Phil. Mag._ July, 1903; August, 1904.
Footnote 27:
Runge, _Phil. Mag._ December, 1903.
Footnote 28:
Debierne, _C. R._ 129, p. 593, 1899; 130, p. 206, 1900.
Footnote 29:
Giesel, _Ber. d. D. Chem. Ges._ p. 3608, 1902; p. 342, 1903.
Footnote 30:
Debierne, _C. R._ 139, p. 538, 1904. Miss Brooks, _Phil. Mag._ Sept.
1904. Giesel, _Phys. Zeit._ 5, p. 822, 1904. _Jahrbuch. d.
Radioaktivität_, no. 4, p. 345, 1904.
Footnote 31:
Giesel, _Ber. d. D. Chem. Ges._ 37, p. 1696, 1904; Hartmann, _Phys.
Zeit._ 5, No. 18, p. 570, 1904.
Footnote 32:
Mme Curie, _C. R._ 127, p. 175, 1898.
Footnote 33:
Mme Curie, _Thèse_, Paris, 1903.
Footnote 34:
Crookes, _Proc. Roy. Soc._ May, 1900.
Footnote 35:
Berndt, _Phys. Zeit._ 2, p. 180, 1900.
Footnote 36:
Marckwald, _Phys. Zeit._ 4, No. 1 b, p. 51.
Footnote 37:
Marckwald, _Ber. d. D. Chem. Ges._ p. 2662, No. 12, 1903.
Footnote 38:
Elster and Geitel, _Annal. d. Phys._ 69, p. 83, 1899.
Footnote 39:
Giesel, _Ber. d. D. Chem. Ges._ p. 3775, 1901.
Footnote 40:
Hofmann and Strauss, _Ber. d. D. Chem. Ges._ p. 3035, 1901.
Footnote 41:
Hofmann, Gonder and Wölfl, _Annal. d. Phys._ No. 13, p. 615, 1904.
Footnote 42:
Hofmann and Zerban, _Ber. d. D. Chem. Ges._ No. 12, p. 3093, 1903.
Footnote 43:
Baskerville and Zerban, _Amer. Chem. Soc._ 26, p. 1642, 1904.
CHAPTER II.
IONIZATION THEORY OF GASES.
=25. Ionization of gases by radiation.= The most important property
possessed by the radiations from radio-active bodies is their power of
discharging bodies whether positively or negatively electrified. As this
property has been made the basis of a method for an accurate
quantitative analysis and comparison of the radiations, the variation of
the rate of discharge under different conditions and the processes
underlying it will be considered in some detail.
In order to explain the similar discharging power of Röntgen rays, the
theory[44] has been put forward that the rays produce positively and
negatively charged carriers throughout the volume of the gas surrounding
the charged body, and that the rate of production is proportional to the
intensity of the radiation. These carriers, or ions[45] as they have
been termed, move with a uniform velocity through the gas under a
constant electric field, and their velocity varies directly as the
strength of the field.
[Illustration: Fig. 1.]
Suppose we have a gas between two metal plates _A_ and _B_ (Fig. 1)
exposed to the radiation, and that the plates are kept at a constant
difference of potential. A definite number of ions will be produced per
second by the radiation, and the number produced will depend in general
upon the nature and pressure of the gas. In the electric field the
positive ions travel towards the negative plate, and the negative ions
towards the positive, and consequently a current will pass through the
gas. Some of the ions will also recombine, the rate of recombination
being proportional to the square of the number present. For a given
intensity of radiation, the current passing through the gas will
increase at first with the potential difference between the plates, but
it will reach a limit when all the ions are removed by the electric
field before any recombination occurs.
This theory accounts also for all the characteristic properties of gases
made conducting by the rays from active substances, though there are
certain differences observed between the conductivity phenomena produced
by active substances and by _X_ rays. These differences are for the most
part the result of unequal absorption of the two types of rays. Unlike
Röntgen rays, a large proportion of the radiation from active bodies
consists of rays which are absorbed in their passage through a few
centimetres of air. The ionization of the gas is thus not uniform, but
falls off rapidly with increase of distance from the active substance.
=26. Variation of the current with voltage.= Suppose that a layer of
radio-active matter is spread uniformly on the lower of two horizontal
plates _A_ and _B_ (Fig. 1). The lower plate _A_ is connected with one
pole of a battery of cells the other pole of which is connected with
earth. The plate _B_ is connected with one pair of quadrants of an
electrometer, the other pair being connected with earth.
The current[46] between the plates, determined by the rate of movement
of the electrometer needle, is observed at first to increase rapidly
with the voltage, then more slowly, finally reaching a value which
increases very slightly with a large increase in the voltage. This, as
we have indicated, is simply explained on the ionization theory.
The radiation produces ions at a constant rate, and, before the electric
field is applied, the number per unit volume increases until the rate of
production of fresh ions is exactly balanced by the recombination of the
ions already produced. On application of a small electric field, the
positive ions travel to the negative electrode and the negative to the
positive.
Since the velocity of the ions between the plates is directly
proportional to the strength of the electric field, in a weak field the
ions take so long to travel between the electrodes that most of them
recombine on the way.
The current observed is consequently small. With increase of the voltage
there is an increase of speed of the ions and a smaller number
recombine. The current consequently increases, and will reach a maximum
value when the electric field is sufficiently strong to remove all the
ions before appreciable recombination has occurred. The value of the
current will then remain constant even though the voltage is largely
increased.
This maximum current will be called the “saturation” current, and the
value of the potential difference required to give this maximum current,
the “saturation P.D.”[47]
The general shape of the current-voltage curve is shown in Fig. 2, where
the ordinates represent current and the abscissae volts.
[Illustration: Fig. 2.]
Although the variation of the current with voltage depends only on the
velocity of the ions and their rate of recombination, the full
mathematical analysis is intricate, and the equations, expressing the
relation between current and voltage, are only integrable for the case
of uniform ionization. The question is complicated by the inequality in
the velocity of the ions and by the disturbance of the potential
gradient between the plates by the movement of the ions. J. J.
Thomson[48] has worked out the case for uniform production of ions
between two parallel plates, and has found that the relation between the
current _i_ and the potential difference _V_ applied is expressed by
_Ai² + Bi = V_
where _A_ and _B_ are constants for a definite intensity of radiation
and a definite distance between the plates.
[Illustration: Fig. 3.]
In certain cases of unsymmetrical ionization, which arise in the study
of the radiations from active bodies, the relation between current and
voltage is very different from that expressed by the above equation.
Some of these cases will be considered in section =47=.
=27.= The general shape of the current-voltage curves for gases exposed
to the radiations from active bodies is shown in Fig. 3.
This curve was obtained for ·45 grams of impure radium chloride, of
activity 1000 times that of uranium, spread over an area of 33 sq. cms.
on the lower of two large parallel plates, 4·5 cms. apart. The maximum
value of the current observed, which is taken as 100, was 1·2 × 10⁻⁸
amperes, the current for low voltages was nearly proportional to the
voltage, and about 600 volts between the plates was required to ensure
approximate saturation.
In dealing with slightly active bodies like uranium or thorium,
approximate saturation is obtained for much lower voltages. Tables I.
and II. show the results for the current between two parallel plates
distant 0·5 cms. and 2·5 cms. apart respectively, when one plate was
covered with a thin uniform layer of uranium oxide.
TABLE I.
0·5 cms. apart
Volts Current
·125 18
·25 36
·5 55
1 67
2 72
4 79
8 85
16 88
100 94
335 100
TABLE II.
2·5 cms. apart
Volts Current
·5 7·3
1 14
2 27
4 47
8 64
16 73
37·5 81
112 90
375 97
800 100
The results are shown graphically in Fig. 4.
[Illustration: Fig. 4.]
From the above tables it is seen that the current at first increases
nearly in proportion to the voltage. There is no evidence of complete
saturation, although the current increases very slowly for large
increases of voltage. For example, in Table I. a change of voltage from
·125 to ·25 volts increases the current from 18 to 36% of the maximum,
while a change of voltage from 100 to 335 volts increases the current
only 6%. The variation of the current per volt (assumed uniform between
the range of voltages considered) is thus about 5000 times greater for
the former change.
Taking into consideration the early part of the curves, the current does
not reach a practical maximum as soon as would be expected on the simple
ionization theory. It seems probable that the slow increase with the
large voltages is due either to an action of the electric field on the
rate of production of ions, or to the difficulty of removing the ions
produced near the surface of the uranium before recombination. It is
possible that the presence of a strong electric field may assist in the
separation of ions which otherwise would not initially escape from the
sphere of one another’s attraction. From the data obtained by Townsend
for the conditions of production of fresh ions at low pressures by the
movement of ions through the gas, it seems that the increase of current
cannot be ascribed to an action of the moving ions in the further
ionization of the gas.
=28.= The equation expressing the relation between the current and the
voltage is very complicated even in the case of a uniform rate of
production of ions between the plates. An approximate theory, which is
of utility in interpreting the experimental results, can however be
simply deduced if the disturbance of the potential gradient is
disregarded, and the ionization assumed uniform between the plates.
Suppose that the ions are produced at a constant rate _q_ per cubic
centimetre per second in the gas between parallel plates distant _l_
cms. from each other. When no electric field is applied, the number _N_
present per c.c., when there is equilibrium between the rates of
production and recombination, is given by _q_ = α_N_², where α is a
constant.
If a small potential difference _V_ is applied, which gives only a small
fraction of the maximum current, and consequently has not much effect on
the value of _N_, the current _i_ per sq. cm. of the plate, is given by
_NeuV_
_i_ = -----
_l_
where _u_ is the sum of the velocity of the ions for unit potential
gradient, and _e_ is the charge carried by an ion.
_uV_
----
_l_
is the velocity of the ions in the electric field of strength
_V_
---
_l_
The number of ions produced per second in a prism of length _l_ and unit
area of cross-section is _ql_. The maximum or saturation current _I_ per
sq. cm. of the plate is obtained when all of these ions are removed to
the electrodes before any recombination has occurred.
Thus
_I_ = _q . l . e_,
and
$$ \frac{i}{I} = \frac{NuV}{ql^2} = \frac{uV}{l^2\sqrt{q\alpha}} $$
This equation expresses the fact previously noted that, for small
voltages, the current _i_ is proportional to _V_.
Let
_i/I_ = ρ,
then
$$ V = \frac {\rho l^2 \sqrt {q\alpha}} {u} $$
Now the greater the value of _V_ required to obtain a given value of ρ
(supposed small compared with unity), the greater the potential required
to produce saturation.
It thus follows from the equation that:
(1) For a given intensity of radiation, the saturation P.D. increases
with the distance between the plates. In the equation, for small values
of ρ, _V_ varies as _l²_. This is found to be the case for uniform
ionization, but it only holds approximately for non-uniform ionization.
(2) For a given distance between the plates, the saturation P.D. is
greater, the greater the intensity of ionization between the plates.
This is found to be the case for the ionization produced by radio-active
substances. With a very active substance like radium, the ionization
produced is so intense that very large voltages are required to produce
approximate saturation. On the other hand, only a fraction of a volt per
cm. is necessary to produce saturation in a gas where the ionization is
very slight, for example, in the case of the natural ionization observed
in a closed vessel, where no radio-active substances are present.
For a given intensity of radiation, the saturation P.D. decreases
rapidly with the lowering of the pressure of the gas. This is due to two
causes operating in the same direction, viz. a decrease in the intensity
of the ionization and an increase in the velocity of the ions. The
ionization varies directly as the pressure, while the velocity varies
inversely as the pressure. This will obviously have the effect of
causing more rapid saturation, since the rate of recombination is slower
and the time taken for the ions to travel between the electrodes is
less.
The saturation curves observed for the gases hydrogen and carbon
dioxide[49] are very similar in shape to those obtained for air. For a
given intensity of radiation, saturation is more readily obtained in
hydrogen than in air, since the ionization is less than in air while the
velocity of the ions is greater. Carbon dioxide on the other hand
requires a greater P.D. to produce saturation than does air, since the
ionization is more intense and the velocity of the ions less than in
air.
=29.= Townsend[50] has shown that, for low pressures, the variation of
the current with the voltage is very different from that observed at
atmospheric pressure. If the increase of current with the voltage is
determined for gases, exposed to Röntgen rays, at a pressure of about 1
mm. of mercury, it is found that for small voltages the ordinary
saturation curve is obtained; but when the voltage applied increases
beyond a certain value, depending on the pressure and nature of the gas
and the distance between the electrodes, the current commences to
increase slowly at first but very rapidly as the voltage is raised to
the sparking value. The general shape of the current curve is shown in
Fig. 5.
[Illustration: Fig. 5.]
The portion _OAB_ of the curve corresponds to the ordinary saturation
curve. At the point _B_ the current commences to increase. This increase
of current has been shown to be due to the action of the negative ions
at low pressures in producing fresh ions by collision with the molecules
in their path. The increase of current is not observed in air at a
pressure above 30 mms. until the P.D. is increased nearly to the value
required to produce a spark. This production of ions by collision is
considered in more detail in section 41.
=30. Rate of recombination of the ions.= A gas ionized by the radiation
preserves its conducting power for some time after it is removed from
the presence of the active body. A current of air blown over an active
body will thus discharge an electrified body some distance away. The
duration of this after conductivity can be examined very conveniently in
an apparatus similar to that shown in Fig. 6.
[Illustration: Fig. 6.]
A dry current of air or any other gas is passed at a constant rate
through a long metal tube _TL_. After passing through a quantity of
cotton-wool to remove dust particles, the current of air passes over a
vessel _T_ containing a radio-active body such as uranium, which does
not give off a radio-active emanation. By means of insulated electrodes
_A_ and _B_, charged to a suitable potential, the current between the
tube and one of these electrodes can be tested at various points along
the tube.
A gauze screen, placed over the cross-section of the tube at _D_, serves
to prevent any direct action of the electric field in abstracting ions
from the neighbourhood of _T_.
If the electric field is sufficiently strong, all the ions travel in to
the electrodes at _A_, and no current is observed at the electrode _B_.
If the current is observed successively at different distances along the
tube, all the electrodes except the one under consideration being
connected to earth, it is found that the current diminishes with the
distance from the active body. If the tube is of fairly wide bore, the
loss of the ions due to diffusion is small, and the decrease in
conductivity of the gas is due to recombination of the ions alone.
On the ionization theory, the number _dn_ of ions per unit volume which
recombine in the time _dt_ is proportional to the square of the number
present. Thus
_dn_
---- = α_n²_,
_dt_
where α is a constant.
Integrating this equation,
1 1
--- − --- = α_t_,
_n_ _N_
if _N_ is the initial number of ions, and _n_ the number after a time
_t_.
The experimental results obtained[51] have been shown to agree very well
with this equation.
In an experiment similar to that illustrated in Fig. 6, using uranium
oxide as a source of ionization, it was found that half the number of
ions present in the gas recombined in 2·4 seconds, and that at the end
of 8 seconds one-fourth of the ions were still uncombined.
Since the rate of recombination is proportional to the square of the
number present, the time taken for half of the ions present in the gas
to recombine decreases very rapidly with the intensity of the
ionization. If radium is used, the ionization is so intense that the
rate of recombination is extremely rapid. It is on account of this
rapidity of recombination that large voltages are necessary to produce
saturation in the gases exposed to very active preparations of radium.
The value of α, which may be termed the _coefficient of recombination_,
has been determined in absolute measure by Townsend[52], McClung[53] and
Langevin[54] by different experimental methods but with very concordant
results. Suppose, for example, with the apparatus of Fig. 6, the time
_T_, taken for half the ions to recombine after passing by the electrode
_A_, has been determined experimentally. Then
1
--- = α_T_,
_N_
where _N_ is the number of ions per c.c. present at _A_. If the
saturation current _i_ is determined at the electrode _A_, _i = NVe_,
where _e_ is the charge on an ion and _V_ is the volume of uniformly
ionized gas carried by the electrode _A_ per second. Then
_Ve_
α = ----- .
_iT_
The following table shows the value of α obtained for different gases.
_Value of_ α.
Gas Townsend McClung Langevin
Air 3420 × _e_ 3384 × _e_ 3200 × _e_
Carbon 3500 × _e_ 3492 × _e_ 3400 × _e_
Dioxide
Hydrogen 3020 × _e_
The latest determination of the value of _e_ (see section 36) is 3·4 ×
10⁻¹⁰ E.S. units; thus α = 1·1 × 10⁻⁶.
Using this value, it can readily be shown from the equation of
recombination that, if 10⁶ ions are present per c.c., half of them
recombine in about 0·9 sec. and 99% in 90 secs.
McClung (_loc. cit._) showed that the value of α was approximately
independent of the pressure between ·125 and three atmospheres. In later
observations, Langevin has found that the value of α decreases rapidly
when the pressure is lowered below the limits used by McClung.
=31.= In experiments on recombination it is essential that the gas
should be free from dust or other suspended particles. In dusty air, the
rate of recombination is much more rapid than in dust-free air, as the
ions diffuse rapidly to the comparatively large dust particles
distributed throughout the gas. The effect of the suspension of small
particles in a conducting gas is very well illustrated by an experiment
of Owens[55]. If tobacco smoke is blown between two parallel plates as
in Fig. 1, the current at once diminishes to a small fraction of its
former value, although a P.D. is applied sufficient to produce
saturation under ordinary conditions. A much larger voltage is then
necessary to produce saturation. If the smoke particles are removed by a
stream of air, the current returns at once to its original value.
=32. Mobility of the ions.= Determinations of the mobility of the ions,
_i.e._ the velocity of the ions under a potential gradient of 1 volt per
cm., have been made by Rutherford[56], Zeleny[57], and Langevin[58] for
gases exposed to Röntgen rays. Although widely different methods have
been employed, the results have been very concordant, and fully support
the view that the ions move with a velocity proportional to the strength
of the field. On the application of an electric field, the ions almost
instantly attain the velocity corresponding to the field and then move
with a uniform speed.
Zeleny[59] first drew attention to the fact that the positive and
negative ions had different velocities. The velocity of the negative ion
is always greater than that of the positive, and varies with the amount
of water vapour present in the gas.
The results, previously discussed, of the variation of the current with
voltage and of the rate of recombination of the ions do not of
themselves imply that the ions produced in gases by the radiations from
active bodies are of the same size as those produced by Röntgen rays
under similar conditions. They merely show that the conductivity under
various conditions can be satisfactorily explained by the view that
charged ions are produced throughout the volume of the gas. The same
general relations would be observed if the ions differed considerably in
size and velocity from those produced by Röntgen rays. The most
satisfactory method of determining whether the ions are identical in the
two cases is to determine the velocity of the ions under similar
conditions.
In order to compare the velocity of the ions[60], the writer has used an
apparatus similar to that shown in Fig. 6 on p. 40.
The ions were carried with a rapid constant stream of air past the
charged electrode _A_, and the conductivity of the gas tested
immediately afterwards at an electrode _B_, which was placed close to
_A_. The insulated electrodes _A_ and _B_ were fixed centrally in the
metal tube _L_, which was connected with earth.
For convenience of calculation, it is assumed that the electric field
between the cylinders is the same as if the cylinders were infinitely
long.
Let _a_ and _b_ be the radii of the electrode _A_, and of the tube _L_
respectively, and let _V_ = potential of _A_.
The electromotive intensity _X_ (without regard to sign) at a distance
_r_ from the centre of the tube is given by
$$ X = \frac {V} {r \log_e \frac {b} {a}} $$
Let _u₁_ and _u₂_ be the velocities of the positive and negative ions
for a potential gradient of 1 volt per cm. If the velocity is
proportional to the electric force at any point, the distance _dr_
traversed by the negative ion in the time _dt_ is given by
_dr_ = _Xu₂_ _dt_,
or
$$ dt = \frac {\log_e \frac {b}{a} r dr} {Vu_{2}} $$
Let _r₂_ be the greatest distance measured from the axis of the tube
from which the negative ion can just reach the electrode _A_ in the time
_t_ taken for the air to pass along the electrode.
Then
$$ t = \frac {r_{2}^2 − a^2} {2 Vu_{2}} \log_e \frac {b}{a} $$
If ρ₂ be the ratio of the number of the negative ions that reach the
electrode _A_ to the total number passing by, then
$$ \rho_{2} = \frac {r_{2}^2 − a^2} {b^2 − a^2} $$
Therefore
$$ u_{2} = \frac {\rho_{2} (b^2 − a^2) \log_e \frac {b}{a}} {2 Vt} $$
Similarly the ratio ρ₁ of the number of positive ions that give up their
charge to the external cylinder to the total number of positive ions is
given by
$$ u_{1} = \frac {\rho_{1} (b^2 − a^2) \log_e \frac {b}{a}} {2 Vt} $$
In the above equations it is assumed that the current of air is uniform
over the cross-section of the tube, and that the ions are uniformly
distributed over the cross-section; also, that the movement of the ions
does not appreciably disturb the electric field. Since the value of _t_
can be calculated from the velocity of the current of air and the length
of the electrode, the values of the velocities of the ions under unit
potential gradient can at once be determined.
The equation (1) shows that ρ₂ is proportional to _V_,—_i.e._ that the
rate of discharge of the electrode _A_ varies directly as the potential
of _A_, provided that the value of _V_ is not large enough to remove all
the ions from the gas as it passes by the electrode. This was found
experimentally to be the case.
In the comparison of the velocities, the potential _V_ was adjusted to
such a value that ρ₂ was about one half, when uranium oxide was placed
in the tube at _L_. The active substance was then removed, and an
aluminium cylinder substituted for the brass tube. X rays were allowed
to fall on the centre of this aluminium cylinder, and the strength of
the rays adjusted to give about the same conductivity to the gas as the
uranium had done. Under these conditions the value of ρ₂ was found to be
the same as for the first experiment.
This experiment shows conclusively that the ions produced by Röntgen
rays and by uranium move with the same velocity and are probably
identical in all respects. The method described above is not very
suitable for an accurate determination of the velocities, but gave
values for the positive ions of about 1·4 cms. per second per volt per
centimetre, and slightly greater values for the negative ions.
=33.= The most accurate determinations of the mobility of the ions
produced by Röntgen rays have been made by Zeleny[61] and Langevin[62].
Zeleny used a method similar in principle to that explained above. His
results are shown in the following table, where _K₁_ is the mobility of
the positive ion and _K₂_ that of the negative ion.
Gas _K₁_ _K₂_ _K₂/K₁_ Temperature
Air, dry 1·36 1·87 1·375 13°·5 C.
„ moist 1·37 1·51 1·10 14°
Oxygen, dry 1·36 1·80 1·32 17°
„ moist 1·29 1·52 1·18 16°
Carbon dioxide, 0·76 0·81 1·07 17°·5
dry
„ „ 0·81 0·75 0·915 17°
moist
Hydrogen, dry 6·70 7·95 1·15 20°
„ moist 5·30 5·60 1·05 20°
Langevin determined the velocity of the ions by a direct method in which
the time taken for the ion to travel over a known distance was observed.
The following table shows the comparative values obtained for air and
carbon dioxide.
Air _K₁_ Air _K₂_ Air CO₂ _K₁_ CO₂ _K₂_ CO₂
K₂/K₁ K₂/K₁
Direct 1·40 1·70 1·22 0·86 0·90 1·05
method
(Langevin)
Current of 1·36 1·87 1·375 0·76 0·81 1·07
gas
(Zeleny)
These results show that for all gases except CO₂, there is a marked
increase in the velocity of the negative ion with the dryness of the
gas, and that, even in moist gases, the velocity of the negative ions is
always greater than that of the positive ions. The velocity of the
positive ion is not much affected by the presence of moisture in the
gas.
The velocity of the ions varies inversely as the pressure of the gas.
This has been shown by Rutherford[63] for the negative ions produced by
ultra-violet light falling on a negatively charged surface, and later by
Langevin[64] for both the positive and negative ions produced by Röntgen
rays. Langevin has shown that the velocity of the positive ion increases
more slowly with the diminution of pressure than that of the negative
ion. It appears as if the negative ion, especially at pressures of about
10 mm. of mercury, begins to diminish in size.
=34. Condensation experiments.= Some experiments will now be described
which have verified in a direct way the theory that the conductivity
produced in gases by the various types of radiation is due to the
production of charged ions throughout the volume of the gas. Under
certain conditions, the ions form nuclei for the condensation of water,
and this property allows us to show the presence of the individual ions
in the gas, and also to count the number present.
It has long been known that, if air saturated with water-vapour be
suddenly expanded, a cloud of small globules of water is formed. These
drops are formed round the dust particles present in the gas, which act
as nuclei for the condensation of water around them. The experiments of
R. von Helmholtz and Richarz[65] had shown that chemical reactions, for
example the combustion of flames, taking place in the neighbourhood,
affected the condensation of a steam-jet. Lenard showed that a similar
action was produced when ultra-violet light fell on a negatively charged
zinc surface placed near the steam-jet. These results suggested that the
presence of electric charges in the gas facilitated condensation.
A very complete study of the conditions of condensation of water on
nuclei has been made by C. T. R. Wilson[66]. An apparatus was
constructed which allowed a very sudden expansion of the air over a wide
range of pressure. The amount of condensation was observed in a small
glass vessel. A beam of light was passed into the apparatus which
allowed the drops formed to be readily observed by the eye.
Preliminary small expansions caused a condensation of the water round
the dust nuclei present in the air. These dust nuclei were removed by
allowing the drops to settle. After a number of successive small
expansions, the air was completely freed from dust, so that no
condensation was produced.
Let _v₁_ = initial volume of the gas in the vessel, _v₂_ = volume after
expansion.
If _v₂_/_v₁_ < 1·25 no condensation is produced in dust-free air. If
however _v₂_/_v₁_ > 1·25 and < 1·38, a few drops appear. This number is
roughly constant until _v₂_/_v₁_ = 1·38, when the number suddenly
increases and a very dense cloud of fine drops is produced.
If the radiation from an X ray tube or a radio-active substance is now
passed into the condensation vessel, a new series of phenomena is
observed. As before, if _v₂_/_v₁_ < 1·25 no drops are formed, but if
_v₂_/_v₁_ = 1·25 there is a sudden production of a cloud. The water
drops of which this cloud is formed are finer and more numerous the
greater the intensity of the rays. The point at which condensation
begins is very marked, and a slight variation of the amount of expansion
causes either a dense cloud or no cloud at all.
It now remains to be shown that the formation of a cloud by the action
of the rays is due to the productions of ions in the gas. If the
expansion vessel is provided with two parallel plates between which an
electric field can be applied, it is seen that the number of drops,
formed by the expansion with the rays acting, decreases with increase of
the electric field. The stronger the field the smaller the number of
drops formed. This result is to be expected if the ions are the centres
of condensation; for in a strong electric field the ions are carried at
once to the electrodes, and thus disappear from the gas. If no electric
field is acting, a cloud can be produced some time after the rays have
been cut off; but if a strong electric field is applied, under the same
conditions, no cloud is formed. This is in agreement with experiments
showing the time required for the ions to disappear by recombination. In
addition it can be shown that each one of the fine drops carries an
electric charge and can be made to move in a strong uniform electric
field.
The small number of drops produced without the action of the rays when
_v₂_/_v₁_ > 1·25 is due to a very slight natural ionization of the gas.
That this ionization exists has been clearly shown by electrical methods
(section 284).
The evidence is thus complete that the ions themselves serve as centres
for the condensation of water around them. These experiments show
conclusively that the passage of electricity through a gas is due to the
presence of charged ions distributed throughout the volume of the gas,
and verify in a remarkable way the hypothesis of the discontinuous
structure of the electric charges carried by matter.
This property of the ions of acting as nuclei of condensation gives a
very delicate method of detecting the presence of ions in the gas. If
only an ion or two is present per c.c., their presence after expansion
is at once observed by the drops formed. In this way the ionization due
to a small quantity of uranium held a yard away from the condensation
vessel is at once made manifest.
=35. Difference between the positive and negative ions.= In the course
of experiments to determine the charge carried by an ion, J. J.
Thomson[67] observed that the cloud formed under the influence of X rays
increased in density when the expansion was about 1·31, and suggested in
explanation that the positive and negative ions had different
condensation points.
[Illustration: Fig. 7.]
This difference in behaviour of the positive and negative ions was
investigated in detail by C. T. R. Wilson[68] in the following way. X
rays were made to pass in a narrow beam on either side of a plate _AB_
(Fig. 7) dividing the condensation vessel into two equal parts. The
opposite poles of a battery of cells were connected with two parallel
plates _C_ and _D_, placed symmetrically with regard to _A_. The middle
point of the battery and the plate _A_ were connected with earth. If the
plate _C_ is positively charged, the ions in the space _CA_ at a short
distance from _A_ are all negative in sign. Those to the right are all
positive. It was found that condensation occurred only for the negative
ions in _AC_ when _v₂_/_v₁_ = 1·25 but did not occur in _AD_ for the
positive ions until _v₂_/_v₁_ = 1·31.
Thus the negative acts more readily than the positive ion as a centre of
condensation. The greater effect of the negative ion in causing
condensation has been suggested as an explanation of the positive charge
always observed in the upper atmosphere. The negative ions under certain
conditions become centres for the formation of small drops of water and
are removed to the earth by the action of gravity, while the positive
ions remain suspended.
With the apparatus described above, it has been shown that the positive
and negative ions are equal in number. If the expansion is large enough
to ensure condensation on both ions, the drops formed on the right and
left of the vessel in Fig. 7 are equal in number and fall at the same
rate, _i.e._ are equal in size.
Since the ions are produced in equal numbers from a gas electrically
neutral, this experiment shows that the charges on positive and negative
ions are equal in value but opposite in sign.
=36. Charge carried by an ion.= For a known sudden expansion of a gas
saturated with water vapour, the amount of water precipitated on the
ions can be calculated readily. The size of the drops can be determined
by observing the rate at which the cloud settles under the action of
gravity. From Stokes’ equation, the terminal velocity _u_ of a small
sphere of radius _r_ and density _d_ falling through a gas of which the
coefficient of viscosity is μ is given by
2 _dgr²_
_u_ = --------
9 μ
where _g_ is the acceleration due to gravity. The radius of the drop and
consequently the weight of water in each drop can thus be determined.
Since the total weight of water precipitated is known, the number of
drops present is obtained at once.
This method has been used by J. J. Thomson[69] to determine the charge
carried by an ion. If the expansion exceeds the value 1·31, both
positive and negative ions become centres of condensation. From the rate
of fall it can be shown that approximately the drops are all of the same
size.
The condensation vessel was similar to that employed by C. T. R. Wilson.
Two parallel horizontal plates were fitted in the vessel and the
radiation from an X ray tube or radio-active substance ionized the gas
between them. A difference of potential _V_, small compared with that
required to saturate the gas, was applied between the parallel plates
distant _l_ cms. from each other. The small current _i_ through the gas
is given (section 28) by
_NuVe_
_i_ = ------
_l_
where
_N_ = number of ions present in the gas,
_e_ = charge on each ion,
_u_ = sum of the velocities of the positive and negative ions.
Since the value of _N_ is the same as the number of drops, and the
velocity _u_ is known, the value of _e_ can be determined.
In his last determination J. J. Thomson found that
_e_ = 3·4 × 10⁻¹⁰ electrostatic units.
A very concordant value, namely, 3·1 × 10⁻¹⁰, has been obtained by H. A.
Wilson[70], by using a modified method of counting the drops. A check on
the size of the drops, determined by their rate of fall, was made by
observing the rate at which the drops moved in a strong electric field,
arranged so as to act with or against gravity.
J. J. Thomson found that the charge on the ions produced in hydrogen and
oxygen is the same. This shows that the nature of the ionization in
gases is distinct from that occurring in the electrolysis of solutions
where the oxygen ion always carries twice the charge of the hydrogen
ion.
=37. Diffusion of the ions.= Early experiments with ionized gases showed
that the conductivity was removed from the gas by passage through a
finely divided substance like cotton-wool, or by bubbling through water.
This loss of conductivity is due to the fact that the ions in passing
through narrow spaces diffuse to the sides of the boundary, to which
they either adhere or give up their charge.
A direct determination of the coefficient of diffusion of the ions
produced in gases by Röntgen rays or by the rays from active substances
has been made by Townsend[71]. The general method employed was to pass a
stream of ionized gas through a diffusion vessel made up of a number of
fine metal tubes arranged in parallel. Some of the ions in their passage
through the tubes diffuse to the sides, the proportion being greater the
slower the motion of the gas and the narrower the tube. Observations
were made of the conductivity of the gas before and after passage
through the tubes. In this way, correcting if necessary for the
recombination during the time taken to pass through the tubes, the
proportion _R_ of either positive or negative ions which are abstracted
can be deduced. The value of _R_ can be expressed mathematically by the
following equation in terms of _K_, the coefficient of diffusion of the
ions into the gas with which they are mixed[72],
$$ R = 4 (\cdot195e \frac{−3\cdot66KZ}{a^2V} + \cdot0243e
\frac{22\cdot3KZ}{a^2V} + &c.) $$
where
_a_ = radius of the tube,
_Z_ = length of the tube,
_V_ = mean velocity of the gas in the tube.
Only the first two terms of the series need be taken into account when
narrow tubes are used.
In this equation _R_, _V_, and _a_ are determined experimentally, and
_K_ can thus be deduced.
The following table shows the results obtained by Townsend when X rays
were used. Almost identical results were obtained later, when the
radiations from active substances replaced the X rays.
_Coefficients of diffusion of ions into gases._
Gas _K_ _K_ Mean Ratio
for + for − value of
ions ions of _K_ values
of _K_
Air, dry ·028 ·043 ·0347 1·54
„ moist ·032 ·035 ·0335 1·09
Oxygen, dry ·025 ·0396 ·0323 1·58
„ moist ·0288 ·0358 ·0323 1·24
Carbonic acid, ·023 ·026 ·0245 1·13
dry
„ „ ·0245 ·0255 ·025 1·04
moist
Hydrogen, dry ·123 ·190 ·156 1·54
„ moist ·128 ·142 ·135 1·11
The moist gases were saturated with water vapour at a temperature of 15°
C.
It is seen that the negative ion in all cases diffuses faster than the
positive. Theory shows that the coefficients of diffusion should be
directly proportional to the velocities of the ions, so that this result
is in agreement with the observations on the greater velocity of the
negative ion.
This difference in the rate of diffusion of the ions at once explains an
interesting experimental result. If ionized gases are blown through a
metal tube, the tube gains a negative charge while the gas itself
retains a positive charge. The number of positive and negative ions
present in the gas is originally the same, but, in consequence of the
more rapid diffusion of the negative ions, more of the negative ions
than of the positive give up their charges to the tube. The tube
consequently gains a negative and the gas a positive charge.
=38.= A very important result can be deduced at once when the velocities
and coefficients of diffusion of the ions are known. Townsend (_loc.
cit._) has shown that the equation of their motion is expressed by the
formula
1 _dp_
--- _pu_ = − ---- + _nXe_,
_K_ _dx_
where _e_ is the charge on an ion,
_n_ = number of ions per c.c.,
_p_ = their partial pressure,
and _u_ is the velocity due to the electric force _X_ in the direction
of the axis of _x_. When a steady state is reached,
_dp_ _nXeK_
---- = 0 and _u_ = ----,
_dx_ _p_
Let _N_ be the number of molecules in a cubic centimetre of gas at the
pressure _P_ and at the temperature 15° C., for which the values of _u_
and _K_ have been determined. Then _N_/_P_ may be substituted for
_n_/_p_, and, since _P_ at atmospheric pressure is 10⁶,
3 × 10⁸_u₁_
_Ne_ = ---------- electrostatic units,
_K_
where _u₁_ is the velocity for 1 volt (_i.e._ ¹⁄₃₀₀ E. S. unit) per cm.
It is known that one absolute electromagnetic unit of electricity in
passing through water liberates 1·23 c.c. of hydrogen at a temperature
of 15° C. and standard pressure. The number of atoms in this volume is
2·46_N_, and, if _e´_ is the charge on the hydrogen atom in the
electrolysis of water,
2·46 _Ne´_ = 3 × 10¹⁰ E. S. units,
_Ne´_ = 1·22 × 10¹⁰ E. S. units.
_e_ _u₁_
Thus --- = 2·46 × 10⁻² ----
_e´_ _K_
For example, substituting the values of _u₁_ and _K_ determined in
moist air for the positive ion,
_e_ 2·46 1·37
--- = ----- × ----- = 1·04.
_e´_ 100 ·032
Values of this ratio, not very different from unity, are obtained for
the positive and negative ions of the gases hydrogen, oxygen, and carbon
dioxide. Taking into consideration the uncertainty in the experimental
values of _u₁_ and _K_, these results indicate that the _charge carried
by an ion in all gases is the same and is equal to that carried by the
hydrogen ion in the electrolysis of liquids_.
=39. Number of the ions.= We have seen that, from experimental data,
Townsend has found that _N_, the number of molecules present in 1 c.c.
of gas at 15° C. and standard pressure, is given by
_Ne_ = 1·22 × 10¹⁰.
Now _e_, the charge on an ion, is equal to 3·4 × 10⁻¹⁰ E. S. units;
thus _N_ = 3·6 × 10⁻¹⁹.
If _I_ is the saturation current through a gas, and _q_ the total rate
of production of ions in the gas,
_I_
_q_ = --- .
_e_
The saturation current through air was found to be 1·2 × 10⁻⁸ amperes,
_i.e._ 36 E.S. units, for parallel plates 4·5 cms. apart, when ·45
gramme of radium of activity 1000 times that of uranium was spread over
an area of 33 sq. cms. of the lower plate. This corresponds to a
production of about 10¹¹ ions per second. Assuming, for the purpose of
illustration, that the ionization was uniform between the plates, the
volume of air acted on by the rays was about 148 c.c., and the number of
ions produced per c.c. per second about 7 × 10⁸. Since _N_ = 3·6 × 10¹⁹,
we see that, if one molecule produces two ions, the proportion of the
gas ionized per second is about 10⁻¹¹ of the whole. For uranium the
fraction is about 10⁻¹⁴, and for pure radium, of activity one million
times that of uranium, about 10⁻⁸. Thus even in the case of pure radium,
only about one molecule of gas is acted on per second in every 100
millions.
The electrical methods are so delicate that the production of one ion
per cubic centimetre per second can be detected readily. This
corresponds to the ionization of about one molecule in every 10¹⁹
present in the gas.
=40. Size and nature of the ions.= An approximate estimate of the mass
of an ion, compared with the mass of the molecule of the gas in which it
is produced, can be made from the known data of the coefficient _K_ of
inter-diffusion of the ions into gases. The value of _K_ for the
positive ions in moist carbon dioxide has been shown to be ·0245, while
the value of _K_ for the inter-diffusion of carbon dioxide with air is
·14. The value of _K_ for different gases is approximately inversely
proportional to the square root of the products of the masses of the
molecules of the two inter-diffusing gases; thus, the positive ion in
carbon dioxide behaves as if its mass were large compared with that of
the molecule. Similar results hold for the negative as well as for the
positive ion, and for other gases besides carbon dioxide.
This has led to the view that the ion consists of a charged centre
surrounded by a cluster of molecules travelling with it, which are kept
in position round the charged nucleus by electrical forces. A rough
estimate shows that this cluster consists of about 30 molecules of the
gas. This idea is supported by the variation in velocity, _i.e._ the
variation of the size of the negative ion, in the presence of water
vapour; for the negative ion undoubtedly has a greater mass in moist
than in dry gases. At the same time it is possible that the apparently
large size of the ion, as determined by diffusion methods, may be in
part a result of the charge carried by the ion. The presence of a charge
on a moving body would increase the frequency of collision with the
molecules of the gas, and consequently diminish the rate of diffusion.
The ion on this view may not actually be of greater size than the
molecule from which it is produced.
The negative and positive ions certainly differ in size, and this
difference becomes very pronounced for low pressures of the gas. At
atmospheric pressure, the negative ion, produced by the action of
ultra-violet light on a negatively charged body, is of the same size as
the ion produced by X rays, but at low pressures J. J. Thomson has shown
that it is identical with the corpuscle or electron, which has an
apparent mass of about ¹⁄₁₀₀₀ of the mass of the hydrogen atom. A
similar result has been shown by Townsend to hold for the negative ion
produced by X rays at a low pressure. It appears that the negative ion
at low pressure sheds its attendant cluster. The result of Langevin,
that the velocity of the negative ion increases more rapidly with the
diminution of pressure than that of the positive ion, shows that this
process of removal of the cluster is quite appreciable at a pressure of
10 mms. of mercury.
We must suppose that the process of ionization in gases consists in a
removal of a negative corpuscle or electron from the molecule of the
gas. At atmospheric pressure this corpuscle immediately becomes the
centre of an aggregation of molecules which moves with it and _is_ the
negative ion. After removal of the negative ion the molecule retains a
positive charge, and probably also becomes the centre of a cluster of
new molecules.
The terms electron and ion as used in this work may therefore be defined
as follows:—
The _electron_ or _corpuscle_ is the body of smallest mass yet known to
science. It carries a negative charge of value 3·4 × 10⁻¹⁰ electrostatic
units. Its presence has only been detected when in rapid motion, when,
for speeds up to about 10¹⁰ cms. a second, it has an apparent mass _m_
given by _e_/_m_ = 1·86 × 10⁷ electromagnetic units. This apparent mass
increases with the speed as the velocity of light is approached (see
section 82).
The ions which are produced in gases at ordinary pressure have an
apparent size, as determined from their rates of diffusion, large
compared with the molecule of the gas in which they are produced. The
negative ion consists of an electron with a cluster of molecules
attached to and moving with it; the positive ion consists of a molecule
from which an electron has been expelled, with a cluster of molecules
attached. At low pressures under the action of an electric field the
electron does not form a cluster. The positive ion is always atomic in
size, even at low pressures of the gas. Each of the ions carries a
charge of value 3·4 × 10⁻¹⁰ electrostatic units.
=41. Ions produced by collision.= The greater part of the radiation from
the radio-active bodies consists of a stream of charged particles
travelling with great velocity. In this radiation, the α particles,
which cause most of the ionization observed in the gas, consist of
positively charged bodies projected with a velocity about one-tenth the
velocity of light. The β rays consist of negatively charged particles,
which are identical with the cathode rays generated in a vacuum tube,
and travel with a speed about one-half the velocity of light (chapter
IV.). Each of these projected particles, in virtue of its great kinetic
energy, sets free a large number of ions by collision with the gas
molecules in its path. No definite experimental evidence has yet been
obtained of the number of ions produced by a single particle, or of the
way in which the ionization varies with the speed, but there is no doubt
that each projected body gives rise to many thousand ions in its path
before its energy of motion is destroyed.
It has already been mentioned (section 29) that at low pressures ions
moving under the action of an electric field are able to produce fresh
ions by collision with the molecules of the gas. At low pressures the
negative ion is identical with the electron set free in a vacuum tube,
or emitted by a radio-active substance.
The mean free path of the ion is inversely proportional to the pressure
of the gas. Consequently, if an ion moves in an electric field, the
velocity acquired between collisions increases with diminution of the
pressure. Townsend has shown that fresh ions are occasionally produced
by collision when the negative ion moves freely between two points
differing in potential by 10 volts. If the difference be about _V_ = 20
volts, fresh ions arise at each collision[73].
Now the energy _W_, acquired by an ion of charge _e_ moving freely
between two points at a difference of potential _V_, is given by
_W_ = _Ve_.
Taking _V_ = 20 volts = ²⁰⁄₃₀₀ E. S. units, and _e_ = 3·4 × 10⁻¹⁰, the
energy _W_ required in the case of a negative ion to produce an ion by
collision is given by
_W_ = 2·3 × 10⁻¹¹ ergs.
The velocity _u_ acquired by the ion of mass _m_ just before a collision
is given by
1
--- _mu²_ = _Ve_,
2
and
$$ u = \sqrt{\frac{2Ve}{m}} $$
Now _e_/_m_ = 1·86 × 10⁷ electromagnetic units for the electron at slow
speeds (section 82).
Taking _V_ = 20 volts, we find that
_u_ = 2·7 × 10⁸ cms. per sec.
This velocity is very great compared with the velocity of agitation of
the molecules of the gas.
In a weak electric field, the negative ions only produce ions by
collision. The positive ion, whose mass is at least 1000 times greater
than the electron, does not acquire a sufficient velocity to generate
ions by collision until an electric field is applied nearly sufficient
to cause a spark through the gas.
An estimate of the energy required for the production of an ion by X
rays has been made by Rutherford and McClung. The energy of the rays
was measured by their heating effect, and the total number of ions
produced determined. On the assumption that _all_ the energy of the rays
is used up in producing ions, it was found that _V_ = 175 volts—a value
considerably greater than that observed by Townsend from data of
ionization by collision. The ionization in the two cases, however, is
produced under very different conditions, and it is impossible to
estimate how much of the energy of the rays is dissipated in the form of
heat.
=42.= Variations are found in the saturation current through gases,
exposed to the radiations from active bodies, when the pressure and
nature of the gas and the distance between the electrodes are varied.
Some cases which are of special importance in measurements will now be
considered. With unscreened active material the ionization of the gas
is, to a large extent, due to the α rays, which are absorbed in their
passage through a few centimetres of air. In consequence of this rapid
absorption, the ionization decreases rapidly from the surface of the
active body, and this gives rise to conductivity phenomena different in
character from those observed with Röntgen rays, where the ionization is
in most cases uniform.
=43. Variation of the current with distance between the plates.= It has
been found experimentally[74] that the intensity of the ionization, due
to a large plane surface of active matter, falls off approximately in an
exponential law with the distance from the plate. On the assumption that
the rate of production of ions at any point is a measure of the
intensity _I_ of the radiation, the value of _I_ at that point is given
by
$$ \frac {i}{i₀} = 1 − e^{–λ x} $$
where λ is a constant, _x_ the distance from the plate, and _I₀_ the
intensity of the radiation at the surface of the plate.
While the exponential law, in some cases, approximately represents the
variation of the ionization with distance, in others the divergence from
it is wide. The ionization, due to a plane surface of polonium, for
example, falls off more rapidly than the exponential law indicates. The
α rays from an active substance like radium are highly complex; the law
of variation of the ionization due to them is by no means simple and
depends upon a variety of conditions. The distribution of ionization is
quite different according as a thick layer or a very thick film of
radio-active matter is employed. The question is fully considered at the
end of chapter IV., but for simplicity, the exponential law is assumed
in the following calculations.
Consider two parallel plates placed as in Fig. 1, one of which is
covered with a uniform layer of radio-active matter. If the distance _d_
between the plates is small compared with the dimensions of the plates,
the ionization near the centre of the plates will be sensibly uniform
over any plane parallel to the plates and lying between them. If _q_ be
the rate of production of ions at any distance _x_ and _q₀_ that at the
surface, then _q_ = _q₀__e_^{–λ_x_}. The saturation current _i_ per unit
area is given by
$$ i = \int₀^d qe' dx $$, where _e´_ is the charge on an ion,
$$ = q₀e' \int₀^d e^{–λ x} dx = \frac{q₀e'}{λ} (1 -
e^{–λ d}) $$
hence, when λ_d_ is small, _i.e._ when the ionization between the plates
is nearly constant,
_i_ = _q₀e´d_.
The current is thus proportional to the distance between the plates.
When λ_d_ is large, the saturation current _i₀_ is equal to _q₀e´_/λ,
and is independent of further increase in the value of _d_. In such a
case the radiation is completely absorbed in producing ions between the
plates, and
$$ \frac {i}{i₀} = 1 − e^{–λ d} $$
For example, in the case of a thin layer of uranium oxide spread over a
large plate, the ionization is mostly produced by rays the intensity of
which is reduced to half value in passing through 4·3 mms. of air,
_i.e._ the value of λ is 1·6. The following table is an example of the
variation of _i_ with the distance between the plates.
Distance Saturation Current
2·5 mms. 32
5 „ 55
7·5 „ 72
10 „ 85
12·5 „ 96
15 „ 100
Thus the increase of current for equal increments of distance between
the plates decreases rapidly with the distance traversed by the
radiation.
The distance of 15 mms. was not sufficient to completely absorb all the
radiation, so that the current had not reached its limiting value.
When more than one type of radiation is present, the saturation current
between parallel plates is given by
$$ i = A (1 − e^{λ d}) + A_1 (1 − e^{–λ_1 d}) $$ &c.
where _A_, _A₁_ are constants, and λ, λ₁ the absorption constants of the
radiations in the gas.
Since the radiations are unequally absorbed in different gases, the
variation of current with distance depends on the nature of the gas
between the plates.
=44. Variation of the current with pressure.= The rate of production of
ions by the radiations from active substances is directly proportional
to the pressure of the gas. The absorption of the radiation in the gas
also varies directly as the pressure. The latter result necessarily
follows if the energy required to produce an ion is independent of the
pressure.
In cases where the ionization is uniform between two parallel plates,
the current will vary directly as the pressure; when however the
ionization is not uniform, on account of the absorption of the radiation
in the gas, the current does not decrease directly as the pressure until
the pressure is reduced so far that the ionization is sensibly uniform.
Consider the variation with pressure of the saturation current _i_
between two large parallel plates, one of which is covered with a
uniform layer of active matter.
Let λ₁ = absorption constant of the radiation in the gas for unit
pressure.
For a pressure _p_, the intensity _I_ at any point _x_ is given by
$$ \frac {I} {I₀} = e^{- p λ_1 x} $$
The saturation current _i_ is thus proportional to
$$ \int₀^d pI dx = \int₀^d pI₀e^{-pλ_1 x} dx =
\frac{I₀}{λ_1} (I − e^{pA_1d}) $$
If _r_ be the ratio of the saturation currents for the pressures _p₁_
and _p₂_,
$$ r = \frac {1 − e^{-p_1λ_1 d}} {1 − e^{-p_2λ_1 d}} $$
The ratio is thus dependent on the distance _d_ between the plates and
the absorption of the radiation by the gas.
The difference in the shape of the pressure-current curves[75] is well
illustrated in Fig. 8, where curves are given for hydrogen, air, and
carbonic acid for plates 3·5 cms. apart.
[Illustration: Fig. 8.]
For the purpose of comparison, the current at atmospheric pressure and
temperature in each case is taken as unity. The actual value of the
current was greatest in carbonic acid and least in hydrogen. In
hydrogen, where the absorption is small, the current over the whole
range is nearly proportional to the pressure. In carbonic acid, where
the absorption is large, the current diminishes at first slowly with the
pressure, but is nearly proportional to it below the pressure of 235
mms. of mercury. The curve for air occupies an intermediate position.
In cases where the distance between the plates is large, the saturation
current will remain constant with diminution of pressure until the
absorption is so reduced that the radiation reaches the other plate.
An interesting result follows from the rapid absorption of radiation by
the gas. If the current is observed between two fixed parallel plates,
distant _d₁_ and _d₂_ respectively from a large plane surface of active
matter, the current at first increases with diminution of pressure,
passes through a maximum value, and then diminishes. In such an
experimental case the lower plate through which the radiations pass is
made either of open gauze or of thin metal foil to allow the radiation
to pass through readily.
The saturation current _i_ is obviously proportional to
$$ \int_{d_1}^{d_2} pI₀e^{-pλ_1 d} $$,
i.e. to
$$ \frac{I₀}{λ_1} (e^{-pλ_1 d_1} − e^{-pλ_1 d_2}) $$
This is a function of the pressure, and is a maximum when
$$ \log_e \frac{d_1}{d_2} = − pλ_1 (d_2 − d_1) $$
For example, if the active matter is uranium, _p_λ₁ = 1·6 for the α rays
at atmospheric pressure. If _d₂_ = 3, and _d₁_ = 1, the saturation
current reaches a maximum when the pressure is reduced to about ⅓ of an
atmosphere. This result has been verified experimentally.
=45. Conductivity of different gases when acted on by the rays.= For a
given intensity of radiation, the rate of production of ions in a gas
varies for different gases and increases with the density of the gas.
Strutt[76] has made a very complete examination of the relative
conductivity of gases exposed to the different types of rays emitted by
active substances. To avoid correction for any difference of absorption
of the radiation in the various gases, the pressure of the gas was
always reduced until the ionization was directly proportional to the
pressure, when, as we have seen above, the ionization must everywhere be
uniform throughout the gas. For each type of rays, the ionization of air
is taken as unity. The currents through the gases were determined at
different pressures, and were reduced to a common pressure by assuming
that the ionization was proportional to the pressure.
With unscreened active material, the ionization is almost entirely due
to α rays. When the active substance is covered with a layer of
aluminium ·01 cm. in thickness, the ionization is mainly due to the β or
cathodic rays, and when covered with 1 cm. of lead, the ionization is
solely due to the γ or very penetrating rays. Experiments on the γ rays
of radium were made by observing the rate of discharge of a special
gold-leaf electroscope filled with the gas under examination and exposed
to the action of the rays. The following table gives the relative
conductivities of gases exposed to various kinds of ionizing radiations.
Gas Relative α β γ Röntgen
Density rays rays rays rays
Hydrogen 0·0693 0·226 0·157 0·169 0·114
Air 1·00 1·00 1·00 1·00 1·00
Oxygen 1·11 1·16 1·21 1·17 1·39
Carbon dioxide 1·53 1·54 1·57 1·53 1·60
Cyanogen 1·86 1·94 1·86 1·71 1·05
Sulphur dioxide 2·19 2·04 2·31 2·13 7·97
Chloroform 4·32 4·44 4·89 4·88 31·9
Methyl iodide 5·05 3·51 5·18 4·80 72·0
Carbon 5·31 5·34 5·83 5·67 45·3
tetrachloride
With the exception of hydrogen, it will be seen that the ionization of
gases is approximately proportional to their density for the α, β, γ
rays of radium. The results obtained by Strutt for Röntgen rays are
quite different; for example, the relative conductivity produced by them
in methyl iodide was more than 14 times as great as that due to the rays
of radium. The relative conductivities of gases exposed to X rays has
been recently re-examined by McClung[77] and Eve[78], who have found
that the conductivity depends upon the penetrating power of the X rays
employed. The results obtained by them will be discussed later (section
107).
This difference of conductivity in gases is due to unequal absorptions
of the radiations. The writer has shown[79] that the total number of
ions produced by the α rays for uranium, when completely absorbed by
different gases, is not very different. The following results were
obtained:
Gas Total
Ionization
Air 100
Hydrogen 95
Oxygen 106
Carbonic acid 96
Hydrochloric acid gas 102
Ammonia 101
The numbers, though only approximate in character, seem to show that the
energy required to produce an ion is probably not very different for the
various gases. Assuming that the energy required to produce an ion in
different gases is about the same, it follows that the relative
conductivities are proportional to the relative absorption of the
radiations.
A similar result has been found by McLennan for cathode rays. He proved
that the ionization was directly proportional to the absorption of the
rays in the gas, thus showing that the same energy is required to
produce an ion in all the gases examined.
=46. Potential Gradient.= The normal potential gradient between two
charged electrodes is always disturbed when the gas is ionized in the
space between them. If the gas is uniformly ionized between two parallel
plates, Child and Zeleny have shown that there is a sudden drop of
potential near the surface of both plates, and that the electric field
is sensibly uniform for the intermediate space between them. The
disturbance of the potential gradient depends upon the difference of
potential applied, and is different at the surface of the two plates.
In most measurements of radio-activity the material is spread over one
plate only. In such a case the ionization is to a large extent confined
to the volume of the air close to the active plate. The potential
gradient in such a case is shown in Fig. 9. The dotted line shows the
variation of potential at any point between the plates when no
ionization is produced between the plates; curve _A_ for weak
ionization, such as is produced by uranium, curve _B_ for the intense
ionization produced by a very active substance. In both cases the
potential gradient is least near the active plate, and greatest near the
opposite plate. For very intense ionization it is very small near the
active surface. The potential gradient varies slightly according as the
active plate is charged positively or negatively.
[Illustration: Fig. 9.]
=47. Variation of current with voltage for surface ionization.=
Some very interesting results, giving the variation of the current with
voltage, are observed when the ionization is intense, and confined to
the space near the surface of one of two parallel plates between which
the current is measured.
The theory of this subject has been worked out independently by
Child[80] and Rutherford[81]. Let _V_ be the potential difference
between two parallel plates at a distance _d_ apart. Suppose that the
ionization is confined to a thin layer near the surface of the plate _A_
(see Fig. 1) which is charged positively. When the electric field is
acting, there is a distribution of positive ions between the plates _A_
and _B_.
Let
_n₁_
= number of positive ions per unit volume at a distance _x_ from the
plate _A_,
_K₁_
= mobility of the positive ions,
_e_ = charge on an ion.
The current _i₁_ per square centimetre through the gas is constant for
all values of _x_, and is given by
$$ i_1 = K_1n_1e \frac{dV}{dx} $$
By Poisson’s equation
$$ \frac {d^2 V} {da^2} = 4\pi n_1 e $$
Then
$$ i_1 = \frac {K_1} {4\pi} \frac {dV} {dx} \frac {d^2 V} {dx^2} $$
Integrating
$$ (\frac {dV} {dx})^2 = \frac {8\pi i_1x} {K_1} + A $$
where _A_ is a constant. Now _A_ is equal to the value of
_dV_
----
_dx_
when _x_ = 0. By making the ionization very intense, the value of
_dV_
----
_dx_
can be made extremely small.
Putting _A_ = 0, we see that
$$ \frac {dV} {dx} = \pm \sqrt {\frac {8\pi i_1 x} {K_1}} $$
This gives the potential gradient between the plates for different
values of _x_.
Integrating between the limits 0 and _d_,
$$ V = \pm \frac{2}{3} \sqrt {\frac {8\pi i_1} {K_1} d^{ rac{3}{2}}} $$
or
$$ i_1 = \frac {9V^2} {32\pi d^3} K_1 $$
If _i₂_ is the value of the current when the electric field is reversed,
and _K₂_ the velocity of the negative ion,
$$ i_2 = \frac {9V^2} {32 \pi d^3} K_2 $$
and
_i₁_ _K₁_
---- = ---- .
_i₂_ _K₂_
The current in the two directions is thus directly proportional to the
velocities of the positive and negative ions. The current should vary
directly as the square of the potential difference applied, and
inversely as the cube of the distance between the plates.
The theoretical condition of surface ionization cannot be fulfilled by
the ionization due to active substances, as the ionization extends some
centimetres from the active plate. If, however, the distance between the
plates is large compared with the distance over which the ionization
extends, the results will be in rough agreement with the theory. Using
an active preparation of radium, the writer has made some experiments on
the variation of current with voltage between parallel plates distant
about 10 cms. from each other[82].
The results showed
(1) That the current through the gas for small voltages increased more
rapidly than the potential difference applied, but not as rapidly as the
square of that potential difference.
(2) The current through the gas depended on the direction of the
electric field; the current was always smaller when the active plate was
charged positively on account of the smaller mobility of the positive
ion. The difference between _i₁_ and _i₂_ was greatest when the gas was
dry, which is the condition for the greatest difference between the
velocities of the ions.
An interesting result follows from the above theory. For given values of
_V_ and _d_, the current cannot exceed a certain definite value, however
much the ionization may be increased. In a similar way, when an active
preparation of radium is used as a source of surface ionization, it is
found that, for a given voltage and distance between the plates, the
current does not increase beyond a certain value however much the
activity of the material is increased.
=48. Magnetic field produced by an ion in motion.= It will be shown
later that the two most important kinds of rays emitted by radio-active
substances consist of electrified particles, spontaneously projected
with great velocity. The easily absorbed rays, known as α rays, are
positively electrified atoms of matter; the penetrating rays, known as β
rays, carry a negative charge, and have been found to be identical with
the cathode rays produced by the electric discharge in a vacuum tube.
The methods adopted to determine the character of these rays are very
similar to those first used by J. J. Thomson to show that the cathode
rays consisted of a stream of negatively electrified particles projected
with great velocity.
The proof that the cathode rays were corpuscular in character, and
consisted of charged particles whose mass was very small compared with
that of the hydrogen atom, marked an important epoch in physical
science: for it not only opened up new and fertile fields of research,
but also profoundly modified our previous conceptions of the
constitution of matter.
A brief account will accordingly be given of the effects produced by a
moving charged body, and also of some of the experimental methods which
have been used to determine the mass and velocity of the particles of
the cathode stream[83].
Consider an ion of radius _a_, carrying a charge of electricity _e_, and
moving with a velocity _u_, small compared with the velocity of light.
In consequence of the motion, a magnetic field is set up around the
charged ion, which is carried with it. The charged ion in motion
constitutes a current element of magnitude _eu_, and the magnetic field
_H_ at any point distant _r_ from the sphere is given by
_eu_ sin θ
_H_ = ---------
_r²_
where θ is the angle the radius vector makes with the direction of
motion. The lines of magnetic force are circles around the axis of
motion. When the ion is moving with a velocity small compared with the
velocity of light, the lines of electric force are nearly radial, but as
the speed of light is approached, they tend to leave the axis of motion
and to bend towards the equator. When the speed of the body is very
close to that of light, the magnetic and electric field is concentrated
to a large extent in the equatorial plane.
The presence of a magnetic field around the moving body implies that
magnetic energy is stored up in the medium surrounding it. The amount of
this energy can be calculated very simply for slow speeds.
In a magnetic field of strength _H_, the magnetic energy stored up in
unit volume of the medium of unit permeability is given by
_H²_
----
8π
Integrating the value of this expression over the region exterior to a
sphere of radius _a_, the total magnetic energy due to the motion of the
charged body is given by
$$ \int_a^{\infty} \frac{H^2}{8\pi} d(vol) = \frac{e^2 u^2}{8\pi}
\int₀^{2\pi} \int₀^{\pi} \int_a^{\infty} \frac{\sin^2
\theta}{r^4} r \sin \theta d\phi rd\theta dr $$
$$ = \frac{e^2 u^2}{4} \int₀^{\pi} \int_a^{\infty} \frac{(1−\cos^2
\theta)}{r^2} \sin \theta d\theta . dr $$
$$ = \frac{e^2 u^2}{3} \int_a^{\infty} \frac{dr}{r^2} = \frac{e^2
u^2}{3a} $$
The magnetic energy, due to the motion, is analogous to kinetic energy,
for it depends upon the square of the velocity of the body. In
consequence of the charge carried by the ion, additional kinetic energy
is associated with it. If the velocity of the ion is changed, electric
and magnetic forces are set up tending to stop the change of motion, and
more work is done during the change than if the ion were uncharged. The
ordinary kinetic energy of the body is
1
--- _mu²_
2
In consequence of its charge, the kinetic energy associated with it is
increased by
_e²u²_
------
3_a_
It thus behaves as if it possessed a mass _m_ + _m₁_ where _m₁_ is _the
electrical mass_, with the value
2_e²_
----
3_a_
We have so far only considered the electrical mass of a charged ion
moving with a velocity small compared with that of light. As the speed
of light is approached, the magnetic energy can no longer be expressed
by the equation already given. The general values of the electrical mass
of a charged body for speed were first worked out by J. J. Thomson[84]
in 1887. A more complete examination was made in 1889 by Heaviside[85],
while Searle[86] worked out the case for a charged ellipsoid. Recently,
the question was again attacked by Abraham[87]. Slightly different
expressions for the variation of electrical mass with speed have been
obtained, depending upon the conditions assumed for the distribution of
the electricity on the sphere. The expression found by Abraham, which
has been utilized by Kaufmann to show that the mass of the electron is
electromagnetic in origin, is given later in section 82.
All the calculations agree in showing that the electrical mass is
practically constant for slow speeds, but increases as the speed of
light is approached, and is theoretically infinite when the speed of
light is reached. The nearer the velocity of light is approached, the
greater is the resisting force to a change of motion. An infinite force
would be required to make an electron actually attain the velocity of
light, so that, according to the present theory, it would be impossible
for an electron to move faster than light, _i.e._ faster than an
electromagnetic disturbance travels in the ether.
The importance of these deductions lies in the fact that an electric
charge in motion, quite independently of any material nucleus, possesses
an apparent mass in virtue of its motion, and that this mass is a
function of the speed. Indeed, we shall see later (see section 82) that
the apparent mass of the particles constituting the cathode stream can
be explained in virtue of their charge, without the necessity of
assuming a material body in which the charge is distributed. This has
led to the suggestion that all mass may be electrical in origin, and due
purely to electricity in motion.
=49. Action of a magnetic field on a moving ion.= Let us consider the
case of an ion of mass _m_ carrying a charge _e_ and moving freely with
a velocity _u_. If _u_ is small compared with the velocity of light, the
ion in motion corresponds to a current element of magnitude _eu_. If the
ion moves in an external magnetic field of strength _H_, it is acted on
by a force at right angles both to the direction of motion, and to that
of the magnetic force and equal in magnitude to _Heu_ sin θ, where θ is
the angle between the direction of the magnetic force and the direction
of motion. Since the force due to the magnetic field is always
perpendicular to the direction of motion, it has no effect upon the
velocity of the particle, but can only alter the direction of its path.
If ρ is the radius of curvature of the path of the ion, the force along
the normal is equal to
_mu²_
-----,
ρ
and this is balanced by the force _Heu_ sin θ.
If
π
θ = ---,
2
_i.e._ if the ion is moving at right angles to the direction of the
magnetic field
_mu²_
_Heu_ = -----
ρ
or
_m_
_H_ρ = ---- _u_
_e_
Since _u_ is constant, ρ is also constant, _i.e._ the particle describes
a circular orbit of radius ρ. The radius of the circular orbit is thus
directly proportional to _u_, and inversely proportional to _H_.
If the ion is moving at an angle θ with the direction of the magnetic
field, it describes a curve which is compounded of a motion of a
particle of velocity _u_ sin θ perpendicular to the field and _u_ cos θ
in the direction of the field. The former describes a circular orbit of
radius ρ, given by
_m_
_H_ρ = --- _u_ sin θ;
_e_
the latter is unaffected by the magnetic field and moves uniformly in
the direction of the magnetic field with a velocity _u_ cos θ. The
motion of the particle is in consequence a helix, traced on a cylinder
of radius
_mu_ sin θ
ρ = ----------,
_eH_
whose axis is in the direction of the magnetic field. Thus an ion
projected obliquely to the direction of a uniform magnetic field always
moves in a helix whose axis is parallel to the lines of magnetic
force[88].
=50. Determination of e/m for the cathode stream.= The cathode rays,
first observed by Varley, were investigated in detail by Crookes. These
rays are projected from the cathode in a vacuum tube at low pressure.
They travel in straight lines, and are readily deflected by a magnet,
and produce strong luminosity in a variety of substances placed in their
path. The rays are deflected by a magnetic field in the same direction
as would be expected for a negatively charged particle projected from
the cathode. In order to explain the peculiar properties of these rays
Crookes supposed that they consisted of negatively electrified
particles, moving with great velocity and constituting, as he
appropriately termed it, “a new or fourth state of matter.” The nature
of these rays was for twenty years a subject of much controversy, for
while some upheld their material character, others considered that they
were a special form of wave motion in the ether.
Perrin and J. J. Thomson showed that the rays always carried with them a
negative charge, while Lenard made the important discovery that the rays
passed through thin metal foil and other substances opaque to ordinary
light. Using this property, he sent the rays through a thin window and
examined the properties of the rays outside the vacuum tube in which
they were produced.
The absorption of the rays by matter was shown to be nearly proportional
to the density over a very wide range, and to be independent of its
chemical constitution.
The nature of these rays was successfully demonstrated by J. J.
Thomson[89] in 1897. If the rays consisted of negatively electrified
particles, they should be deflected in their passage through an electric
as well as through a magnetic field. Such an experiment had been tried
by Hertz, but with negative results. J. J. Thomson, however, found that
the rays were deflected by an electric field in the direction to be
expected for a negatively charged particle, and showed that the failure
of Hertz to detect the same was due to the masking of the electric field
by the strong ionization produced in the gas by the cathode stream. This
effect was got rid of by reducing the pressure of the gas in the tube.
The experimental arrangement used for the electric deflection of the
rays is shown in Fig. 10.
The cathode rays are generated at the cathode _C_, and a narrow pencil
of rays is obtained by passing the rays through a perforated disc _AB_.
The rays then passed midway between two parallel insulated plates _D_
and _E_, _d_ centimetres apart, and maintained at a constant difference
of potential _V_. The point of incidence of the pencil of rays was
marked by a luminous patch produced on a phosphorescent screen placed at
_PP´_.
The particle carrying a negative charge _e_ in passing between the
charged plates, is acted on by a force _Xe_ directed towards the
positive plate, where _X_, the strength of the electric field, is given
by
_V_
---- .
_d_
[Illustration: Fig. 10.]
The application of the electric field thus causes the luminous patch to
move in the direction of the positive plate. If now a uniform magnetic
field is applied at the plates _D_ and _E_, perpendicular to the pencil
of rays, and parallel to the plane of the plates, and in such a
direction that the electric and magnetic forces are opposed to one
another, the patch of light can be brought back to its undisturbed
position by adjusting the strength of the magnetic field. If _H_ is the
strength of the magnetic field, the force on the particle due to the
magnetic field is _Heu_, and when a balance is obtained
_Heu_ = _Xe_,
or
_X_
_u_ = --- (1).
_H_
Now if the magnetic field _H_ is acting alone, the curvature ρ of the
path of the rays between the plates can be deduced from the deflection
of the luminous patch. But we have seen that
_mu_
_H_ = ----- (2).
_e_
From equations (1) and (2), the value of _u_ and _e_/_m_ for the
particle can be determined.
The velocity _u_ is not constant, but depends upon the potential
difference between the electrodes, and this in turn depends upon the
pressure and nature of the residual gas in the tube.
By altering these factors, the cathode particles may be made to acquire
velocities varying between about 10⁹ and 10¹⁰ cms. per second. This
velocity is enormous compared with that which can be impressed
ordinarily upon matter by mechanical means. On the other hand, the value
of _e_/_m_ for the particles is sensibly constant for different
velocities.
As a result of a series of experiments the mean value _e_/_m_ = 7·7 ×
10⁶ was obtained. The value of _e_/_m_ is independent of the nature or
pressure of the gas in the vacuum tube and independent of the metal used
as cathode. A similar value of _e_/_m_ was obtained by Lenard[90] and
others.
Kaufmann[91] and Simon[92] used a different method to determine the
value of _e_/_m_. The potential difference _V_ between the terminals of
the tube was measured. The work done on the charged particle in moving
from one end of the tube to the other is _Ve_, and this must be equal to
the kinetic energy
1
-- _mu²_
2
acquired by the moving particle. Thus
_e_ _u²_
--- = ---- (3).
_m_ 2_V_
By combination of this equation with (2) obtained by measurement of the
magnetic deflexion, both _u_ and _e_/_m_ can be determined.
Simon found by this method that
_e_
--- = 1·865 × 10⁷.
_m_
It will be seen later (section 82) that a similar value was deduced by
Kaufmann for the electrons projected from radium.
These results, which have been based on the effect of a magnetic and
electric field on a moving ion, were confirmed by Weichert, who
determined by a direct method the time required for the particle to
traverse a known distance.
The particles which make up the cathode stream were termed “corpuscles”
by J. J. Thomson. The name “electron,” first employed by Johnstone
Stoney, has also been applied to them and has come into general use[93].
The methods above described do not give the mass of the electron, but
only the ratio of the charge to the mass. A direct comparison can,
however, be made between the ratio _e_/_m_ for the electron and the
corresponding value for the hydrogen atoms set free in the electrolysis
of water. Each of the hydrogen atoms is supposed to carry a charge _e_,
and it is known that 96,000 coulombs of electricity, or, in round
numbers, 10⁴ electromagnetic units of quantity are required to liberate
one gram of hydrogen. If _N_ is the number of atoms in one gram of
hydrogen, then _Ne_ = 10⁴. But if _m_ is the mass of a hydrogen atom,
then _Nm_ = 1. Dividing one by the other _e_/_m_ = 10⁴. We have seen
already that a gaseous ion carries the same charge as a hydrogen atom,
while indirect evidence shows that the electron carries the same charge
as an ion, and consequently the same charge as the atom of hydrogen.
Hence we may conclude that the apparent mass of the electron is only
about ¹⁄₁₀₀₀ of the mass of the hydrogen atom. The electron thus behaves
as the smallest body known to science.
In later experiments J. J. Thomson showed that the negative ions set
free at low pressures by an incandescent carbon filament, and also the
negative ions liberated from a zinc plate exposed to the action of
ultra-violet light, had the same value for _e_/_m_ as the electrons
produced in a vacuum tube. It thus seemed probable that the electron was
a constituent of all matter. This view received strong support from
measurements of quite a different character. Zeeman in 1897 found that
the lines of the spectrum from a source of light exposed in a strong
magnetic field were displaced and doubled. Later work has shown that the
lines in some cases are trebled, in others sextupled, while, in a few
cases, the multiplication is still greater. These results received a
general explanation on the radiation theories previously advanced by
Lorenz and Larmor. The radiation, emitted from any source, was supposed
to result from the orbital or oscillatory motion of the charged parts
constituting the atom. Since a moving ion is acted on by an external
magnetic field, the motion of the charged ions is disturbed when the
source of light is exposed between the poles of a strong magnet. There
results a small change in the period of the emitted light, and a bright
line in the spectrum is, in consequence, displaced by the action of the
magnetic field. According to theory, the small change in the wave-length
of the emitted light depends upon the strength of the magnetic field and
on the ratio _e_/_m_ of the charge carried by the ion to its mass. By
comparison of the theory with the experimental results, it was deduced
that the moving ion carried a negative charge, and that the value of
_e_/_m_ was about 10⁷. The charged ion, responsible for the radiation
from a luminous body, is thus identical with the electron set free in a
vacuum tube.
It thus seems reasonable to suppose that the atoms of all bodies are
complex and are built up, in part at least, of electrons, whose apparent
mass is very small compared with that of the hydrogen atom. The
properties of such disembodied charges has been examined mathematically
among others by Larmor, who sees in this conception the ultimate basis
of a theory of matter. J. J. Thomson and Lord Kelvin have investigated
mathematically certain arrangements of a number of electrons which are
stable for small disturbances. This question will be discussed more in
detail in section 270.
=51. Canal rays.= If a discharge is passed through a vacuum tube
provided with a perforated cathode, within certain limits of pressure,
luminous streams are observed to pass through the holes and to emerge on
the side of the cathode remote from the anode. These rays were first
observed by Goldstein[94] and were called by him the “Canal-strahlen.”
These rays travel in straight lines and produce phosphorescence in
various substances.
Wien[95] showed that the canal rays were deflected by strong magnetic
and electric fields, but the amount of deflection was very small
compared with that of the cathode rays under similar conditions. The
deflection was found to be opposite in direction to the cathode rays,
and this indicates that the canal rays consist of positive ions. Wien
determined their velocity and the ratio _e_/_m_, by measuring the amount
of their magnetic and electric deflection. The value of _e_/_m_ was
found to be variable, depending upon the gas in the tube, but the
maximum value observed was 10⁴. This shows that the positive ion, in no
case, has a mass less than that of the hydrogen atom. It seems probable
that the canal rays consist of positive ions, derived either from the
gas or the electrodes, which travel towards the cathode, and have
sufficient velocity to pass through the holes of the cathode and to
appear in the gas beyond.
It is remarkable that, so far, no case has been observed where the
carrier of a positive charge has an apparent mass less than that of the
hydrogen atom. Positive electricity always appears to be associated with
bodies atomic in size. We have seen that the process of ionization in
gases is supposed to consist of the expulsion of an electron from the
atom. The corresponding positive charge remains behind on the atom and
travels with it. This difference between positive and negative
electricity appears to be fundamental, and no explanation of it has, as
yet, been forthcoming.
=52. Radiation of energy.= If an electron moves uniformly in a straight
line with constant velocity, the magnetic field, which travels with it,
remains constant, and there is no loss of energy from it by radiation.
If, however, its motion is hastened or retarded, the magnetic field is
altered, and there results a loss of energy from the electron in the
form of electromagnetic radiation. The rate of loss of energy from an
accelerated electron was first calculated by Larmor[96] and shown to be
2_e²_
---- × (acceleration)²,
3_V_
where _e_ is the charge on the electron in electromagnetic units, and
_V_ the velocity of light.
Any alteration in the velocity of a moving charge is thus always
accompanied by a radiation of energy from it. Since the electron, set
free in a vacuum tube, increases in velocity in passing through the
electric field, energy must be radiated from it during its passage from
cathode to anode. It can, however, readily be calculated that, in
ordinary cases, this loss of energy is small compared with the kinetic
energy acquired by the electron in passing through the electric field.
An electron moving in a circular orbit is a powerful radiator of energy,
since it is constantly accelerated towards the centre. An electron
moving in an orbit of radius equal to the radius of an atom (about 10⁻⁸
cms.) would lose most of its kinetic energy of motion in a small
fraction of a second, even though its velocity was originally nearly
equal to the velocity of light. If, however, a number of electrons are
arranged at equal angular intervals on the circumference of a circle and
move with constant velocity round the ring, the radiation of energy is
much less than for a single electron, and rapidly diminishes with an
increase in the number of electrons round the ring. This result,
obtained by J. J. Thomson, will be discussed in more detail later when
the stability of systems composed of rotating electrons is under
consideration.
Since the radiation of energy is proportional to the square of the
acceleration, the proportion of the total energy radiated depends upon
the suddenness with which an electron is started or stopped. Now some of
the cathode ray particles are stopped abruptly when they impinge on the
metal cathode, and, in consequence, give up a fraction of their kinetic
energy in the form of electromagnetic radiation. Stokes and Weichert
suggested that this radiation constituted the X rays, which are known to
have their origin at the surface on which the cathode rays impinge. The
mathematical theory has been worked out by J. J. Thomson[97]. If the
motion of an electron is suddenly arrested, a thin spherical pulse in
which the magnetic and electric forces are very intense travels out from
the point of impact with the velocity of light. The more suddenly the
electron is stopped, the thinner and more intense is the pulse. On this
view the X rays are not corpuscular like the cathode rays, which produce
them, but consist of transverse disturbances in the ether, akin in some
respects to light waves of short wave-length. The rays are thus made up
of a number of pulses, which are non-periodic in character, and which
follow one another at irregular intervals.
On this theory of the nature of the X rays, the absence of direct
deflection, refraction, or polarization is to be expected, if the
thickness of the pulse is small compared with the diameter of an atom.
It also explains the non-deflection of the path of the rays by a
magnetic or electric field. The intensity of the electric and magnetic
force in the pulse is so great that it is able to cause a removal of an
electron from some of the atoms of the gas, over which the pulse passes,
and thus causes the ionization observed.
The cathode rays produce X rays, and these in turn give rise to a
secondary radiation whenever they impinge on a solid body. This
secondary radiation is emitted equally in all directions, and consists
partly of a radiation of the X ray type and also of electrons projected
with considerable velocity. This secondary radiation gives rise to a
tertiary radiation and so on.
Barkla[98] has shown that the secondary radiation emitted from a gas
through which the rays pass consists in part of scattered X rays of
about the same penetrating power as the primary rays as well as some
easily absorbed rays.
Part of the cathode rays is diffusely reflected on striking the cathode.
These scattered rays consist in part of electrons of the same speed as
in the primary beam, but also include some others of much less velocity.
The amount of diffuse reflection depends upon the nature of the cathode
and the angle of incidence of the rays.
We shall see later (chapter IV.) that similar effects are produced when
the rays from radio-active substances impinge upon solid bodies.
* * * * *
In this chapter an account of the ionization theory of gases has been
given to the extent that is necessary for the interpretation of the
measurements of radio-activity by the electric method. It would be out
of place here to discuss the development of that theory in detail, to
explain the passage of electricity through flames and vapours, the
discharge of electricity from hot bodies, and the very complicated
phenomena observed in the passage of electricity through a vacuum tube.
For further information on this important subject, the reader is
referred to J. J. Thomson’s _Conduction of Electricity through Gases_,
in which the whole subject is treated in a full and complete manner. A
simple account of the effect of moving charges and the electronic theory
of matter was given by the same author in the Silliman Lectures of Yale
University and published under the title _Electricity and Matter_
(Scribner, New York, 1904).
Footnote 44:
J. J. Thomson and Rutherford, _Phil. Mag._ Nov. 1896.
Footnote 45:
The word ion has now been generally adopted in the literature of the
subject. In using this word, it is not assumed that the ions in gases
are the same as the corresponding ions in the electrolysis of
solutions.
Footnote 46:
A minute current is observed between the plates even if no
radio-active matter be present. This has been found to be due mainly
to a slight natural radio-activity of the matter composing them. (See
chapter XIV.)
Footnote 47:
This nomenclature has arisen from the similarity of the shape of the
current-voltage curves to the magnetization curves for iron. Since, on
the ionization theory, the maximum current is a result of the
_removal_ of all the ions from the gas, before recombination occurs,
the terms are not very suitable. They have however now come into
general use and will be retained throughout this work.
Footnote 48:
J. J. Thomson, _Phil. Mag._ 47, p. 253, 1899; _Conduction of
Electricity through Gases_, p. 73, 1903.
Footnote 49:
Rutherford, _Phil. Mag._ Jan. 1899.
Footnote 50:
Townsend, _Phil. Mag._ Feb. 1901.
Footnote 51:
Rutherford, _Phil. Mag._ Nov. 1897, p. 144, Jan. 1899.
Footnote 52:
Townsend, _Phil. Trans._ A, p. 157, 1899.
Footnote 53:
McClung, _Phil. Mag._ March, 1902.
Footnote 54:
Langevin, _Thèse présentée à la Faculté des Sciences_, p. 151, Paris,
1902.
Footnote 55:
Owens, _Phil. Mag._ Oct. 1899.
Footnote 56:
Rutherford, _Phil. Mag._ p. 429, Nov. 1897.
Footnote 57:
Zeleny, _Phil. Trans._ A, p. 193, 1901.
Footnote 58:
Langevin, _C. R._ 134, p. 646, 1902.
Footnote 59:
Zeleny, _Phil. Mag._ July, 1898.
Footnote 60:
Rutherford, _Phil. Mag._ Feb. 1899.
Footnote 61:
Zeleny, _Phil. Trans._ 195, p. 193, 1900.
Footnote 62:
Langevin, _C. R._ 134, p. 646, 1902, and Thesis, p. 191, 1902.
Footnote 63:
Rutherford, _Proc. Camb. Phil. Soc._ 9, p. 410, 1898.
Footnote 64:
Langevin, Thesis, p. 190, 1902.
Footnote 65:
Helmholtz and Richarz, _Annal. d. Phys._ 40, p. 161, 1890.
Footnote 66:
Wilson, _Phil. Trans._ p. 265, 1897; p. 403, 1899; p. 289, 1900.
Footnote 67:
Thomson, _Phil. Mag._ p. 528, Dec. 1898.
Footnote 68:
Wilson, _Phil. Trans._ A, 193, p. 289, 1899.
Footnote 69:
Thomson, _Phil. Mag._ p. 528, Dec. 1898, and March, 1903. _Conduction
of Electricity through Gases_, Camb. Univ. Press, 1903, p. 121.
Footnote 70:
Wilson, _Phil. Mag._ April, 1903.
Footnote 71:
Townsend, _Phil. Trans._ A, p. 129, 1899.
Footnote 72:
Townsend, _loc. cit._ p. 139.
Footnote 73:
Some difference of opinion has been expressed as to the value of _V_
required to produce ions at each collision. Townsend considers it to
be about 20 volts; Langevin 60 volts and Stark about 50 volts.
Footnote 74:
Rutherford, _Phil. Mag._ Jan. 1899.
Footnote 75:
Rutherford, _Phil. Mag._ Jan. 1899.
Footnote 76:
Strutt, _Phil. Trans._ A, p. 507, 1901 and _Proc. Roy. Soc._ p. 208,
1903.
Footnote 77:
McClung, _Phil. Mag._ Sept. 1904.
Footnote 78:
Eve, _Phil. Mag._ Dec. 1904.
Footnote 79:
Rutherford, _Phil. Mag._ p. 137, Jan. 1899.
Footnote 80:
Child, _Phys. Rev._ Vol. 12, 1901.
Footnote 81:
Rutherford, _Phil. Mag._ p. 210, August, 1901; _Phys. Rev._ Vol. 13,
1901.
Footnote 82:
Rutherford, _Phil. Mag._ Aug. 1901.
Footnote 83:
A simple and excellent account of the effects produced by the motion
of a charged ion and also of the electronic theory of matter was given
by Sir Oliver Lodge in 1903 in a paper entitled “Electrons”
(_Proceedings of the Institution of Electrical Engineers_, Part 159,
Vol. 32, 1903). See also J. J. Thomson’s _Electricity and Matter_
(Scribner, New York, 1904).
Footnote 84:
J. J. Thomson, _Phil. Mag._ April, 1887.
Footnote 85:
Heaviside, _Collected Papers_, Vol. II. p. 514.
Footnote 86:
Searle, _Phil. Mag._ Oct. 1897.
Footnote 87:
Abraham, _Phys. Zeit._ 4, No. 1 b, p. 57, 1902.
Footnote 88:
A full account of the path described by a moving ion under various
conditions is given by J. J. Thomson, _Conduction of Electricity in
Gases_ (Camb. Univ. Press, 1903), pp. 79–90.
Footnote 89:
J. J. Thomson, _Phil. Mag._ p. 293, 1897.
Footnote 90:
Lenard, _Annal. d. Phys._ 64, p. 279, 1898.
Footnote 91:
Kaufmann, _Annal. d. Phys._ 61, p. 544; 62, p. 596, 1897; 65, p. 431,
1898.
Footnote 92:
Simon, _Annal. d. Phys._ 69, p. 589, 1899.
Footnote 93:
A complete discussion of the various methods employed to measure the
velocity and mass of electrons and also of the theory on which they
are based will be found in J. J. Thomson’s _Conduction of Electricity
through Gases_.
Footnote 94:
Goldstein, _Berlin Sitzber._ 39, p. 691, 1896; _Annal. d. Phys._ 64,
p. 45, 1898.
Footnote 95:
Wien, _Annal. d. Phys._ 65, p. 440, 1898.
Footnote 96:
Larmor, _Phil. Mag._ 44, p. 593, 1897.
Footnote 97:
J. J. Thomson, _Phil. Mag._ Feb. 1897.
Footnote 98:
Barkla, _Phil. Mag._ June, 1903.
CHAPTER III.
METHODS OF MEASUREMENT.
=53. Methods of Measurement.= Three general methods have been employed
for examination of the radiations from radio-active bodies, depending on
(1) The action of the rays on a photographic plate.
(2) The ionizing action of the rays on the surrounding gas.
(3) The fluorescence produced by the rays on a screen of platinocyanide
of barium, zinc sulphide, or similar substance.
The third method is very restricted in its application, and can only be
employed for intensely active substances like radium or polonium.
The photographic method has been used very widely, especially in the
earlier development of the subject, but has gradually been displaced by
the electrical method, as a quantitative determination of the radiations
became more and more necessary. In certain directions, however, it
possesses distinct advantages over the electrical method. For example,
it has proved a very valuable means of investigating the curvature of
the path of the rays, when deflected by a magnetic or electric field,
and has allowed us to determine the constants of these rays with
considerable accuracy.
On the other hand, as a general method of study of the radiations, it is
open to many objections. A day’s exposure is generally required to
produce an appreciable darkening of the sensitive film when exposed to a
weak source of radiation like uranium and thorium. It cannot, in
consequence, be employed to investigate the radiations of those active
products which rapidly lose their activity. Moreover, W. J. Russell has
shown that the darkening of a photographic plate can be produced by many
agents which do not give out rays like those of the radio-active bodies.
This darkening of the plate is produced under the most varied
conditions, and very special precautions are necessary when long
exposures to a weak source of radiation are required.
The main objection to the photographic method, however, lies in the fact
that the radiations which produce the strongest electrical effect are
very weak photographically. For example, Soddy[99] has shown that the
photographic action of uranium is due almost entirely to the more
penetrating rays, and that the easily absorbed rays produce in
comparison very little effect. Speaking generally, the penetrating rays
are the most active photographically, and, under ordinary conditions,
the action on the plate is almost entirely due to them.
Most of the energy radiated from active bodies is in the form of easily
absorbed rays which are comparatively inactive photographically. These
rays are difficult to study by the photographic method, as the layer of
black paper which, in many cases, is required in order to absorb the
phosphorescent light from active substances, cuts off at the same time
most of the rays under examination. These easily absorbed rays will be
shown to play a far more important part in the processes occurring in
radio-active bodies than the penetrating rays which are more active
photographically.
The electrical method, on the other hand, offers a rapid and accurate
method of quantitatively examining the radiations. It can be used as a
means of measurement of all the types of radiation emitted, excluding
light waves, and is capable of accurate measurement over an extremely
wide range. With proper precautions it can be used to measure effects
produced by radiations of extremely small intensity.
=54. Electrical Methods.= The electrical methods employed in studying
radio-activity are all based on the property of the radiation in
question of ionizing the gas, _i.e._ of producing positively and
negatively charged carriers throughout the volume of the gas. The
discussion of the application of the ionization theory of gases to
measurements of radio-activity has been given in the last chapter. It
has been shown there that the essential condition to be fulfilled for
comparative measurements of the intensity of the radiations is that the
electrical field shall in all cases be strong enough to obtain the
maximum or saturation current through the gas.
The electric field required to produce practical saturation varies with
the intensity of the ionization and consequently with the activity of
the preparations to be examined. For preparations which have an activity
not more than 500 times that of uranium, under ordinary conditions, a
field of 100 volts per cm. is sufficient to produce a practical
saturation current. For very active samples of radium, it is often
impossible to obtain conveniently a high enough electromotive force to
give even approximate saturation. Under such conditions comparative
measurement can be made by measuring the current under diminished
pressure of the gas, when saturation is more readily obtained.
The method to be employed in the measurement of this ionization current
depends largely on the intensity of the current to be measured. If some
very active radium is spread on the lower of two insulated plates as in
Fig. 1, and a saturating electric field applied, the current may readily
be measured by a sensitive galvanometer of high resistance. For example,
a weight of ·45 gr. of radium chloride of activity 1000 times that of
uranium oxide, spread over a plate of area 33 sq. cms., gave a maximum
current of 1·1 × 10⁻⁸ amperes when the plates were 4·5 cms. apart. In
this case the difference of potential to be applied to produce practical
saturation was about 600 volts. Since most of the ionization is due to
rays which are absorbed in passing through a few centimetres of air, the
current is not much increased by widening the distance between the two
plates. In cases where the current is not quite large enough for direct
deflection, the current may be determined by connecting the upper
insulated plate with a well insulated condenser. After charging for a
definite time, say one or more minutes, the condenser is discharged
through the galvanometer, and the current can readily be deduced.
=55.= In most cases, however, when dealing with less active substances
like uranium or thorium, or with small amounts of active material, it is
necessary to employ methods for measuring much smaller currents than can
be detected conveniently by an ordinary galvanometer. The most
convenient apparatus to employ for this purpose is one of the numerous
types of quadrant electrometer or an electroscope of special design. For
many observations, especially where the activity of the two substances
is to be compared under constant conditions, an electroscope offers a
very certain and easy method of measurement. As an example of a simple
apparatus of this kind, a brief description will be given of the
electroscope used by M. and Mme Curie in many of their earlier
observations.
[Illustration: Fig. 11.]
The connections are clearly seen from Fig. 11. The active material is
placed on a plate laid on top of the fixed circular plate _P_, connected
with the case of the instrument and with earth. The upper insulated
plate _P´_ is connected with the insulated gold-leaf system _LL´_. _S_
is an insulating support and _L_ the gold-leaf.
The system is first charged to a suitable potential by means of the rod
_C_. The rate of movement of the gold-leaf is observed by means of a
microscope. In comparisons of the activity of two specimens, the time
taken by the gold leaf to pass over a certain number of divisions of the
micrometer scale in the eye-piece is observed. Since the capacity of the
charged system is constant, the average rate of movement of the
gold-leaf is directly proportional to the ionization current between _P_
and _P´_, _i.e._ to the intensity of the radiation emitted by the active
substance. Unless very active material is being examined, the difference
of potential between _P_ and _P´_ can easily be made sufficient to
produce saturation.
When necessary, a correction can be made for the rate of leak when no
active material is present. In order to avoid external disturbances, the
plates _PP´_ and the rod _C_ are surrounded by metal cylinders, _E_ and
_F_, connected with earth.
=56.= A modified form of the gold-leaf electroscope can be used to
determine extraordinarily minute currents with accuracy, and can be
employed in cases where a sensitive electrometer is unable to detect the
current. A special type of electroscope has been used by Elster and
Geitel, in their experiments on the natural ionization of the
atmosphere. A very convenient type of electroscope to measure the
current due to minute ionization of the gas is shown in Fig. 12.
[Illustration: Fig. 12.]
This type of instrument was first used by C. T. R. Wilson[100] in his
experiments of the natural ionization of air in closed vessels. A brass
cylindrical vessel is taken of about 1 litre capacity. The gold-leaf
system, consisting of a narrow strip of gold-leaf _L_ attached to a flat
rod _R_, is insulated inside the vessel by the small sulphur bead or
piece of amber _S_, supported from the rod _P_. In a dry atmosphere a
clean sulphur bead or piece of amber is almost a perfect insulator. The
system is charged by a light bent rod _CC´_ passing through an ebonite
cork[101]. The rod _C_ is connected to one terminal of a battery of
small accumulators of 200 to 300 volts. If these are absent, the system
can be charged by means of a rod of sealing-wax. The charging rod _CC´_
is then removed from contact with the gold-leaf system. The rods _P_ and
_C_ and the cylinder are then connected with earth.
The rate of movement of the gold-leaf is observed by a reading
microscope through two holes in the cylinder, covered with thin mica. In
cases where the natural ionization due to the enclosed air in the
cylinder is to be measured accurately, it is advisable to enclose the
supporting and charging rod and sulphur bead inside a small metal
cylinder _M_ connected to earth, so that only the charged gold-leaf
system is exposed in the main volume of the air.
In an apparatus of this kind the small leakage over the sulphur bead can
be eliminated almost completely by keeping the rod _P_ charged to the
average potential of the gold-leaf system during the observation. This
method has been used with great success by C. T. R. Wilson (_loc.
cit._). Such refinements, however, are generally unnecessary, except in
investigations of the natural ionization of gases at low pressures, when
the conduction leak over the sulphur bead is comparable with the
discharge due to the ionized gas.
=57.= The electric capacity _C_ of a gold-leaf system about 4 cms. long
is usually about 1 electrostatic unit. If _V_ is the decrease of
potential of the gold-leaf system in t seconds, the current i through
the gas is given by
_CV_
_i_ = ----
_t_.
With a well cleaned brass electroscope of volume 1 litre, the fall of
potential due to the natural ionization of the air was found to be about
6 volts per hour. Since the capacity of the gold-leaf system was about 1
electrostatic unit
6
_i_ = 1 × ---------- = 5·6 × 10⁻⁶ E.S. units = 1·9 × 10⁻¹⁵ amperes.
3600 × 300
With special precautions a rate of discharge of ⅒ or even ¹⁄₁₀₀ of
this amount can be measured accurately.
The number of ions produced in the gas can be calculated if the charge
on an ion is known. J. J. Thomson has shown that the charge _e_ on an
ion is equal to 3·4 × 10⁻¹⁰ electrostatic units or 1·13 × 10⁻¹⁹
coulombs.
Let _q_ = number of ions produced per second per cubic centimetre
throughout the volume of the electroscope,
_S_ = volume of electroscope in cubic centimetres.
If the ionization be uniform, the saturation current _i_ is given by _i_
= _qSe_.
Now for an electroscope with a volume of 1000 c.c., _i_ was equal to
about 1·9 × 10⁻¹⁵ amperes. Substituting the values given above
_q_ = 17 ions per cubic centimetre per second.
With suitable precautions an electroscope can thus readily measure an
ionization current corresponding to the production of 1 ion per cubic
centimetre per second.
The great advantage of an apparatus of this kind lies in the fact that
the current measured is due to the ionization inside the vessel and is
not influenced by the ionization of the external air or by electrostatic
disturbances[102]. Such an apparatus is very convenient for
investigating the very penetrating radiations from the radio-elements,
since these rays pass readily through the walls of the electroscope.
When the electroscope is placed on a lead plate 3 or 4 mms. thick, the
ionization in the electroscope, due to a radio-active body placed under
the lead, is due entirely to the very penetrating rays, since the other
two types of rays are completely absorbed in the lead plate. If a
circular opening is cut in the base of the electroscope and covered with
thin aluminium of sufficient thickness to absorb the α rays,
measurements of the intensity of the β rays from an active substance
placed under it, can be made with ease and certainty.
=58.= A modified form of electroscope, which promises to be of great
utility for measuring currents even more minute than those to be
observed with the type of instrument already described, has recently
been devised by C. T. R. Wilson[103]. The construction of the apparatus
is shown in Fig. 13.
The case consists of a rectangular brass box 4 cms. × 4 cms. × 3 cms. A
narrow gold-leaf _L_ is attached to a rod _R_ passing through a clean
sulphur cork. Opposite the gold-leaf is fixed an insulated brass plate
_P_, placed about 1 mm. from the wall of the box. The movement of the
gold-leaf is observed through two small windows by means of a microscope
provided with a micrometer scale. The plate _P_ is maintained at a
constant potential (generally about 200 volts). The electrometer case is
placed in an inclined position as shown in the figure, the angle of
inclination and the potential of the plate being adjusted to give the
desired sensitiveness. The gold-leaf is initially connected to the case,
and the microscope adjusted so that the gold-leaf is seen in the centre
of the scale. For a given potential of the plate, the sensitiveness
depends on the angle of tilt of the case. There is a certain critical
inclination below which the gold-leaf is unstable. The most sensitive
position lies just above the critical angle. In a particular experiment
Wilson found that with an angle of tilt of 30° and with the plate at a
constant potential of 207 volts, the gold-leaf, when raised to a
potential of one volt above the case, moved over 200 scale divisions of
the eye-piece, 54 divisions corresponding to one millimetre.
[Illustration: Fig. 13.]
In use, the rod _R_ is connected with the external insulated system
whose rise or fall of potential is to be measured. On account of the
small capacity of the system and the large movement of the gold-leaf for
a small difference of potential, the electroscope is able to measure
extraordinarily minute currents. The apparatus is portable. If the plate
_P_ be connected to one pole of a dry pile the gold-leaf is stretched
out towards the plate, and in this position can be carried without risk
of injury.
=59. Electrometers.= Although the electroscope can be used with
advantage in special cases, it is limited in its application. The most
generally convenient apparatus for measurement of ionization currents
through gases is one of the numerous types of quadrant electrometer.
With the help of auxiliary capacities, the electrometer can be used to
measure currents with accuracy over a wide range, and can be employed
for practically every kind of measurement required in radio-activity.
The elementary theory of the symmetrical quadrant electrometer as given
in the text-books is very imperfect. It is deduced that the sensibility
of the electrometer—measured by the deflection of the needle for 1 volt
P.D. between the quadrants—varies directly as the potential of the
charged needle, provided that this potential is high compared with the
P.D. between the quadrants. In most electrometers however, the
sensibility rises to a maximum, and then decreases with increase of
potential of the needle. For electrometers in which the needle lies
close to the quadrants, this maximum sensibility is obtained for a
comparatively low potential of the needle. A theory of the quadrant
electrometer, accounting for this action, has been recently given by G.
W. Walker[104]. The effect appears to be due to the presence of the air
space that necessarily exists between adjoining quadrants.
[Illustration: Fig. 14.]
Suppose that it is required to measure with an electrometer the
ionization current between two horizontal metal plates _A_ and _B_ (Fig.
14) on the lower of which some active material has been spread. If the
saturation current is required, the insulated plate _A_ is connected
with one pole of a battery of sufficient E.M.F. to produce saturation,
the other pole being connected to earth. The insulated plate _B_ is
connected with one pair of quadrants of the electrometer, the other pair
being earthed. By means of a suitable key _K_, the plate _B_ and the
pair of quadrants connected with it may be either insulated or connected
with earth. When a measurement is to be taken, the earth connection is
broken. If the positive pole of the battery is connected with _A_, the
plate _B_ and the electrometer connections immediately begin to be
charged positively, and the potential, if allowed, will steadily rise
until it is very nearly equal to the potential of _A_. As soon as the
potential of the electrometer system begins to rise, the electrometer
needle commences to move at a uniform rate. Observations of the angular
movement of the needle are made either by the telescope and scale or by
the movement of the spot of light on a scale in the usual way. If the
needle is damped so as to give a uniform motion over the scale, the rate
of movement of the needle, _i.e._ the number of divisions of the scale
passed over per second, may be taken as a measure of the current through
the gas. The rate of movement is most simply obtained by observing with
a stop-watch the time taken for the spot of light, after the motion has
become steady, to pass over 100 divisions of the scale. As soon as the
observation is made, the plate _B_ is again connected with earth, and
the electrometer needle returns to its original position.
In most experiments on radio-activity, only comparative measurements of
saturation currents are required. If these measurements are to extend
over weeks or months, as is sometimes the case, it is necessary to adopt
some method of standardizing the electrometer from day to day, so as to
correct for variation in its sensibility. This is done most simply by
comparing the current to be measured with that due to a standard sample
of uranium oxide, which is placed in a definite position in a small
testing vessel, always kept in connection with the electrometer. Uranium
oxide is a very constant source of radiation, and the saturation current
due to it is the same from day to day. By this method of comparison
accurate observations may be made on the variation of activity of a
substance over long intervals of time, although the sensibility of the
electrometer may vary widely between successive measurements.
=60. Construction of electrometers.= As the quadrant electrometer has
gained the reputation of being a difficult and uncertain instrument for
accurate measurements of current, it may be of value to give some
particular details in regard to the best method of construction and
insulation. In most of the older types of quadrant electrometers the
needle system was made unnecessarily heavy. In consequence of this, if a
sensibility of the order of 100 mms. deflection for 1 volt was required,
it was necessary to charge the Leyden jar connected to the needle to a
fairly high potential. This at once introduced difficulties, for at a
high potential it is not easy to insulate the Leyden jar satisfactorily,
or to charge it to the same potential from day to day. This drawback is
to a large extent avoided in the White pattern of the Kelvin
electrometer, which is provided with a replenisher and attracted disc
for keeping the potential of the needle at a definite value. If
sufficient trouble is taken in insulating and setting up this type of
electrometer, it proves a very useful instrument of moderate
sensibility, and will continue in good working order for a year or more
without much attention.
Simpler types of electrometer of greater sensibility can however be
readily constructed to give accurate results. The old type of quadrant
electrometer, to be found in every laboratory, can readily be modified
to prove a useful and trustworthy instrument. A light needle can be made
of thin aluminium, of silvered paper or of a thin plate of mica, covered
with gold-leaf to make it conducting. The aluminium wire and mirror
attached should be made as light as possible. The needle should be
supported either by a fine quartz fibre or a long bifilar suspension of
silk. A very fine phosphor bronze wire of some length is also very
satisfactory. A magnetic control is not very suitable, as it is
disturbed by coils or dynamos working in the neighbourhood. In addition,
the zero point of the needle is not as steady as with the quartz or
bifilar suspension.
When an electrometer is used to measure a current by noting the rate of
movement of the needle, it is essential that the needle should be damped
sufficiently to give a uniform motion of the spot of light over the
scale. The damping requires fairly accurate adjustment. If it is too
little, the needle has an oscillatory movement superimposed on the
steady motion; if it is too great, it moves too sluggishly from rest and
takes some time to attain a state of uniform motion. With a light
needle, very little, if any, extra damping is required. A light platinum
wire with a single loop dipping in sulphuric acid is generally
sufficient for the purpose.
With light needle systems and delicate suspensions, it is only necessary
to charge the needle to a potential of a few hundred volts to give a
sensibility of several thousand divisions for a volt. With such low
potentials, the difficulty of insulation of the condenser, with which
the needle is in electrical connection, is much reduced. It is
convenient to use a condenser such that the potential of the needle does
not fall more than a few per cent. per day. The ordinary short glass jar
partly filled with sulphuric acid is, in most cases, not easy to
insulate to this extent. It is better to replace it by an ebonite (or
sulphur) condenser[105] such as is shown in Fig. 15.
[Illustration: Fig. 15.]
A circular plate of ebonite about 1 cm. thick is turned down until it is
not more than ½ mm. thick in the centre. Into this circular recess a
brass plate _B_ fits loosely. The ebonite plate rests on another brass
plate _C_ connected with earth. The condenser thus formed has a
considerable capacity and retains a charge for a long time. In order to
make connection with the needle, a small glass vessel _D_, partly filled
with sulphuric acid, is placed on the plate _B_ and put in connection
with the needle by means of a fine platinum wire. The platinum wire from
the needle dips into the acid, and serves to damp the needle. In a dry
atmosphere, a condenser of this kind will not lose more than 20 per
cent. of its charge in a week. If the insulation deteriorates, it can
readily be made good by rubbing the edge of the ebonite _A_ with
sand-paper, or removing its surface in a lathe.
If a sufficient and steady E.M.F. is available, it is much better to
keep the battery constantly connected with the needle, and to avoid the
use of the condenser altogether. If a battery of small accumulators is
used, their potential can be kept at a constant value, and the
electrometer always has a constant sensibility.
=61.= A very useful electrometer of great sensibility has been devised
by Dolezalek[106]. It is of the ordinary quadrant type with a very light
needle of silvered paper, spindle shaped, which lies fairly close to the
quadrants. A very fine quartz suspension is employed. In consequence of
the lightness of the needle and its nearness to the quadrants, it acts
as its own damper. This is a great advantage, for difficulties always
arise when the wire dips into sulphuric acid, on account of the thin
film which collects after some time on the surface of the acid. This
film obstructs the motion of the platinum wire dipping into the acid,
and has to be removed at regular intervals. These instruments can
readily be made to give a sensibility of several thousand divisions for
a volt when the needle is charged to about one hundred volts. The
sensibility of the electrometer passes through a maximum as the
potential of the needle is increased. It is always advisable to charge
the needle to about the value of this critical potential. The capacity
of the electrometer is in general high (about 50 electrostatic units)
but the increased sensibility more than compensates for this. The needle
may either be charged by lightly touching it with one terminal of a
battery, or it may be kept charged to a constant potential through the
quartz suspension.
Dolezalek states that the fibre can be made sufficiently conducting for
the purpose by dipping it into a dilute solution of calcium chloride or
phosphoric acid. I have not found this method satisfactory in dry
climates as in many cases the fibre practically loses its conductivity
after a few days exposure to dry air.
In addition to its great sensibility, the advantage of this instrument
is in the steadiness of the zero and in the self-damping.
A sensibility of 10,000 millimetre divisions per volt can be readily
obtained with this electrometer, if a very fine fibre be used. The use
of such high sensibilities cannot, however, be recommended except for
very special experiments. The period of swing of the needle under these
conditions is several minutes and the natural leak of the testing
vessels employed, as well as electrostatic and other disturbances, make
themselves only too manifest. If measurements of minute currents are
required, an electroscope of the type described in Section 56 is much to
be preferred to a very sensitive electrometer. The electroscope readings
in such a case are more accurate than similar measurements made by an
electrometer.
For most measurements in radio-activity, an electrometer which has a
sensibility of 100 divisions per volt is very suitable, and no advantage
is gained by using an electrometer of greater sensibility. If still
smaller effects require to be measured, the sensibility may be increased
to several thousand divisions per volt.
=62. Adjustment and screening.= In adjusting an electrometer, it is
important to arrange that the needle shall lie symmetrically with regard
to the quadrants. This is best tested by observing whether the needle is
deflected on charging, the quadrants all being earthed. In most
electrometers there is an adjustable quadrant, the position of which may
be altered until the needle is not displaced on charging. When this
condition is fulfilled, the zero reading of the electrometer remains
unaltered as the needle loses its charge, and the deflection on both
sides of the zero should be the same for equal and opposite quantities
of electricity.
The supports of the quadrants require to be well insulated. Ebonite rods
are as a rule more satisfactory for this purpose than glass. In testing
for the insulation of the quadrants and the connections attached, the
system is charged to give a deflection of about 200 scale divisions. If
the needle does not move more than one or two divisions after standing
for one minute, the insulation may be considered quite satisfactory.
When a suitable desiccator is placed inside the tight-fitting
electrometer case, the insulation of the quadrants should remain good
for months. If the insulation of the ebonite deteriorates, it can easily
be made good by removing the surface of the ebonite in a lathe.
In working with a sensitive instrument like the Dolezalek electrometer,
it is essential that the electrometer and the testing apparatus should
be completely enclosed in a screen of wire-gauze connected with earth,
in order to avoid electrostatic disturbances. If an apparatus is to be
tested at some distance from the electrometer, the wires leading to it
should be insulated in metal cylinders connected with earth. The size of
the insulators used at various points should be made as small as
possible, in order to avoid disturbances due to their electrification.
In damp climates, paraffin, amber, or sulphur insulates better than
ebonite. The objection to paraffin as an insulator for sensitive
electrometers lies in the difficulty of getting entirely rid of any
electrification on its surface. When paraffin has been once charged, the
residual charge, after diselectrifying it with a flame, continues to
leak out for a long interval. All insulators should be diselectrified by
means of a spirit-lamp or still better by leaving some uranium near
them. Care should be taken not to touch the insulation when once
diselectrified.
In accurate work it is advisable to avoid the use of gas jets or Bunsen
flames in the neighbourhood of the electrometer, as the flame gases are
strongly ionized and take some time to lose their conductivity. If
radio-active substances are present in the room, it is necessary to
enclose the wires leading to the electrometer in fairly narrow tubes,
connected with earth. If this is not done, it will be found that the
needle does not move at a constant rate, but rapidly approaches a steady
deflection where the rate of loss of charge of the electrometer and
connections, due to the ionization of the air around them, is balanced
by the current to be measured. This precaution must always be taken when
observations are made on the very penetrating rays from active
substances. These rays readily pass through ordinary screens, and ionize
the air around the electrometer and connecting wires. For this reason it
is impossible to make accurate measurements of small currents in a room
which is used for the preparation of radio-active material. In course of
time the walls of the room become radio-active owing to the
dissemination of dust and the action of the radio-active
emanations[107].
=63. Electrometer key.= For work with electrometers of high sensibility,
a special key is necessary to make and break from a distance the
connection of the quadrants with earth in order to avoid electrostatic
disturbances at the moment the current is to be measured. The simple key
shown in Fig. 16 has been found very satisfactory for this purpose. A
small brass rod _BM_, to which a string is attached, can be moved
vertically up and down in a brass tube _A_, which is rigidly attached to
a bent metal support connected with earth. When the string is released,
this rod makes contact with the mercury _M_, which is placed in a small
metal vessel resting on a block of ebonite _P_. The electrometer and
testing vessel are connected with the mercury. When the string is
pulled, the rod _BM_ is removed from the mercury and the earth
connection of the electrometer system is broken. On release of the
string, the rod _BM_ falls and the electrometer is again earthed. By
means of this key, which may be operated at any distance from the
electrometer, the earth connection may be made and broken at definite
intervals without any appreciable disturbance of the needle.
[Illustration: Fig. 16.]
=64. Testing apparatus.= The arrangement shown in Fig. 17 is very
convenient for many measurements in radio-activity. Two parallel
insulated metal plates _A_ and _B_ are placed inside a metal vessel _V_,
provided with a side door. The plate _A_ is connected with one terminal
of a battery of small storage cells, the other pole of which is earthed;
the plate _B_ with the electrometer, and the vessel _V_ with earth. The
shaded areas in the figure indicate the position of ebonite insulators.
The active material to be tested is spread uniformly in a shallow groove
(about 5 cms. square and 2 mms. deep) in the brass plate _A_. In order
to avoid breaking the battery connection every time the plate _A_ is
removed, the wire from the battery is permanently connected with the
metal block _N_ resting on the ebonite support. In this arrangement
there is no possibility of a conduction leak from the plate _A_ to _B_,
since the earth-connected vessel _V_ intervenes.
[Illustration: Fig. 17.]
An apparatus of this kind is very convenient for testing the absorption
of the radiations by solid screens, as well as for making comparative
studies of the activity of different bodies. Unless very active
preparations of radium are employed, a battery of 300 volts is
sufficient to ensure saturation when the plates are not more than 5
centimetres apart. If substances which give off a radio-active emanation
are being tested, the effect of the emanation can be eliminated by
passing a steady current of air from a gas bag between the plates. This
removes the emanation as fast as it is produced.
If a clean plate is put in the place of _A_, a small movement of the
electrometer needle is always observed. If there is no radio-active
substance in the neighbourhood, this effect is due to the small natural
ionization of the air. We can correct for this natural leak when
necessary.
=65.= We have often to measure the activity due to the emanations of
thorium or radium, or the excited activity produced by those emanations
on rods or wires. A convenient apparatus for this purpose is shown in
Fig. 18. The cylinder _B_ is connected with the battery in the usual
way, and the central conductor _A_ with the electrometer. This central
rod is insulated from the external cylinder by an ebonite cork, which is
divided into two parts by a metal ring _CC´_ connected to earth. This
ring acts the part of a guard-ring, and prevents any conduction leak
between _B_ and _A_. The ebonite is thus only required to insulate
satisfactorily for the small rise of potential produced on _A_ during
the experiment. In all accurate measurements of current in
radio-activity the guard-ring principle should always be used to ensure
good insulation. This is easily secured when the ebonite is only
required to insulate for a fraction of a volt, instead of for several
hundred volts, as is the case when the guard-ring is absent.
[Illustration: Fig. 18.]
=66.= For measurements of radio-activity with an electrometer, a steady
source of E.M.F. of at least 300 volts is necessary. This is best
obtained by a battery of small cells simply made by immersing strips of
lead in dilute sulphuric acid, or by a battery of small accumulators of
the usual construction. Small accumulators of capacity about one-half
ampere-hour can now be obtained at a moderate price, and are more
constant and require less attention than simple lead cells.
In order to measure currents over a wide range, a graduated series of
capacities is required. The capacity of an electrometer and testing
apparatus is usually about 50 electrostatic units or ·000056
microfarads. Subdivided condensers of mica are constructed in which
capacities varying from ·001 to ·2 microfarads are provided. With such a
condenser, another extra capacity is required to bridge over the gap
between the capacity of the electrometer and the lowest capacity of the
condenser. This capacity of value about 200 electrostatic units can
readily be made by using parallel plates or still better concentric
cylinders. With this series of capacities, currents may be measured
between 3 × 10⁻¹⁴ and 3 × 10⁻⁸ amperes—a range of over one million.
Still larger currents can be measured if the sensibility of the
electrometer is reduced, or if larger capacities are available.
In a room devoted to electrometer measurements of radio-activity, it is
desirable to have no radio-active matter present except that to be
tested. The room should also be as free from dust as possible. The
presence of a large quantity of dust in the air (see section 31) is a
very disturbing factor in all radio-active measurements. A larger E.M.F.
is required to produce saturation on account of the diffusion of the
ions to the dust particles. The presence of dust in the air also leads
to uncertainty in the distribution of excited activity in an electric
field (see section 181).
=67. Measurement of Current.= In order to determine the current in the
electrometer circuit by measuring the rate of movement of the needle, it
is necessary to know both the capacity of the circuit and the
sensibility of the electrometer.
Let _C_ = capacity of electrometer and its connections in E.S.
units,
_d_ = number of divisions of the scale passed over per second,
_D_ = sensibility of the electrometer measured in scale
divisions
for 1 volt P.D. between the quadrants.
The current _i_ is given by the product of the capacity of the system
and the rate of rise of potential.
Thus
_Cd_
_i_ = ----- E.S. units,
300_D_
_Cd_
= ----------- amperes.
9 × 10¹¹ _D_
Suppose, for example,
_C_ = 50, _d_ = 5, _D_ = 1000;
then _i_ = 2·8 × 10⁻¹³ amperes.
Since the electrometer can readily measure a current corresponding to a
movement of half a scale division per second, we see that an
electrometer can measure a current of 3 × 10⁻¹⁴ amperes, which is
considerably below the range of the most sensitive galvanometer.
The capacity of the electrometer itself must not be considered as equal
to that of the pair of quadrants and the needle when in a position of
rest. The actual capacity is very much larger than this, on account of
the motion of the charged needle. Suppose, for example, that the needle
is charged to a high negative potential, and kept at the zero position
by an external constraint. If a quantity _Q_ of positive electricity is
given to the electrometer and its connections, the whole system is
raised to a potential _V_, such that _Q_ = _CV_, where _C_ is the
capacity of the system. When however the needle is allowed to move, it
is attracted into the charged pair of quadrants. This corresponds to the
introduction of a negatively charged body between the quadrants, and in
consequence the potential of the system is lowered to _V´_. The actual
capacity _C´_ of the system when the needle moves is thus greater than
_C_, and is given by
_C´V´_ = _CV_.
Thus the capacity of the electrometer is not a constant, but depends on
the potential of the needle, _i.e._ on the sensibility of the
electrometer.
An interesting result of practical importance follows from the variation
of the capacity of the electrometer with the potential of the needle. If
the external capacity attached to the electrometer is small compared
with that of the electrometer itself, the rate of movement of the needle
for a constant current is, in some cases, independent of the
sensibility. An electrometer may be used for several days or even weeks
to give nearly equal deflections for a constant current, without
recharging the needle, although its potential has been steadily falling
during the interval. In such a case the decrease in sensibility is
nearly proportional to the decrease in capacity of the electrometer, so
that the deflection for a given current is only slightly altered. The
theory of this action has been given by J. J. Thomson[108].
=68.= The capacity of the electrometer and its connections cannot be
measured by any of the commutator methods used for the determination of
small capacities, for in such cases the needle does not move, and the
capacity measured is not that of the electrometer system when in actual
use. The value of the capacity may, however, be determined by the method
of mixtures.
Let _C_ = capacity of electrometer and connections,
_C₁_ = capacity of a standard condenser.
The electrometer and its connections are charged to a potential _V₁_ by
a battery, and the deflection _d₁_ of the needle is noted. By means of
an insulated key, the capacity of the standard condenser is added in
parallel with the electrometer system. Let _V₂_ be the potential of the
system, and _d₂_ the new deflection.
Then _CV₁_ = (_C_ + _C₁_) _V₂_,
_C_ + _C₁_ _V₁_ _d₁_
-------- = ---- = -----
_C_ _V₂_ _d₂_
_d₂_
and _C_ = _C₁_ ----------
_d₁_ − _d₂_
[Illustration: Fig. 19.]
A simple standard capacity for this purpose can be constructed of two
concentric brass tubes the diameters of which can be accurately
measured. The external cylinder _D_ (Fig. 19) is mounted on a wooden
base, which is covered with a sheet of metal or tinfoil connected to
earth. The tube _C_ is supported centrally on ebonite rods at each end.
The capacity is given approximately by the formula
$$ C = \frac {l} {2 \log_e \frac {b} {a}} $$,
where _b_ is the internal diameter of _D_, _a_ the external diameter of
_C_, and _l_ the length of the tubes.
The following method can be used in some cases with advantage. While a
testing vessel is in connection with the electrometer, a sample of
uranium is placed on the lower plate _A_. Let _d₂_ and _d₁_ be the
number of divisions passed over per second by the needle with and
without the standard capacity in connection.
_C_ + _C₁_ _d₁_
Then ---------- = ----,
_C_ _d₂_
_d₂_
and _C_ = _C₁_ ---------
_d₁_ − _d₂_
This method has the advantage that the relative capacities are expressed
in terms of the motion of the needle under the actual conditions of
measurement.
=69. Steady deflection method.= The methods of measurement previously
described depend upon the rate of angular movement of a suspended
gold-leaf or of an electrometer needle. The galvanometer can only be
employed for measurements with intensely active matter. A need, however,
has long been felt for a method in which ordinary ionization currents
can be measured by means of a steady deflection of an electrometer
needle. This is especially the case, where measurements have to be made
with active substances whose activity alters rapidly in the course of a
few minutes.
This can obviously be secured if the electrometer system (one pair of
quadrants being earthed) is connected to earth through a suitable high
resistance. A steady deflection of the electrometer needle will be
obtained when the rate of supply of electricity to the electrometer
system is balanced by the loss due to conduction through the resistance.
If the high resistance obeys Ohm’s law, the deflection should be
proportional to the ionization current to be measured.
A simple calculation shows that the resistance required is very great.
Suppose, for example, that a current is to be measured corresponding to
a rate of movement of the needle of 5 divisions per second, with a
sensibility of 1000 divisions per volt, and where the capacity of the
electrometer system is 50 electrostatic units. This current is equal to
2·8 × 10⁻¹³ amperes. If a steady deflection of 10 divisions is required,
which corresponds to a rise of potential of the system of ¹⁄₁₀₀ of a
volt, the resistance should be 36,000 megohms. For a deflection of 100
divisions, the resistance should be 10 times as large. Dr Bronson[109],
working in the laboratory of the writer, has recently made some
experiments in order to devise a practical method for measurements of
this character. It is difficult to obtain sufficiently high and constant
resistances to answer the purpose. Tubes of xylol had too great a
resistance, while special carbon resistances were not sufficiently
constant. The difficulty was finally got over by the use of what may be
called an “air resistance.” The arrangement of the experiment is shown
in Fig. 20.
[Illustration: Fig. 20.]
The electrometer system was connected with the upper of two insulated
parallel plates _AB_, on the lower of which was spread a layer of a very
active substance. An active bismuth plate, coated with radio-tellurium,
which had been obtained from Sthamer of Hamburg, proved very convenient
for this purpose.
The lower plate _B_ was connected to earth. The charge communicated to
the upper plate of the testing vessel _CD_ and the electrometer system
leaked away in consequence of the strong ionization between the plates
_AB_, and a steady deflection was obtained when the rate of supply was
equal to the rate of discharge.
This air resistance obeyed Ohm’s law over a considerable range, _i.e._
the steady deflection was proportional to the current. It is advisable,
in such an arrangement, to test whether the deflection is proportional
to the ionization current over the range required for measurement. This
can readily be done by the use of a number of metal vessels filled with
a constant radio-active substance like uranium oxide. The effect of
these, when placed in the testing vessel, can be tested separately and
in groups, and in this way the scale can be calibrated accurately.
The plates _AB_ were placed inside a closed vessel to avoid air
currents. The contact difference of potential between the plates _AB_,
which shows itself by a steady deflection when no radio-active matter is
present in _CD_, was for the most part eliminated by covering the
surface of the plates _A_ and _B_ with very thin aluminium foil.
This method proved very accurate and convenient for measurement of rapid
changes in activity, and possesses many advantages over the ordinary
rate-method of use of an electrometer. A thin layer of radium of
moderate activity would probably serve in place of the radio-tellurium,
but the emanation and the β and γ rays emitted from it would be a
possible source of disturbance to the measurements. The deflection of
the electrometer needle in this arrangement is independent of the
capacity of the electrometer system, and thus comparative measurements
of current can be made without the necessity of determining the capacity
in each case.
=70.= =Quartz piezo-electrique.= In measurements of the strength of
currents by electrometers, it is always necessary to determine the
sensibility of the instrument and the capacity of the electrometer and
the apparatus attached thereto. By means of the quartz piezo-electrique
devised by the brothers MM. J. and P. Curie[110], measurements of the
current can be made with rapidity and accuracy over a wide range. These
measurements are quite independent of the capacity of the electrometer
and external circuit.
The essential part of this instrument consists of a plate of quartz
which is cut in a special manner. When this plate is placed under
tension, there is a liberation of electricity equal in amount but
opposite in sign on the two sides of the plate. The plate of quartz _AB_
(Fig. 21) is hung vertically and weights are added to the lower end. The
plate is cut so that the optic axis of the crystal is horizontal and at
right angles to the plane of the paper.
[Illustration: Fig. 21.]
The two faces _A_ and _B_ are normal to one of the binary axes (or
electrical axes) of the crystal. The tension must be applied in a
direction normal to the optic and electric axes. The two faces _A_ and
_B_ are silvered, but the main portion of the plate is electrically
insulated by removing a narrow strip of the silvering near the upper and
lower ends of the plate. One side of the plate is connected with the
electrometer and with the conductor, the rate of leak of which is to be
measured. The quantity of electricity set free on one face of the plate
is accurately given by
_L_
_Q_ = ·063 ---- _F_
_b_
where _L_ is the length of the insulated portion of the plate, _b_ the
thickness _AB_, and _F_ the weight attached in kilogrammes. _Q_ is then
given in electrostatic units.
Suppose, for example, that it is required to measure the current between
the plates _CD_ (Fig. 21) due to some radio-active material on the plate
_C_, for a given difference of potential between _C_ and _D_. At a given
instant the connection of the quadrants of the electrometer with the
earth is broken. The weight is attached to the quartz plate, and is held
in the hand so as to apply the tension gradually. This causes a release
of electricity opposite in sign to that given to the plate _D_. The
electrometer needle is kept at the position of rest as nearly as
possible by adjusting the tension by hand. The tension being fully
applied, the moment the needle commences to move steadily from zero is
noted. The current between the plates _CD_ is then given by _Q_/_t_
where _t_ is the time of the observation. The value of _Q_ is known from
the weight attached.
In this method the electrometer is only used as a detector to show that
the system is kept at zero potential. No knowledge of the capacity of
the insulated system is required. With practice, measurements of the
current can be made in this way with rapidity and certainty.
Footnote 99:
Soddy, _Trans. Chem. Soc._ Vol. 81, p. 860, 1902.
Footnote 100:
Wilson, _Proc. Roy. Soc._ Vol. 68, p. 152, 1901.
Footnote 101:
If the apparatus is required to be air-tight, the gold-leaf system can
be charged by means of a piece of magnetized steel wire, which is made
to touch the rod _R_ by the approach of a magnet.
Footnote 102:
It is sometimes observed that the motion of the gold-leaf, immediately
after charging, is irregular. In many cases, this can be traced to air
currents set up in the electroscope in consequence of unsymmetrical
heating by the source of light used for illumination.
Footnote 103:
Wilson, _Proc. Camb. Phil. Soc._ Vol. 12, Part II. 1903.
Footnote 104:
Walker, _Phil. Mag._ Aug. 1903.
Footnote 105:
Strutt, _Phil. Trans._ A, p. 507, 1901.
Footnote 106:
Dolezalek, _Instrumentenkunde_, p. 345, Dec. 1901.
Footnote 107:
It is very desirable that care should be taken not to release large
quantities of the radium emanation inside a laboratory. This emanation
has a slow rate of decay and is carried by currents of air throughout
the whole building and finally leaves behind an active deposit of very
slow rate of change (see chapter XI.). Eve (_Nature_, March 16, 1905)
has drawn attention to the difficulty of making refined radio-active
measurements under such conditions.
Footnote 108:
J. J. Thomson, _Phil. Mag._ 46, p. 537, 1898.
Footnote 109:
Bronson, _Amer. Journ. Science_, Feb. 1905.
Footnote 110:
J. and P. Curie, _C. R._ 91, pp. 38 and 294, 1880. See also Friedel
and J. Curie, _C. R._ 96, pp. 1262 and 1389, 1883, and Lord Kelvin,
_Phil. Mag._ 36, pp. 331, 342, 384, 414, 453, 1893.
CHAPTER IV.
NATURE OF THE RADIATIONS.
PART I.
Comparison of the Radiations.
=71. The Three Types of Radiation.= All the radio-active substances
possess in common the power of acting on a photographic plate and of
ionizing the gas in their immediate neighbourhood. The intensity of the
radiations may be compared by means of their photographic or electrical
action; and, in the case of the strongly radio-active substances, by the
power they possess of lighting up a phosphorescent screen. Such
comparisons, however, do not throw any light on the question whether the
radiations are of the same or of different kinds, for it is well known
that such different types of radiations as the short waves of
ultra-violet light, Röntgen and cathode rays, all possess the property
of producing ions throughout the volume of a gas, lighting up a
fluorescent screen, and acting on a photographic plate. Neither can the
ordinary optical methods be employed to examine the radiations under
consideration, as they show no trace of regular reflection, refraction,
or polarization.
Two general methods can be used to distinguish the types of the
radiations given out by the same body, and also to compare the
radiations from the different active substances. These methods are as
follows:
(1) By observing whether the rays are appreciably deflected in a
magnetic field.
(2) By comparing the relative absorption of the rays by solids and
gases.
Examined in these ways, it has been found that there are three different
types of radiation emitted from radio-active bodies, which for brevity
and convenience have been termed by the writer the α, β, and γ rays.
(i) The α rays are very readily absorbed by thin metal foil and by a few
centimetres of air. They have been shown to consist of positively
charged bodies projected with a velocity of about ⅒ the velocity of
light. They are deflected by intense magnetic and electric fields, but
the amount of deviation is minute in comparison with the deviation,
under the same conditions, of the cathode rays produced in a vacuum
tube.
(ii) The β rays are far more penetrating in character than the α rays,
and consist of negatively charged bodies projected with velocities of
the same order as the velocity of light. They are far more readily
deflected than the α rays, and are in fact identical with the cathode
rays produced in a vacuum tube.
(iii) The γ rays are extremely penetrating, and non-deviable by a
magnetic field. Their true nature is not definitely settled, but they
are analogous in most respects to very penetrating Röntgen rays.
The three best known radio-active substances, uranium, thorium, and
radium, all give out these three types of rays, each in an amount
approximately proportional to its relative activity measured by the α
rays. Polonium stands alone in giving only the α or easily absorbed
rays[111].
=72. Deflection of the rays.= The rays emitted from the active bodies
thus present a very close analogy with the rays which are produced in a
highly exhausted vacuum tube when an electric discharge passes through
it. The α rays correspond to the canal rays, discovered by Goldstein,
which have been shown by Wien to consist of positively charged bodies
projected with great velocity (see section 51). The β rays are the same
as the cathode rays, while the γ rays resemble the Röntgen rays. In a
vacuum tube, a large amount of electric energy is expended in producing
the rays, but, in the radio-active bodies, the rays are emitted
spontaneously, and at a rate uninfluenced by any chemical or physical
agency. The α and β rays from the active bodies are projected with much
greater velocity than the corresponding rays in a vacuum tube, while the
γ rays are of much greater penetrating power than Röntgen rays.
The effect of a magnetic field on a pencil of rays from a radio-active
substance giving out the three kinds of rays is very well illustrated in
Fig. 22[112].
[Illustration: Fig. 22.]
Some radium is placed in the bottom of a narrow cylindrical lead vessel
_R_. A narrow pencil of rays consisting of α, β, and γ rays escapes from
the opening. If a strong uniform magnetic field is applied at right
angles to the plane of the paper, and directed towards the paper, the
three types of rays are separated from one another. The γ rays continue
in a straight line without any deviation. The β rays are deflected to
the right, describing circular orbits the radii of which vary within
wide limits. If the photographic plate _AC_ is placed under the radium
vessel, the β rays produce a diffuse photographic impression on the
right of the vessel _R_. The α rays are bent in the direction opposite
to that of the β rays, and describe a portion of the arc of a circle of
large radius, but they are rapidly absorbed after traversing a distance
of a few centimetres from the vessel _R_. The amount of the deviation of
the α rays compared with that of the β rays is much exaggerated in the
figure.
=73. Ionizing and penetrating power of the rays.= Of the three kinds of
rays, the α rays produce most of the ionization in the gas and the γ
rays the least. With a thin layer of unscreened active material spread
on the lower of two parallel plates 5 cms. apart, the amount of
ionization due to the α, β, and γ rays is of the relative order 10,000,
100, and 1. These numbers are only rough approximations, and the
differences become less marked as the thickness of the radio-active
layer increases.
The average penetrating power of the rays is shown below. In the first
column is given the thickness of the aluminium, which cuts each
radiation down to half its value, and in the second the relative power
of penetration of the rays.
Radiation Thickness of Relative
Aluminium in cms. power of
which cuts off half penetration
the radiation
α rays 0·0005 cms. 1
β „ 0·05 cms. 100
γ „ 8 cms. 10000
The relative power of penetration is thus approximately inversely
proportional to the relative ionization. These numbers, however, only
indicate the order of relative penetrating power. This power varies
considerably for the different active bodies.
The α rays from uranium and polonium are the least penetrating, and
those from thorium the most. The β radiations from thorium and radium
are very complex, and consist of rays widely different in penetrating
power. Some of the β rays from these substances are much less and others
much more penetrating than those from uranium, which gives out fairly
homogeneous rays.
=74. Difficulties of comparative measurements.= It is difficult to make
quantitative or even qualitative measurements of the relative intensity
of the three types of rays from active substances. The three general
methods employed depend upon the action of the rays in ionizing the gas,
in acting on a photographic plate, and in causing phosphorescent or
fluorescent effects in certain substances. In each of these methods the
fraction of the rays which is absorbed and transformed into another form
of energy is different for each type of ray. Even when one specific kind
of ray is under observation, comparative measurements are rendered
difficult by the complexity of that type of rays. For example, the β
rays from radium consist of negatively charged particles projected with
a wide range of velocity, and, in consequence, they are absorbed in
different amounts in passing through a definite thickness of matter. In
each case, only a fraction of the energy absorbed is transformed into
the particular type of energy, whether ionic, chemical, or luminous,
which serves as a means of measurement.
The rays which are the most active electrically are the least active
photographically. Under ordinary conditions, most of the photographic
action of uranium, thorium, and radium, is due to the β or cathodic
rays. The α rays from uranium and thorium, on account of their weak
action, have not yet been detected photographically. With active
substances like radium and polonium, the α rays readily produce a
photographic impression. So far the γ rays have been detected
photographically from radium only. That no photographic action of these
rays has yet been established for uranium and thorium is probably due
merely to the fact that the effect sought for is very small, and during
exposures for long intervals it is very difficult to avoid fogging of
the plates owing to other causes. Considering the similarity of the
radiations in other respects, there can be little doubt that the γ rays
do produce some photographic action, though it is too small to observe
with certainty.
These differences in the photographic and ionizing properties of the
radiations must always be taken into account in comparing results
obtained by the two methods. The apparent contradiction of results
obtained by different observers using these two methods is found to be
due to their differences in relative photographic and ionizing action.
For example, with the unscreened active material, the ionization
observed by the electrical method is due almost entirely to α rays,
while the photographic action under the same condition is due almost
entirely to the β rays.
It is often convenient to know what thickness of matter is sufficient to
absorb a specific type of radiation. A thickness of aluminium or mica of
·01 cms. or a sheet of ordinary writing-paper is sufficient to absorb
completely all the α rays. With such a screen over the active material,
the effects are due only to the β and γ rays, which pass through with a
very slight absorption. Most of the β rays are absorbed in 5 mms. of
aluminium or 2 mms. of lead. The radiation passing through such screens
consists very largely of the γ rays. As a rough working rule, it may be
taken that a thickness of matter required to absorb any type of rays is
inversely proportional to the density of the substance, _i.e._ the
absorption is proportional to the density. This rule holds approximately
for light substances, but, in heavy substances like mercury and lead,
the radiations are about twice as readily absorbed as the density rule
would lead us to expect.
PART II.
The β or Cathodic Rays.
=75. Discovery of the β rays.= A discovery which gave a great impetus to
the study of the radiations from active bodies was made in 1899, almost
simultaneously in Germany, France, and Austria. It was observed that
preparations of radium gave out some rays which were deviable by a
magnetic field, and very similar in character to the cathode rays
produced in a vacuum tube. The observation of Elster and Geitel that a
magnetic field altered the conductivity produced in air by radium rays,
led Giesel[113] to examine the effect of a magnetic field on the
radiations. In his experiments, the radio-active preparation was placed
in a small vessel between the poles of an electromagnet. The vessel was
arranged to give a pencil of rays which was approximately perpendicular
to the field. The rays caused a small fluorescent patch on the screen.
On exciting the electromagnet, the fluorescent zone was observed to
broaden out on one side. On reversing the field, the extension of the
zone was in the opposite direction. The deviation of the rays thus
indicated was in the same direction and of the same order of magnitude
as that for cathode rays.
S. Meyer and Schweidler[114] also obtained similar results. They showed,
in addition, the deviation of the rays by the alteration of the
conductivity of the air when a magnetic field was applied.
Becquerel[115], a little later, showed the magnetic deflection of the
radium rays by using the photographic method. P. Curie[116], by the
electrical method, showed furthermore that the rays from radium
consisted of two kinds, one apparently non-deviable and easily absorbed
(now known as the α rays), and the other penetrating and deviable by a
magnetic field (now known as the β rays). The ionization effect due to
the β rays was only a small fraction of that due to the α rays. At a
later date Becquerel, by the photographic method, showed that uranium
gave out some deflectable rays. It had been shown previously[117] that
the rays from uranium consisted of α and β rays. The deflected rays in
Becquerel’s experiment consisted entirely of β rays, as the α rays from
uranium produce no appreciable photographic action. Rutherford and
Grier[118], using the electric method, showed that compounds of thorium,
like those of uranium, gave out, besides α rays, some penetrating β
rays, deviable in a magnetic field. As in the case of radium, the
ionization due to the α rays of uranium and thorium is large compared
with that due to the β rays.
=76. Examination of the magnetic deviation by the photographic method.=
Becquerel has made a very complete study, by the photographic method, of
the β rays from radium, and has shown that they behave in all respects
like cathode rays, which are known to be negatively charged particles
moving with a high velocity. The motion of a charged ion acted on by a
magnetic field has been discussed in section 49. It has been shown that
if a particle of mass _m_ and charge _e_ is projected with a velocity
_u_, at an angle α with the direction of a uniform field of strength
_H_, it will describe a helix round the magnetic lines of force. This
helix is wound on a cylinder of radius _R_, with the axis parallel to
the field, where _R_ is given by
_mu_
_R_ = ---- sin α.
_He_
When α = π/2, _i.e._ when the rays are projected normally to the field,
the particles describe circles of radius
_mu_
_R_ = ----
_He_
The planes of these circles are normal to the field. Thus, for a
particular velocity _u_, the value of _R_ varies inversely as the
strength of the field. In a uniform field the rays projected normally to
the field describe circles, and their directions of projection are the
tangents at the origin.
This conclusion has been verified experimentally by Becquerel for the β
rays of radium, by an arrangement similar to that shown in Fig. 23.
[Illustration: Fig. 23.]
A photographic plate _P_, with the film downwards, is enveloped in black
paper and placed horizontally in the uniform horizontal magnetic field
of an electromagnet. The magnetic field is supposed to be uniform, and,
in the figure, is at right angles to the plane of the paper. The plate
was covered with a sheet of lead, and on the edge of the plate, in the
centre of the magnetic field, is placed a small lead vessel _R_
containing the radio-active matter.
On exciting the magnet, so that the rays are bent to the left of the
figure, it is observed that a photographic impression is produced
directly below the source of the rays, which have been bent round by the
magnetic field. The active matter sends out rays equally in all
directions. The rays perpendicular to the field describe circles, which
strike the plate immediately under the source. A few of these rays,
_A₁_, _A₂_, _A₃_, are shown in the figure. The rays, normal to the
plate, strike the plate almost normally, while the rays nearly parallel
to the plate strike the plate at grazing incidence. The rays, inclined
to the direction of the field, describe spirals and produce effects on
an axis parallel to the field passing through the source. In consequence
of this, any opaque screen placed in the path of the rays has its shadow
thrown near the edge of the photographic plate.
=77. Complexity of the rays.= The deviable rays from radium are complex,
_i.e._ they are composed of a flight of particles projected with a wide
range of velocity. In a magnetic field every ray describes a path, of
which the radius of curvature is directly proportional to the velocity
of projection. The complexity of the radiation has been shown very
clearly by Becquerel[119] in the following way.
An uncovered photographic plate, with the film upwards, was placed
horizontally in the horizontal uniform magnetic field of an
electromagnet. A small, open, lead box, containing the radio-active
matter, was placed in the centre of the field, on the photographic
plate. The light, due to the phosphorescence of the radio-active matter,
therefore, could not reach the plate. The whole apparatus was placed in
a dark room. The impression on the plate took the form of a large,
diffuse, but continuous band, elliptic in shape, produced on one side of
the plate.
Such an impression is to be expected if the rays are sent out in all
directions, even if their velocities of projection are the same, for it
can readily be shown theoretically, that the path of the rays is
confined within an ellipse whose minor axis, which is at right angles to
the field, is equal to 2_R_, and whose major axis is equal to π_R_. If,
however, the active matter is placed in the bottom of a deep lead
cylinder of small diameter, the rays have practically all the same
direction of projection, and in that case each part of the plate is
acted on by rays of a definite curvature.
In this case also, a diffuse impression is observed on the plate,
giving, so to speak, a continuous spectrum of the rays and showing that
the radiation is composed of rays of widely different curvatures. Fig.
24 shows a photograph of this kind obtained by Becquerel, with strips
of paper, aluminium, and platinum placed on the plate.
[Illustration: Fig. 24.]
If screens of various thickness are placed on the plate, it is observed
that the plate is not appreciably affected within a certain distance
from the active matter, and that this distance increases with the
thickness of the screen. This distance is obviously equal to twice the
radius of curvature of the path of the rays, which are just able to
produce an impression through the screen.
These experiments show very clearly that the most deviable rays are
those most readily absorbed by matter. By observations of this kind
Becquerel has determined approximately the inferior limit of the value
of _HR_ for rays which are transmitted through different thicknesses of
matter.
The results are given in the table below:
Substance Thickness Inferior limit
in mms. of _HR_ for
transmitted
rays
Black paper 0·065 650
Aluminium 0·010 350
„ 0·100 1000
„ 0·200 1480
Mica 0·025 520
Glass 0·155 1130
Platinum 0·030 1310
Copper 0·085 1740
Lead 0·130 2610
If _e_/_m_ is a constant for all the rays, the value of _HR_ is
proportional to the velocity of the rays, and it follows from the table
that the velocity of the rays which just produce an effect on the plate
through ·13 mms. of lead is about 7 times that of the rays which just
produce an impression through ·01 mm. of aluminium. It will be shown,
however, in section 82, that _e_/_m_ is not a constant for all speeds,
but decreases with increase of velocity of the rays. The difference in
velocity between the rays is in consequence not as great as this
calculation would indicate. On examination of the rays from uranium,
Becquerel found that the radiation is not as complex as that from
radium, but consists wholly of rays for which the value of _HR_ is about
2000.
=78. Examination of the β rays by the electric method.= The presence of
easily deviable rays given off from an active substance can most readily
be shown by the photographic method, but it is necessary, in addition,
to show that the penetrating rays which produce the ionization in the
gas are the same as those which cause the photographic action. This can
be conveniently tested in an arrangement similar to that shown in Fig.
25.
[Illustration: Fig. 25.]
The radio-active matter _A_ is placed on a lead block _B´´_ between the
two parallel lead plates _BB´_. The rays pass between the parallel
plates and ionize the gas between the plates _PP´_ of the testing
vessel. The magnetic field is applied at right angles to the plane of
the paper. The dotted rectangle _EEEE_ represents the position of the
pole piece. If a compound of radium or thorium is under investigation, a
stream of air is required to prevent the diffusion of the radio-active
emanations into the testing vessel. When a layer of uranium, thorium or
radium compound is placed at _A_, the ionization in the testing vessel
is due mainly to the action of the α and β rays. The α rays are cut off
by adding a layer of aluminium ·01 cm. thick over the active material.
When the layer of active matter is not more than a few millimetres
thick, the ionization due to the γ rays is small compared with that
produced by the β rays, and may be neglected. On the application of a
magnetic field at right angles to the mean direction of the rays, the
ionization in the testing vessel due to the rays steadily decreases as
the strength of the field increases, and in a strong field it is reduced
to a very small fraction of its original value. In this case the rays
are bent so that none of them enter the testing vessel.
Examined in this way, it has been found that the β rays of uranium,
thorium, and radium consist entirely of rays readily deflected by a
magnetic field. The rays from polonium consist entirely of α rays, the
deviation of which can be detected only in very intense magnetic fields.
When the screen covering the active material is removed, in a strong
magnetic field, the ionization in the vessel is mainly due to the α
rays. On account of the slight deviation of the α rays under ordinary
experimental conditions, a still greater increase of the magnetic field
does not appreciably alter the current due to them in the testing
vessel.
The action of a magnetic field on a very active substance like radium is
easily shown by the electrical method, as the ionization current due to
the deviable rays is large. With substances of small activity like
uranium and thorium, the ionization current due to the deviable rays is
very small, and a sensitive electrometer or an electroscope is required
to determine the variation, in a magnetic field, of the very small
current involved. This is especially the case for thorium oxide, which
gives out only about ⅕ of the amount of deviable rays given out by the
same weight of uranium oxide.
=79. Experiments with a fluorescent screen.= The β rays from a few
milligrams of pure radium bromide produce intense fluorescence in barium
platinocyanide and other substances which can be made luminous under
the influence of the cathode rays. Using a centigram of radium bromide,
the luminosity on a screen, placed upon it, is bright enough to be
observed in daylight. With the aid of such a screen in a dark room many
of the properties of the β rays may be simply illustrated and their
complex nature clearly shown. A small quantity of radium is placed in
the bottom of a short, narrow, lead tube open at one end. This is placed
between the pole pieces of an electromagnet, and the screen placed below
it. With no magnetic field, a faint luminosity of the screen is observed
due to the very penetrating γ rays which readily pass through the lead.
When the magnetic field is put on, the screen is brightly lighted up on
one side over an area elliptical in shape (section 77). The direction of
deviation is reversed by reversal of the field. The broad extent of the
illumination shows the complex nature of the β rays. On placing a
metallic object at various points above the screen, the trajectory of
the rays can readily be traced by noticing the position of the shadow
cast upon the screen. By observing the density of the shadow, it can be
seen that the rays most easily deviated are the least penetrating.
Comparison of the β rays with cathode rays.
=80. Means of comparison.= In order to prove the identity of the β rays
from active bodies with the cathode rays produced in a vacuum tube, it
is necessary to show
(1) That the rays carry with them a negative charge;
(2) That they are deviated by an electric as well as by a magnetic
field;
(3) That the ratio _e_/_m_ is the same as for the cathode rays.
=Electric charge carried by the β rays.= The experiments of Perrin and
J. J. Thomson have shown that the cathode rays carry with them a
negative charge. In addition, Lenard has shown that the rays still carry
a charge after traversing thin layers of matter. When the rays are
absorbed, they give up their charge to the body which absorbs them. The
total amount of charge carried by the β rays from even a very active
preparation of radium is, in general, small compared with that carried
by the whole of the cathode rays in a vacuum tube, and can be detected
only by delicate methods.
* * * * *
Suppose that a layer of very active radium is spread on a metal plate
connected to earth, and that the β rays are absorbed by a parallel plate
connected with an electrometer. If the rays are negatively charged, the
top plate should receive a negative charge increasing with the time. On
account, however, of the great ionization produced by the rays between
the plates, any charge given to one of them is almost instantly
dissipated. In many cases, the plate does become charged to a definite
positive or negative potential depending on the metal, but this is due
to the contact difference of potential between the plates, and would be
produced whether the rays were charged or not. The ionization of the gas
is greatly diminished by placing over the active material a metal screen
which absorbs the α rays, but allows the β rays to pass through with
little absorption.
The rapid loss of any charge communicated to the top plate can be very
much reduced, either by diminishing the pressure of the gas surrounding
it or by enclosing the plate with suitable insulators. In their
experiments to determine the amount of charge carried by the radium
rays, M. and Mme Curie[120] used the second method.
A metal disc _MM_ (Fig. 26) is connected with an electrometer by the
wire _T_. The disc and wire are completely surrounded by insulating
matter _ii_. The whole is surrounded by a metal envelope _EEEE_
connected with earth. On the lower side of the disc, the insulator and
the metallic covering are very thin. This side is exposed to the rays of
the radium _R_ placed in a depression in a lead plate _AA_.
[Illustration: Fig. 26.]
The rays of the radium pass through the metal cover and insulator with
little absorption, but they are completely absorbed by the disc _MM_. It
was observed that the disc received a negative charge which increased
uniformly with the time, showing that the rays carry with them a
negative charge. The current observed was very small. With an active
preparation of radium[121], forming a layer 2·5 sq. cms. in area and 2
mms. thick, a current of the order of 10⁻¹¹ amperes was observed after
the rays had traversed a layer of aluminium ·01 mm. thick and a layer of
ebonite ·3 mm. thick. The current was the same with discs of lead,
copper, and zinc, and also when the ebonite was replaced by paraffin.
Curie also observed in another experiment of a similar character that
the radium itself acquired a positive charge. This necessarily follows
if the rays carry with them a negative charge. If the β rays alone
carried with them a charge, a pellet of radium, if perfectly insulated,
and surrounded by a non-conducting medium, would in the course of time
be raised to a high positive potential. Since, however, the α rays carry
with them a charge opposite in sign to the β rays, the ratio of the
charge carried off by the two types of rays must be determined, before
it can be settled whether the radium would acquire a positive or a
negative charge. If, however, the radium is placed in an insulated metal
vessel of a thickness sufficient to absorb all the α rays, but not too
thick to allow most of the β rays to escape, the vessel will acquire a
positive charge in a vacuum.
An interesting experimental result bearing upon this point has been
described by Dorn[122]. A small quantity of radium was placed in a
sealed glass tube and left for several months. On opening the tube with
a file, a bright electric spark was observed at the moment of fracture,
showing that there was a large difference of potential between the
inside of the tube and the earth.
In this case the α rays were absorbed in the walls of the tube, but a
large proportion of the β rays escaped. The inside of the tube thus
became charged, in the course of time, to a high positive potential; a
steady state would be reached when the rate of escape of negative
electricity was balanced by the leakage of positive electricity through
the walls of the tube. The external surface of the glass would be always
practically at zero potential, on account of the ionization of the air
around it.
Strutt[123] has recently described a simple and striking experiment to
illustrate still more clearly that a radium preparation acquires a
positive charge, if it is enclosed in an envelope thick enough to absorb
all the α particles, but thin enough to allow most of the β particles to
escape. The experimental arrangement is clearly seen in Fig. 27. A
sealed tube _AA_ containing the radium, was attached at one end to a
pair of thin gold leaves in metallic connection with the radium, and was
insulated inside a larger tube by means of a quartz rod _B_. The inner
surface of the tube was coated with tinfoil _EE_ connected to earth. The
glass surface of _AA_ was made conducting by a thin coating of
phosphoric acid. The air in the outer tube was exhausted as completely
as possible by means of a mercury pump, in order to reduce the
ionization in the gas, and consequently the loss of any charge gained by
the gold leaves. After an interval of 20 hours, the gold leaves were
observed to diverge to their full extent, indicating that they had
acquired a large positive charge. In this experiment Strutt used ½ gram
of radiferous barium of activity only 100 times that of uranium.
[Illustration: Fig. 27.]
If the tube is filled with 30 mgrs. of pure radium bromide, the leaves
diverge to their full extent in the course of about a minute. If it is
arranged that the gold leaf, at a certain angle of divergence, comes in
contact with a piece of metal connected with earth, the apparatus can be
made to work automatically. The leaf diverges, touches the metal, and at
once collapses, and this periodic movement of the leaf will continue, if
not indefinitely, at any rate as long as the radium lasts. This “radium
clock” should work at a sensibly uniform rate for many years, but, from
evidence considered later (Section 261), there is reason to believe that
the number of β particles emitted would decrease exponentially with the
time, falling to half value in about 1200 years. The period of movement
of the leaf should thus gradually increase with the time, and ultimately
the effect would become too small to observe.
The action of this radium clock is the nearest approach to an apparent
perpetual motion that has so far been observed.
A determination of the amount of the charge carried off by the β rays of
radium has been made by Wien[124]. A small quantity of radium, placed in
a sealed platinum vessel, was hung by an insulating thread inside a
glass cylinder, which was exhausted to a low pressure. A connection
between the platinum vessel and an electrode sealed on to the external
glass cylinder could be made, when required, by tilting the tube. Wien
found that in a good vacuum the platinum vessel became charged to about
100 volts. The rate of escape of negative electricity from the platinum
vessel containing 4 milligrams of radium bromide corresponded to 2·91 ×
10⁻¹² amperes. If the charge on each particle is taken as 1·1 × 10⁻²⁰
electromagnetic units, this corresponds to an escape of 2·66 × 10⁷
particles per second. From 1 gram of radium bromide the corresponding
number would be 6·6 × 10⁹ per second. Since some of the β rays are
absorbed in their passage through the walls of the containing vessel and
through the radium itself, the actual number projected per second from 1
gram of radium bromide must be greater than the above value. This has
been found by the writer to be the case. The method employed reduced the
absorption of the β rays to a minimum, and the total number emitted per
second by 1 gram of radium bromide in radio-active equilibrium was found
to be 4·1 × 10¹⁰, or about six times the number found by Wien. A
detailed account of the method employed cannot be given with advantage
at this stage, but will be found later in Section 253.
=81. Determination of= _e_/_m_. We have seen (Section 50) that, in their
passage between the plates of a condenser, the cathode rays are
deflected towards the positive plate. Shortly after the discovery of the
magnetic deviation of the β rays from radium, Dorn[125] and
Becquerel[126] showed that they also were deflected by an electric
field.
By observing separately the amount of the electric and magnetic
deviation, Becquerel was able to determine the ratio of _e_/_m_ and the
velocity of the projected particles. Two rectangular copper plates, 3·45
cms. high and 1 cm. apart, were placed in a vertical plane and insulated
on paraffin blocks. One plate was charged to a high potential by means
of an influence machine, and the other was connected with earth. The
active matter was placed in a narrow groove cut in a lead plate parallel
to the copper plates and placed midway between them. The photographic
plate, enveloped in black paper, was placed horizontally above the plate
containing the active substance. The large and diffuse pencil of rays
thus obtained was deflected by the electric field, but the deviation
amounted to only a few millimetres and was difficult to measure. The
method finally adopted was to place vertically above the active matter a
thin screen of mica, which cut the field into two equal parts. Thus, in
the absence of an electric field, a narrow rectangular shadow was
produced on the plate.
When the electric field was applied, the rays were deflected and a part
of the pencil of rays was stopped by the mica screen. A shadow was thus
cast on the plate which showed the direction of deviation and
corresponded to the least deviable rays which gave an impression through
the black paper.
If a particle of mass _m_, charge _e_, and velocity _u_, is projected
normally to an electric field of strength _X_, the acceleration α is in
the direction of the field, and is given by
_Xe_
α = ----- .
_m_
Since the particle moves with a constant acceleration parallel to the
field, the path of the particle is the same as that of a body projected
horizontally from a height with a constant velocity and acted on by
gravity. The path of the particle is thus a parabola, whose axis is
parallel to the field and whose apex is at the point where the particle
enters the electric field. The linear deviation _d₁_ of the ray parallel
to the field after traversing a distance _l_ is given by
1 _Xe_ _l²_
_d₁_ = -- -- ---- ---- .
2 _m_ _u²_
On leaving the electric field, the particle travels in the direction of
the tangent to the path at that point. If θ is the angular deviation of
the path at that point
_eXl_
tan θ = ----- .
_mu²_
The photographic plate was at a distance _h_ above the extremity of the
field. Thus the particles struck the plate at a distance _d₂_ from the
original path given by
$$ d_2 = h \tan \theta + d_1 $$
$$ = \frac {Xle} {mu^2} (\frac {l} {2} + h) $$
In the experimental arrangement the values were
_d₂_ = ·4 cms.;
_X_ = 1·02 × 10¹²;
_l_ = 3·45 cms.;
_h_ = 1·2 cms.
If the radius _R_ of curvature of the path of the same rays is observed
in a magnetic field of strength _H_ perpendicular to the rays,
_e_ _V_
--- = ----
_m_ _HR_
Combining these two equations we get
$$ u = \frac {X . l (\frac {l} {2} + h)} {H . R . d_2} $$ .
A difficulty arose in identifying the part of the complex pencil of rays
for which the electric and magnetic deviations were determined.
Becquerel estimated that the value of _HR_ for the rays deflected by the
electric field was about 1600 C.G.S. units. Thus
_u_ = 1·6 × 10¹⁰ cms. per second,
and
_e_
---- = 10⁷.
_m_
Thus these rays had a velocity more than half the velocity of light, and
an apparent mass about the same as the cathode ray particles, _i.e._
about ¹⁄₁₀₀₀ of the mass of the hydrogen atom. The β ray is therefore
analogous in all respects to the cathode ray, except that it differs in
velocity. In a vacuum tube the cathode rays generally have a velocity of
about 2 × 10⁹ cms. per sec. In special tubes with strong fields the
velocity may be increased to about 10¹⁰ cms. per sec. These β particles,
then, behave like isolated units of negative electricity, identical with
the electrons set free by an electric discharge in a vacuum tube. The
electrons projected from radium have velocities varying from about
0·2_V_ to at least 0·96_V_, where _V_ is the velocity of light, and thus
have an average speed considerably greater than that of the electrons
produced in a vacuum tube. These moving electrons are able to pass
through much greater thicknesses of matter before they are absorbed than
the slower electrons produced in a vacuum tube, but the difference is
one merely of degree and not of kind. Since electrons are continuously
and spontaneously expelled from radium with enormous velocities, they
must acquire their energy of motion from the matter itself. It is
difficult to avoid the conclusion, that this velocity has not been
suddenly impressed on the electron. Such a sudden gain of velocity would
mean an immense and sudden concentration of energy on a small particle,
and it is more probable that the electron before its expulsion has been
in rapid orbital or oscillatory motion in the atom, and, by some means,
suddenly escapes from its orbit. According to this view, the energy of
the electron is not suddenly created but is only made obvious by its
escape from the system to which it belongs.
=82. Variation of= _e_/_m_ =with the velocity of the electron=. The fact
that radium throws off electrons with rates of speed varying from ⅕ to
⁹⁄₁₀ the velocity of light has been utilised by Kaufmann[127] to examine
whether the ratio _e_/_m_ of the electrons varies with the speed. We
have seen (Section 48) that, according to the electromagnetic theory, a
charge of electricity in motion behaves as if it had apparent mass. For
small speeds, this additional electrical mass is equal to
2 _e²_
-- ----,
3 _a_
where _a_ is the radius of the body, but it increases rapidly as the
speed of light is approached. It is very important to settle whether the
mass of the electron is due partly to mechanical and partly to
electrical mass, or whether it can be explained by virtue of electricity
in motion independently of the usual conception of mass.
Slightly different formulae expressing the variation of mass with speed
have been developed by J. J. Thomson, Heaviside, and Searle. To
interpret his results Kaufmann used a formula developed by M.
Abraham[128].
Let _m₀_ = mass of electron for slow speeds;
_m_ = apparent mass of electron at any speed;
_u_ = velocity of electron;
_V_ = velocity of light.
Let β = _u_/_V_; then it can be shown that
$$ \frac {m} {m₀} = \frac {3} {4} \psi (\beta) $$ (1),
where
$$ \psi (\beta) = \frac {1}{\beta^2} (\frac {1 + \beta^2} {2\beta}
\log \frac {1 + \beta} {1 − \beta} − 1) $$ (2).
The experimental method employed to determine _e_/_m_ and _u_ is similar
to the method of crossed spectra. Some strongly active radium was placed
at the bottom of a brass box. The rays from this passed between two
brass plates insulated and about 1·2 mm. apart. These rays fell on a
platinum diaphragm, containing a small tube about 0·2 mm. in diameter,
which allowed a narrow bundle of rays to pass. The rays then struck a
photographic plate enveloped in a thin layer of aluminium.
In the experiments the diaphragm was about 2 cms. from the active
material and at the same distance from the photographic plate. When the
whole apparatus was placed in a vacuum, a P.D. of from 2000 to 5000
volts could be applied between the plates without a spark. The rays were
deflected in their passage through the electric field, and produced what
may be termed an electric spectrum on the plate.
[Illustration: Fig. 28.]
If a magnetic field is superimposed parallel to the electric field by
means of an electromagnet, a magnetic spectrum is obtained perpendicular
to the electric spectrum. The combination of the two spectra gives rise
to a curved line on the plate. The double trace obtained on the
photographic plate with reversal of the magnetic field is shown in Fig.
28. Disregarding some small corrections, it can readily be shown that if
_y_ and _z_ are the electric and magnetic deviations respectively,
_z_
β = κ₁ ----- (3),
_y_
and
_e_ _z²_
--- = κ ---- (4).
_m_ _y_
From these two equations, combined with (1), we obtain
$$ \frac {y} {z^2 \psi (\kappa_1 \frac {z} {y})} = \kappa_2 $$ ...
(5),
where κ, κ₁, κ₂ are constants.
Equation (5) gives the curve that should be obtained on the plate
according to the electromagnetic theory. This is compared by trial with
the actual curve obtained on the plate.
In this way Kaufmann[129] found that the value of _e_/_m_ decreased with
the speed, showing that, assuming the charge constant, the mass of the
electron increased with the speed.
The following numbers give some of the preliminary results obtained by
this method.
Velocity of electron _e_/_m_
2·36 × 10¹⁰ cms. per 1·31 × 10⁷
sec.
2·48 „ 1·17 × 10⁷
„
2·59 „ 0·97 × 10⁷
„
2·72 „ 0·77 × 10⁷
„
2·85 „ 0·63 × 10⁷
„
For the cathode rays S. Simon[130] obtained a value for _e_/_m_ of 1·86
× 10⁷ for an average speed of about 7 × 10⁹ cms. per second.
In a later paper[131] with some very active radium, more satisfactory
photographs were obtained, which allowed of accurate measurement. The
given equation of the curve was found to agree satisfactorily with
experiment.
The table given below, deduced from the results given by Kaufmann, shows
the agreement between the theoretical and experimental values, _u_ being
the velocity of the electron and _V_ that of light.
The average percentage error between the observed and calculated value
is thus not much more than one per cent. It is remarkable how nearly the
velocity of the electron has to approach the velocity of light before
the value of _m_/_m₀_ becomes large. This is shown in the following
table which gives the calculated values of _m_/_m₀_ for different
velocities of the electron.
Value of Observed value Percentage
_u_/_V_ of _m_/_m₀ difference
from
theoretical
values
Small 1
·732 1·34 −1·5 %
·752 1·37 −0·9 „
·777 1·42 −0·6 „
·801 1·47 +0·5 „
·830 1·545 +0·5 „
·860 1·65 0 „
·883 1·73 +2·8 „
·933 2·05 −7·8 „ ?
·949 2·145 −1·2 „
·963 2·42 +0·4 „
Value of small ·1 ·5 ·9 ·99 ·999 ·9999 ·999999
_u_/_V_
Calculated 1·00 1·015 1·12 1·81 3·28 4·96 6·68 10·1
value
m/m₀
Thus for velocities varying from 0 to ⅒ the velocity of light, the
mass of the electron is practically constant. The increase of mass
becomes appreciable at about half the velocity of light, and increases
steadily as the velocity of light is approached. Theoretically the mass
becomes infinite at the velocity of light, but even when the velocity of
the electron only differs from that of light by one part in a million,
its mass is only 10 times the value for slow speeds.
The above results are therefore in agreement with the view that the mass
of the electron is altogether electrical in origin and can be explained
purely by electricity in motion. The value of _e_/_m₀_, for slow speeds,
deduced from the results was 1·84 × 10⁷, which is in very close
agreement with the value obtained by Simon for the cathode rays, viz.
1·86 × 10⁷.
If the electricity carried by the electron is supposed to be distributed
uniformly over a sphere of radius _a_, for speeds slow compared with the
velocity of light, the apparent mass
2 _e²_
_m₀_ = --- ----
3 _a_
Therefore
2 _e_
_a_ = --- ---- . _e_
3 _m₀_
Taking the value of _e_ as 1·13 × 10⁻²⁰, _a_ is 1·4 × 10⁻¹³ cms.
Thus the diameter of an electron is minute compared with the diameter of
an atom.
=83. Distribution of velocity amongst the β particles=. Some interesting
experiments have been recently made by Paschen[132] to determine the
relative number of β particles which are expelled from radium at the
different speeds. The experimental arrangement is shown in Fig. 29.
[Illustration: Fig. 29.]
A small thin silvered glass tube _b_, containing 15 mgrs. of radium
bromide, was placed in the axis of a number of lead vanes arranged round
a cylinder of diameter 2 cms. and length 2·2 cms. When no magnetic field
was acting, the β particles from the radium passed through the openings
and were absorbed in an outer concentric cylinder _aa_ of lead of inner
diameter 3·7 cms. and of thickness 5·5 mms. This outer cylinder was
rigidly connected to the inner cylinder _cc_ by quartz rods _ii_, which
also served to insulate it. The cylinder _c_ and the radium were
connected with earth. A gold-leaf electroscope _E_ was attached to _a_,
and the whole apparatus was enclosed in a glass vessel which was
exhausted to a low vacuum by means of a mercury pump. The glass vessel
was placed in the uniform field of a large electromagnet, so that the
axis of the lead cylinder was parallel to the lines of force.
The outer cylinder gains a negative charge on account of the particles
which are absorbed in it. This negative charge, which is indicated by
the movement of the gold-leaf, tends to be dissipated by the small
ionization produced in the residual gas by the passage of the β rays.
This action of the gas can be eliminated by observing the rate of
movement of the gold leaf when charged alternately to an initial
positive and negative potential. The mean of the two rates is
proportional to the number of β particles which give up their charge to
the lead cylinder. This is evidently the case, since, when the charge is
positive, the ionization of the gas assists the rate of movement of the
gold-leaf, and, when negative, diminishes it to an equal extent.
When a magnetic field is applied, each of the particles describes a
curved path, whose radius of curvature depends on the velocity of the
particle. For weak fields, only the particles of smallest velocity will
be deflected sufficiently not to strike the outer cylinder, but, as the
field is raised, the number will increase until finally all the β
particles fail to reach the outer cylinder. The decrease of the charge
communicated to the outer cylinder with the increase of the strength of
the magnetic field is shown graphically in Fig. 30, Curve I.
The ordinates represent in arbitrary units the charge communicated to
the lead cylinder per second, and thus serve as a measure of the number
of β particles which reach the cylinder. Knowing the dimensions of the
apparatus, and assuming the value _e_/_m_ found by Kaufmann, the
velocity of the particles which just fail to reach the lead cylinder can
be deduced from any strength of the magnetic field. Curve II, Fig. 30 is
the first differential of Curve I, and the ordinates represent the
relative number of β particles which are projected at each velocity.
[Illustration: Fig. 30.]
From the data given by Kaufmann (see section 82) Paschen deduced that
the group of rays examined by the former, which had velocities lying
between 2·12 × 10¹⁰ and 2·90 × 10¹⁰ cms. per second, corresponded to the
group of rays between the points _A_ and _B_, that is, to the group of
rays which were completely deflected from the lead cylinder between the
magnetic fields of strengths of 1875 and 4931 C.G.S. units. Since radium
gives off β particles which require a field of strength over 7000 units
to deflect them, Paschen concluded that β particles are expelled from
radium with still greater velocities than the highest recorded by
Kaufmann.
Paschen considered that the small charge observed in still higher fields
was mainly due to the γ rays. The effect is small and is probably not
due to an actual charge carried by the γ rays but to a secondary effect
produced by them. This question will be discussed in more detail in
section 112.
There is a group of low velocity β particles emitted by radium (see Fig.
30) which have about the same speed as the electrons set free in a
vacuum tube. In consequence of their small velocity, these probably
produce a large proportion of the ionization due to the β rays at short
distances from the radium, for it will be shown (section 103) that the
ionization produced by an electron per unit length of path steadily
decreases with increase of its velocity above a small limiting value.
This observation is confirmed by experiments on the absorption of the β
rays in passing through matter.
In Paschen’s experiments, the glass tube containing the radium was ·5
mms. thick, so that a considerable proportion of the low velocity β
particles must have been stopped by it. This is borne out by some later
experiments of Seitz which will be described in section 85.
=84. Absorption of the β rays by matter=. The β particles produce ions
in their passage through the gas and their energy of motion is
consequently diminished. A similar action takes place also when the β
rays pass through solid and liquid media, and the mechanism of
absorption is probably similar in all cases. Some of the particles in
their passage through matter are completely stopped, while others have
their velocity reduced. In addition, there is a considerable scattering
or diffuse reflection of the rays in traversing matter. The amount of
this scattering depends upon the density of the substance and also upon
the angle of incidence of the rays. This scattering of the rays will be
discussed later in section 111.
There are two general methods of determining the absorption of the β
rays. In the first method, the variation of the ionization current is
observed in a testing vessel when the active matter is covered by
screens differing in material and thickness. This ionization in the
vessel depends upon two quantities, viz. the number of β particles which
pass through the matter and also upon the number of ions produced by
them per unit path. In the absence of any definite information in regard
to the variation of ionization by the electron with its velocity, no
very definite conclusions can be drawn from such experiments.
The advent of pure radium-bromide has made it possible to determine the
actual number of electrons which are absorbed in their passage through a
definite thickness of matter, by measuring the negative charge carried
by the issuing rays. Experiments of this character have been made by
Seitz and will be considered later.
These two methods of determining the absorption of β rays are quite
distinct in principle, and it is not to be expected that the values of
the coefficients of absorption obtained in the two cases should be the
same. The whole question of the absorption of electrons by matter is
very complicated, and the difficulty is still further increased by the
complexity of the β rays emitted by the radio-active substances. Many of
the results obtained by different methods, while pointing to the same
general conclusion, are quantitatively in wide disagreement. Before any
definite advance can be made to a better understanding of the mechanism
of absorption, it will be necessary to determine the variation of the
ionization with the speed of the electron over a very wide range. Some
work has already been done in this direction but not between
sufficiently wide limits.
Ionization method.
We shall first consider the results obtained on the absorption of β rays
by measuring the variation of the ionization current, when screens of
different thickness are placed over the active substance. When the
active matter is covered with aluminium foil of thickness ·1 mm., the
current in a testing vessel such as is shown in Fig. 17, is due almost
entirely to the β rays. If a uranium compound is used, it is found that
the saturation current decreases with the thickness of matter traversed
nearly according to an exponential law. Taking the saturation current as
a measure of the intensity of the rays, the intensity _I_ after passing
through a thickness _d_ of matter is given by
$$ \frac {I} {I₀} = e^{–λ d} $$,
where λ is the constant of absorption of the rays and _I₀_ is the
initial intensity. For uranium rays, the current is reduced to half its
value after passing through about ·5 mm. of aluminium.
If a compound of thorium or radium is examined in the same way, it is
found that the current does not decrease regularly according to the
above equation. Results of this kind for radium rays have been given by
Meyer and Schweidler[133]. The amount of absorption of the rays by a
certain thickness of matter decreases with the thickness traversed. This
is exactly opposite to what is observed for the α rays. This variation
in the absorption is due to the fact that the β rays are made up of rays
which vary greatly in penetrating power. The rays from uranium are
fairly homogeneous in character, _i.e._ they consist of rays projected
with about the same velocity. The rays from radium and thorium are
complex, _i.e._ they consist of rays projected with a wide range of
velocity and consequently with a wide range of penetrating power. The
electrical examination of the deviable rays thus leads to the same
results as their examination by the photographic method.
Results on the absorption of cathode rays have been given by
Lenard[134], who has shown that the absorption of cathode rays is nearly
proportional to the density of the absorbing matter, and is independent
of its chemical state. If the deviable rays from active bodies are
similar to cathode rays, a similar law of absorption is to be expected.
Strutt[135], working with radium rays, has determined the law of
absorption, and has found it roughly proportional to the density of
matter over a range of densities varying from 0·041 for sulphur dioxide
to 21·5 for platinum. In the case of mica and cardboard, the values of λ
divided by the density were 3·94 and 3·84 respectively, while the value
for platinum was 7·34. In order to deduce the absorption coefficient, he
assumed that the radiation fell off according to an exponential law with
the distance traversed. As the rays from radium are complex, we have
seen that this is only approximately the case.
Since the β rays from uranium are fairly homogeneous, and are at the
same time penetrating in character, they are more suitable for such a
determination than the complex rays of radium. I have in consequence
made some experiments with uranium rays to determine the dependence of
absorption on the density. The results obtained are given in the
following table, where λ is the coefficient of absorption.
Substance λ Density λ/Density
Glass 14·0 2·45 5·7
Mica 14·2 2·78 5·1
Ebonite 6·5 1·14 5·7
Wood 2·16 ·40 5·4
Cardboard 3·7 ·70 5·3
Iron 44 7·8 5·6
Aluminium 14·0 2·60 5·4
Copper 60 8·6 7·0
Silver 75 10·5 7·1
Lead 122 11·5 10·8
Tin 96 7·3 13·2
It will be observed that the value of the absorption constant divided by
the density is very nearly the same for such different substances as
glass, mica, ebonite, wood, iron and aluminium. The divergences from the
law are great, however, for the other metals examined, viz. copper,
silver, lead and tin. In tin the value of λ divided by the density is
2·5 times its value for iron and aluminium. These differences show that
a law for the absorption of the β rays depending only on the density
does not hold for all substances. With an exception in the case of tin,
the value of λ divided by the density for the metals increases in the
same order as their atomic weights.
The absorption of the β rays by matter decreases very rapidly with
increase of speed. For example, the absorption of cathode rays in
Lenard’s experiment (_loc. cit._) is about 500 times as great as for the
uranium β rays. The velocity of the β rays of uranium was found by
Becquerel to be about 1·6 × 10¹⁰ cms. per sec. The velocity of the
cathode rays used in Lenard’s experiment was certainly not less than
⅒ of this, so that, for a decrease of speed of less than 10 times,
the absorption has increased over 500 times.
=85. Number of electrons stopped by matter.= An account will now be
given of the experiments made by Seitz[136], to determine the relative
number of electrons which are stopped in their passage through different
thicknesses of matter. The experimental arrangement is shown in Fig. 31.
[Illustration: Fig. 31.]
The radium was placed outside a glass vessel containing an insulated
brass plate _P_, the connection of which with a wire leading to the
electrometer could be made or broken by a simple electromagnetic device.
The β rays from the radium _R_, after passing through openings in a
brass plate _A_, covered with thin aluminium foil, were absorbed in the
plate _P_. The glass vessel was exhausted, and the charge communicated
to _P_ by the β rays was measured by an electrometer.
In a good vacuum, the magnitude of the current observed is a measure of
the number of β particles absorbed by the upper plate[137]. The
following table shows the results obtained when different thicknesses of
tin foil were placed over the radium. The second table gives the ratio
_I_/_I₀_ where _I₀_ is the rate of discharge observed before the
absorbing screen is introduced. The mean value of the absorption
constant λ was deduced from the equation
$$ \frac {I} {I₀} = e^{–λ d} $$
where _d_ is the thickness of matter traversed.
The values included in the brackets have not the same accuracy as the
others. There is thus a wide difference in penetrating power of the β
particles emitted from radium, and some of them are very readily
absorbed.
When a lead screen 3 mms. thick was placed over the radium—a thickness
sufficient to absorb all the readily deflectable β rays—a small negative
charge was still given to the plate, corresponding to ·29 per cent. of
the maximum. This is a very much smaller value than was observed by
Paschen (see Fig. 30).
Thickness of _I_/_I₀_ λ
Tin in mms.
0·00834 ·869 175
0·0166 ·802 132·5
0·0421 ·653 101·5
0·0818 ·466 93·5
0·124 ·359 82·5
0·166 ·289 74·9
0·205 ·230 71·5
0·270 ·170 65·4
0·518 ·065 53
0·789 ·031 44
1·585 ·0059 32
2·16 ·0043 25
This difference may, in part, be due to the fact that, in Paschen’s
experiments, a large proportion of the slow velocity electrons were
absorbed in the glass tube of ·5 mm. thickness containing the radium.
Seitz also determined the relative thickness, compared with tin, of
different substances which reduced the negative charge communicated to
_P_ by a definite amount. A few of the numbers are given below, and
expressed in terms of tin as unity.
Substance Thickness
Tin = 1
Lead ·745
Gold ·83
Platinum ·84
Silver 1
Steel 1·29
Aluminium 1·56
Water 1·66
Paraffin 1·69
The thickness required to stop a given proportion of the β rays thus
decreases with the density, but not nearly so fast as the density
increases. These results are difficult to reconcile with the density-law
of absorption found by Lenard from the cathode rays, or with the results
of the ionization method already considered. A further experimental
examination of the whole question is very much to be desired.
=86. Variation of the amount of radiation with the thickness of the
layer of radiating material.= The radiations are sent out equally from
all portions of the active mass, but the ionization of the gas which is
measured is due only to the radiations which escape into the air. The
depth from which the radiations can reach the surface depends on the
absorption of the radiation by the active matter itself.
Let λ be the absorption constant of the homogeneous radiation by the
active material. It can readily be shown that the intensity _I_ of the
rays issuing from a layer of active matter, of thickness _d_, is given
by
$$ \frac {I} {I_{0}} = 1 − e^{–λ d} $$,
where _I₀_ is the intensity at the surface due to a very thick layer.
This equation has been confirmed experimentally by observing the current
due to the β rays for different thicknesses of uranium oxide. In this
case _I_ = (½)_I₀_ for a thickness of oxide corresponding to ·11 gr. per
sq. cm. This gives a value of λ divided by density of 6·3. This is a
value slightly greater than that observed for the absorption of the same
rays in aluminium. Such a result shows clearly that the substance which
gives rise to the β rays does not absorb them to a much greater extent
than does ordinary matter of the same density.
The value of λ will vary, not only for the different active substances,
but also for the different compounds of the same substance.
PART III.
The α Rays.
=87. The α rays=. The magnetic deviation of the β rays was discovered
towards the end of 1899, at a comparatively early stage in the history
of radio-activity, but three years elapsed before the true character of
the α rays was disclosed. It was natural that great prominence should
have been given in the early stages of the subject to the β rays, on
account of their great penetrating power and marked action in causing
phosphorescence in many substances. The α rays were, in comparison, very
little studied, and their importance was not generally recognized. It
will, however, be shown that the α rays play a far more important part
in radio-active processes than the β rays, and that the greater portion
of the energy emitted in the form of ionizing radiations is due to them.
=88. The nature of the α rays=. The nature of the α rays was difficult
to determine, for a magnetic field sufficient to cause considerable
deviation of the β rays produced no appreciable effect on the α rays. It
was suggested by several observers that they were, in reality, secondary
rays set up by the β or cathode rays in the active matter from which
they were produced. Such a view, however, failed to explain the
radio-activity of polonium, which gave out α rays only. Later work also
showed that the matter, which gave rise to the β rays from uranium,
could be chemically separated from the uranium, while the intensity of
the α rays was unaffected. These and other results show that the α and β
rays are produced quite independently of one another. The view that they
are an easily absorbed type of Röntgen rays fails to explain a
characteristic property of the α rays, viz. that the absorption of the
rays in a given thickness of matter, determined by the electrical
method, increases with the thickness of matter previously traversed. It
does not seem probable that such an effect could be produced by a
radiation like X rays, but the result is to be expected if the rays
consist of projected bodies, which fail to ionize the gas when their
velocity is reduced below a certain value. From observations of the
relative ionization produced in gases by the α and β rays, Strutt[138]
suggested in 1901 that the α rays might consist of positively charged
bodies projected with great velocity. Sir William Crookes[139], in 1902,
advanced the same hypothesis. From a study of the α rays of polonium
Mme. Curie[140] in 1900 suggested the probability that these rays
consisted of bodies, projected with great velocity, which lost their
energy by passing through matter.
The writer was led independently to the same view by a mass of indirect
evidence which received an explanation only on the hypothesis that the
rays consisted of matter projected with great velocity. Preliminary
experiments with radium of activity 1000 showed that it was very
difficult to determine the magnetic deviation of the α rays. When the
rays were passed through slits sufficiently narrow to enable a minute
deviation of the rays to be detected, the ionizing effect of the issuing
rays was too small to be measured with certainty. It was not until
radium of activity 19,000 was obtained that it was possible to detect
the deviation of these rays in an intense magnetic field. How small the
magnetic deviation is may be judged from the fact that the α rays,
projected at right angles to a magnetic field of 10,000 C.G.S. units,
describe the arc of a circle of about 39 cms. radius, while under the
same conditions the cathode rays produced in a vacuum tube would
describe a circle of about ·01 cm. radius. It is therefore not
surprising that the α rays were for some time thought to be non-deviable
in a magnetic field.
=89. Magnetic deviation of the α rays=. The general method employed[141]
to detect the magnetic deviation of the α rays was to allow the rays to
pass through narrow slits and to observe whether the rate of discharge
of an electroscope, due to the issuing rays, was altered by the
application of a strong magnetic field. Fig. 32 shows the general
arrangement of the experiment. The rays from a thin layer of radium of
activity 19,000 passed upwards through a number of narrow slits _G_, in
parallel, and then through a thin layer of aluminium foil, ·00034 cm.
thick, into the testing vessel _V_. The ionization produced by the rays
in the testing vessel was measured by the rate of movement of the leaves
of a gold-leaf electroscope _B_. The gold-leaf system was insulated
inside the vessel by a sulphur bead _C_, and could be charged by means
of a movable wire _D_, which was afterwards earthed. The rate of
movement of the gold-leaf was observed through small mica windows in the
testing vessel by means of a microscope provided with a micrometer
eye-piece.
[Illustration: Fig. 32.]
In order to increase the ionization in the testing vessel, the rays
passed through 20 to 25 slits of equal width, placed side by side. This
was arranged by cutting grooves at regular intervals in side-plates into
which brass plates were slipped. The width of the slit varied in
different experiments between ·042 cm. and ·1 cm. The magnetic field was
applied perpendicular to the plane of the paper, and parallel to the
plane of the slits. The rays are thus deflected in a direction
perpendicular to the plane of the slits and a very small amount of
deviation is sufficient to cause the rays to impinge on the sides of the
plate where they are absorbed.
The testing vessel and system of plates were waxed to a lead plate _P_
so that the rays entered the vessel _V_ only through the aluminium foil.
It is necessary in these experiments to have a steady stream of gas
passing downwards between the plates in order to prevent the diffusion
of the emanation from the radium upwards into the testing vessel. The
presence in the testing vessel of a small amount of this emanation,
which is always given out by radium, would produce great ionization and
completely mask the effect to be observed. For this purpose, a steady
current of dry electrolytic hydrogen of about 2 c.c. per second was
passed into the testing vessel; it then streamed through the porous
aluminium foil, and passed between the plates carrying the emanation
with it away from the apparatus. The use of a stream of hydrogen instead
of air greatly simplifies the experiment, for it _increases_ the
ionization current due to the α rays in the testing vessel, and at the
same time greatly _diminishes_ that due to the β and γ rays. This is
caused by the fact that the α rays are much more readily absorbed in air
than in hydrogen, while the rate of production of ions due to the β and
γ rays is much less in hydrogen than in air. The intensity of the α rays
after passing between the plates is consequently greater when hydrogen
is used; and since the rays pass through a sufficient distance of
hydrogen in the testing vessel to be largely absorbed, the total amount
of ionization produced by them is greater with hydrogen than with air.
The following is an example of an observation on the magnetic
deviation:—
Pole-pieces 1·90 × 2·50 cms.
Strength of field between pole-pieces 8370 units.
Apparatus of 25 parallel plates of length 3·70 cms., width ·70 cm.,
with an average air-space between plates of ·042 cm.
Distance of radium below plates 1·4 cm.
Rate of discharge of
electroscope in
volts per minute
(1) Without magnetic field 8·33
(2) With magnetic field 1·72
(3) Radium covered with thin layer of 0·93
mica to absorb all α rays
(4) Radium covered with mica and 0·92
magnetic field applied
The mica plate, ·01 cm. thick, was of sufficient thickness to absorb
completely all the α rays, while it allowed the β rays and γ rays to
pass through without appreciable absorption. The difference between (1)
and (3), 7·40 volts per minute, gives the rate of discharge due to the α
rays alone; the difference between (2) and (3), 0·79 volts per minute,
that due to the α rays not deviated by the magnetic field employed.
The amount of α rays not deviated by the field is thus about 11% of the
total. The small difference between (3) and (4) measures the small
ionization due to the β rays, for they would be completely deviated by
the magnetic field; (4) comprises the effect of the γ rays together with
the natural leak of the electroscope in hydrogen.
In this experiment there was a good deal of stray magnetic field acting
on the rays before they reached the pole-pieces. The diminution of the
rate of discharge due to the α rays was found to be proportional to the
strength of field between the pole-pieces. With a more powerful magnetic
field, the whole of the α rays were deviated, showing that they
consisted _entirely_ of projected charged particles.
In order to determine the _direction_ of deviation of the rays, the rays
were passed through slits one mm. in width, each of which was half
covered with a brass strip. The diminution of the rate of discharge in
the testing vessel for a given magnetic field in such a case depends
upon the _direction_ of the field. In this way it was found that the
rays were deviated in the _opposite sense_ to the cathode rays. Since
the latter consist of negatively charged particles, the α rays must
consist of _positively_ charged particles.
These results were soon after confirmed by Becquerel[142], by the
photographic method, which is very well adapted to determine the
character of the path of the rays acted on by a magnetic field. The
radium was placed in a linear groove cut in a small block of lead. Above
this source, at a distance of about 1 centimetre, was placed a metallic
screen, formed of two plates, leaving between them a narrow opening
parallel to the groove. Above this was placed the photographic plate.
The whole apparatus was placed in a strong magnetic field parallel to
the groove. The strength of the magnetic field was sufficient to deflect
the β rays completely away from the plate. When the plate was parallel
to the opening, there was produced on it an impression, due to the α
rays alone, which became more and more diffuse as the distance from the
opening increased. This distance should not exceed 1 or 2 centimetres on
account of the absorption of the rays in air. If, during the exposure,
the magnetic field is reversed for equal lengths of time, on developing
the plate two images of the α rays are observed which are deflected in
opposite directions. This deviation, even in a strong field, is small
though quite appreciable and is opposite in sense to the deviation
observed for the β or cathodic rays from the same material.
M. Becquerel[143], by the same method, found that the α rays from
polonium were deviated in the same direction as the α rays from radium;
and thus that they also consist of projected positive bodies. In both
cases, the photographic impressions were sharply marked and did not show
the same diffusion which always appears in photographs of the β rays.
=90. Electrostatic deviation of the α rays=. If the rays are charged
bodies, they should be deflected in passing through a strong electric
field. This was found by the writer to be the case, but the electric
deviation is still more difficult to detect than the magnetic deviation,
as the intensity of the electric field must of necessity be less than
that required to produce a spark in the presence of radium. The
apparatus was similar to that employed for the magnetic deviation (Fig.
32) with this exception, that the brass sides which held the plates in
position, were replaced by ebonite. Alternate plates were connected
together and charged to a high potential by means of a battery of small
accumulators. The discharge in the electroscope, due to the α rays, was
found to be diminished by application of the electric field. With plates
·055 cm. apart and 4·5 cms. high, the diminution was only 7% with a P.D.
of 600 volts between the slits. With a special arrangement of plates,
with slits only ·01 cm. apart, the discharge was diminished about 45%
with an electric field corresponding to 10,000 volts per cm.
=91. Determination of the constants of the rays.= If the deviation of
the rays in both an electric and magnetic field is known, the values of
the velocity of the rays, and the ratio _e_/_m_ of the charge of the
particle to its mass can be determined by the method, first used by J.
J. Thomson for the cathode rays, which is described in section 50. From
the equations of a moving charged body, the radius of curvature ρ of the
path of the rays in a magnetic field of strength _H_ perpendicular to
the path of the rays is given by
_m_
_H_ρ = ---- _V_ .
_e_
If the particle, after passing through a uniform magnetic field for a
distance _l₁_, is deviated through a small distance _d₁_ from its
original direction,
2ρ_d₁_ = _l₁²_
or
_l₁²_ _e_ _H_
_d₁_ = ----- ---- ---- (1).
2 _m_ _V_
If the rays pass through a uniform electric field of strength _X_ and
length _l₂_ with a deviation _d₂_,
1 _Xel₂²_
_d₂_ = --- ------ (2),
2 _mV²_
since _Xe_/_m_ is the acceleration of the particle, at right angles to
its direction, and _l₂_/_V_ is the time required to travel through the
electric field.
From equations (1) and (2)
_d₁_ _l₂²_ _X_
_V_ = ---- ----- -----,
_d₂_ _l₁²_ _H_
and
_e_ 2_d₁_ _V_
---- = ---- --- .
_m_ _l₁²_ _H_
The values of _V_ and _e_/_m_ are thus completely determined from the
combined results of the electric and magnetic deviation. It was found
that
_V_ = 2·5 × 10⁹ cms. per sec.
_e_/_m_ = 6 × 10³.
On account of the difficulty of obtaining a large electrostatic
deviation, these values are only approximate in character.
The results on the magnetic and electric deviation of the α rays of
radium have been confirmed by Des Coudres[144], by the photographic
method. Some pure radium bromide was used as a source of radiation. The
whole apparatus was enclosed in a vessel which was exhausted to a low
vacuum. In this way, not only was he able to determine the photographic
action of the rays at a much greater distance from the source, but he
was also able to apply a stronger electric field without the passage of
a spark. He found values of the constants given by
_V_ = 1·65 × 10⁹ cms. per sec.
_e_/_m_ = 6·4 × 10³.
These values are in very good agreement with the numbers found by the
electric method. The α rays from radium are complex, and probably
consist of a stream of positively charged bodies projected at velocities
lying between certain limits. The amount of deviation of the particles
in a magnetic field will thus differ according to the velocity of the
particle. The photographic results of Becquerel seem to indicate that
the velocity of the rays of radium can vary only within fairly narrow
limits, since the trajectory of the rays in a magnetic field is sharply
marked and not nearly as diffuse as in similar experiments with the β
rays. The evidence, however, discussed in the following section, shows
that the velocities of the α particles from a thick layer of radium vary
over a considerable range.
=92.= Becquerel[145] has examined the amount of magnetic deviation of
the α rays at different distances from the source of the rays in a very
simple way. A narrow vertical pencil of the rays, after its passage
through a narrow slit, fell on a photographic plate, which was inclined
at a small angle to the vertical and had its lower edge perpendicular to
the slit. The trajectory of the rays is shown by a fine line traced on
the plate. If a strong magnetic field is applied parallel to the slit,
the trajectory of the rays is displaced to the right or left according
to the direction of the field. If equal times of exposure are given for
the magnetic field in the two directions, on developing the plate two
fine diverging lines are found traced on the plate. The distance between
these lines at any point is a measure of twice the average deviation at
that point, corresponding to the value of the magnetic field. By
measuring the distance between the trajectories at various points,
Becquerel found that the radius of curvature of the path of the rays
_increased_ with the distance from the slit. The product _H_ρ of the
strength of the field and the radius of curvature of the path of the
rays is shown in the following table.
Distance in mms. _H_ρ
from the slit
1 2·91 × 10⁵
3 2·99 „
5 3·06 „
7 3·15 „
8 3·27 „
9 3·41 „
The writer (_loc. cit._) showed that the _maximum_ value of _H_ρ for
complete deviation of the α rays was 390,000. The results are thus in
good agreement. Since
_m_
_H_ρ = ----- _V_
_e_
these results show that the values either of _V_ or of _e_/_m_ for the
projected particles vary at different distances from the source.
Becquerel considered that the rays were homogeneous, and, in order to
explain the results, has suggested that the charge on the projected
particles may gradually decrease with the distance traversed, so that
the radius of curvature of the path steadily increases with the distance
from the source. It, however, seems more probable that the rays consist
of particles projected with different velocities, and that the slower
particles are more quickly absorbed in the gas. In consequence of this,
only the swifter particles are present some distance from the source.
This conclusion is borne out by some recent experiments of Bragg and
Kleeman[146] on the nature of the absorption of α particles by matter,
which are discussed in more detail in sections 103 and 104. They found
that the α particles from a thick layer of radium are complex, and have
a wide range of penetrating power and presumably of velocity. This is
due to the fact that the α particles emitted from the radium come from
different depths. Since their velocity is reduced in their transit
through matter, a pencil of α rays will consist of particles which
differ considerably in speed. Those which are just able to emerge from
the radium will be absorbed in a very short depth of air, while those
that come from the surface will be able to pass through several
centimetres of air before they lose their power of ionizing the gas.
Since the α particles have different velocities, they will be unequally
deflected by the magnetic field, the slower moving particles describing
a more curved path than the swifter ones. Consequently, the outer edge
of the trace of the pencil of rays on the photographic plate, as
obtained by Becquerel, will be the locus of the points where the
photographic action of the α particles end. It was found that the α
particles are most efficient as ionizers of the gas just before their
power of ionizing ends. The loss of ionizing power of the α particles
seems to be fairly abrupt, and, for particles of the same velocity, to
occur always after traversing a definite distance in air. On the
assumption that the photographic as well as the ionizing action is most
intense just before the particles are stopped, and ceases fairly
abruptly, Bragg has been able to account numerically for the
measurements (see above table) recorded by Becquerel. Quite apart from
the special assumptions required for such a quantitative comparison of
theory with experiment, there can be little doubt that the increase of
value of _H_ρ with distance can be satisfactorily explained as a
consequence of the complex character of the pencil of rays[147].
Becquerel states that the amount of deviation, in a given magnetic
field, was the same for the α rays of polonium and of radium. This shows
that the value of
_m_
---- _V_
_e_
is the same for the α rays from the two substances. Since the α rays
from polonium are far more readily absorbed than the α rays from radium,
this result would indicate that the value of _m_/_e_ is greater for the
α particles of polonium than of radium. Further experimental evidence is
required on this important point.
=93. Charge carried by the α rays=. We have seen that the negative
charge carried by the β particles has been readily measured. Since there
is reason to believe (section 229) that four α particles are expelled
from radium for each β particle, it is to be expected that the positive
charge carried by the α particles should be determined still more
readily. All the initial experiments, however, made to detect this
charge, gave negative results; and, before successful results were
obtained, it was found necessary to eliminate some secondary actions,
which at first completely masked the effects to be looked for.
In consequence of the importance of this question, a brief account will
be given of the methods of measurement adopted and the special
experimental difficulties which have arisen.
In the first place, it must be remembered that only a small fraction of
the α rays, emitted from a layer of powdered radium bromide, escape into
the surrounding gas. On account of the ease with which the α rays are
stopped in their passage through matter, only those escape which are
expelled from a superficial layer, and the rest are absorbed by the
radium itself. On the other hand, a much larger proportion of the β rays
escape, on account of their greater power of penetration. In the second
place, the α particle is a far more efficient ionizer of the gas than
the β particle, and, in consequence, if the charge carried by the α rays
is to be determined by methods similar to those employed for the β rays
(see section 80), the pressure of the gas surrounding the conductor to
be charged must be very small in order to eliminate, as far as possible,
the loss of charge resulting from the ionization of the residual gas by
the α rays[148].
The experimental arrangement used by the writer is shown in Fig. 33.
A thin film of radium was obtained on a plate _A_ by evaporation of a
radium solution containing a known weight of radium bromide. Some hours
after evaporation, the activity of the radium, measured by the α rays,
is about 25 per cent. of its maximum value, and the β rays are almost
completely absent. The activity measured by the α and β rays is then
slowly regained, and recovers its original value after about a month’s
interval (see chapter XI.). The experiments were made on the active
plate when its activity was a minimum, in order to avoid complications
due to the presence of β rays. The film of radium was so thin that only
a very small fraction of the α rays was absorbed.
[Illustration: Fig. 33.]
The active plate _A_ was insulated in a metal vessel _D_, and was
connected to one pole of the battery, the other pole being earthed. The
upper electrode, which was insulated and connected with a Dolezalek
electrometer, consisted of a rectangular copper vessel _BC_, the lower
part of which was covered with a thin sheet of aluminium foil. The α
rays passed through the foil, but were stopped by the copper sides of
the vessel. This arrangement was found to reduce the secondary
ionization produced at the surface of the upper plate. The outside
vessel _D_ could be connected with either _A_ or _B_ or with earth. By
means of a mercury pump, the vessel was exhausted to a very low
pressure. If the rays carry a positive charge, the current between the
two plates measured by the electrometer should be greater when _A_ is
charged positively. No certain difference, however, between the currents
in the two directions was observed, even when a very good vacuum was
obtained. In some arrangements, it was found that the current was even
greater when the lower plate was negative than when it was positive. An
unexpected experimental result was also noticed. The current between the
parallel plates at first diminished with the pressure, but soon reached
a limiting value which was not altered however good a vacuum was
produced. For example, in one experiment, the current between the two
parallel plates, placed about 3 mms. apart, was initially 6·5 × 10⁹
amperes and fell off directly as the pressure. The current reached a
limiting value of about 6 × 10⁻¹² amperes, or about ¹⁄₁₀₀₀ of the value
at atmospheric pressure. The magnitude of this limiting current was not
much altered if the air was replaced by hydrogen.
Experiments of a similar character have been made by Strutt[149] and J.
J. Thomson[150]; using an active bismuth plate coated with
radio-tellurium (polonium) after Marckwald’s method. This substance
emits only α rays, and is thus especially suitable for experiments of
this kind. Strutt employed the method used by him to show the charge
carried by the β rays (Fig. 27). He found, however, that, even in the
lowest possible vacuum, the electroscope rapidly lost its charge and at
the same rate whether it was charged positively or negatively. This is
in agreement with the results found by the writer with radium.
In the experiments of J. J. Thomson, the electroscope was attached to a
metal disc placed 3 cms. from the plate of radio-tellurium. A very low
vacuum was produced by Dewar’s method by absorbing the residual gas in
cocoanut charcoal immersed in liquid air. When the electroscope was
charged negatively, an extremely slow rate of leak was observed, but
when charged positively the leak was about 100 times greater. This
showed that the polonium gave out large quantities of negative
electricity, but not enough positive to be detected. By placing the
apparatus in a strong magnetic field, the negative particles were
prevented from reaching the electroscope and the positive leak was
stopped.
These results indicate that these negative particles are not projected
with sufficient velocity to move against the repulsion exerted by the
electrified body, and are bent by a magnetic field. There thus seems
little doubt that a stream of negative particles (electrons) is
projected from the active surface at a very slow speed. Such low
velocity electrons are also projected from uranium and radium. It is
probable that these electrons are a type of secondary radiation, set up
at the surfaces on which the α rays fall. The particles would be
extremely readily absorbed in the gas, and their presence would be
difficult to detect except in low vacua. J. J. Thomson at first obtained
no evidence that the α particles of polonium were charged; but in later
experiments, where the plates were closer together, the electroscope
indicated that the α rays did carry a positive charge.
In order to see whether the positive charge due to the α rays from
radium could be detected when the slow moving ions were prevented from
escaping by a magnetic field, I placed the apparatus of Fig. 33 between
the pole-pieces of a large electromagnet, so that the magnetic field was
parallel to the plane of the plates[151]. A very marked alteration was
observed both on the magnitude of the positive and negative currents. In
a good vacuum, the upper plate received a positive charge, independently
of whether the lower plate was charged positively or negatively or was
connected with earth. After the magnetic field had reached a certain
value, a great increase in its strength had no appreciable effect on the
magnitude of the current.
The following table illustrates the results obtained when the two plates
were 3 mms. apart, and were both coated with thin aluminium foil.
Potential Current in arbitrary units
of lower
plate
Without magnetic With magnetic field
field
0 — +·36
+2 volts 2·0 +·46}
} ·39
−2 „ 2·5 +·33}
+4 „ 2·8 +·47}
} ·41
−4 „ 3·5 +·35}
+8 „ 3·1 +·56}
} ·43
−8 „ 4·0 +·31}
+84 „ 3·5 +·77}
} ·50
−84 „ 5·2 +·24}
Let _n_ be the number of α particles, carrying a charge _e_, which are
absorbed in the upper plate. Let ι₀ be the current due to the slight
ionization of the residual gas.
If only a small potential is applied to the lower plate, this current
should be equal in magnitude but opposite in sign when the potential is
reversed. Let ι₁ be the charge per sec. communicated to the upper
electrode when the lower plate is charged positively and ι₂ the value
when charged negatively. Then
ι₁ = ι₀ + _ne_,
ι₂ = ι₀ + _ne_;
adding we get
ι₁ + ι₂
_ne_ = ------ .
2
Now in the third column of the above table it is seen that (ι₁ + ι₂)/2
has the values ·39, ·41, ·43 for 2, 4, and 8 volts respectively. The
numbers are thus in fairly good agreement. Similar results were obtained
when a brass plate was substituted for the upper electrode shown in the
figure. Taking into consideration that the magnitude of _ne_ is
independent of the strength of the magnetic field above a certain small
value, and the good agreement of the numbers obtained with variation of
voltage, I think that there can be no doubt that the positive charge
communicated to the upper electrode was carried by the α particles. This
positive charge was not small, for using a weight of ·48 mgrs. radium
bromide spread in a thin foil over an area of about 20 sq. cms., the
charge communicated by the particles corresponded to a current 8·8 ×
10⁻¹³ amperes, and, with the Dolezalek electrometer employed, it was
necessary to add a capacity of ·0024 microfarads to the electrometer
system.
In these experiments, the film of radium bromide was so thin, that only
a very small percentage of the α particles was stopped by the radium
itself. Assuming that each α particle carries the same charge as an ion,
viz. 1·1 × 10⁻¹⁹ coulombs, and remembering that half of the α particles
are absorbed in the lower plate, the total number _N_ of α particles
expelled per second from one gram of radium bromide (at its minimum
activity) can be deduced. In two separate experiments where the amount
of radium used was ·194 and ·484 mgrs. respectively, the values of _N_
were in close agreement and equal to 3·6 × 10¹⁰. Now it will be shown
later that in radium there are three other products in radio-active
equilibrium, each of which probably gives out the same number of α
particles as radium itself. If this is the case, the total number of α
particles expelled per second from 1 gram of radium bromide _in
radio-active equilibrium_ is 4_N_ or 1·44 × 10¹¹. Assuming the
composition of radium bromide as RaBr₂, the number per second per gram
of radium is 2·5 × 10¹⁰. This number will be found to be in very good
agreement with that deduced from indirect data (chapter XIII.). The
value of _N_ is of great importance in determining the magnitude of
various quantities in radio-active calculations.
=94. Mass and energy of the α particle.= It has been pointed out that
the α rays from radium and polonium are analogous to the Canal rays of
Goldstein, for both carry a positive charge and are difficult to deflect
by a magnetic field. The experiments of Wien have shown that the
velocity of projection of the canal rays varies with the gas in the tube
and the intensity of the electric field applied, but it is generally
about ⅒ of the velocity of the α particle from radium. The value of
_e_/_m_ is also variable, depending upon the gas in the tube.
It has been shown that for the α rays of radium
_e_
V = 2·5 × 10⁹ and ---- = 6 × 10³.
_m_
Now the value of _e_/_m_ for the hydrogen atom, liberated in the
electrolysis of water, is 10⁴. Assuming the charge carried by the α
particle to be the same as that carried by the hydrogen atom, the mass
of the α particle is about twice that of the hydrogen atom. Taking into
consideration the uncertainty attaching to the experimental value of
_e_/_m_ for the α particle, if the α particle consists of any known kind
of matter, this result indicates that it consists either of projected
helium or hydrogen. Further evidence on this important question is given
in section 260.
The α rays from all the radio-active substances and their products, such
as the radio-active emanations and the matter causing excited activity,
possess the same general properties and do not vary very much in
penetrating power. It is thus probable that in all cases the α rays from
the different radio-active substances consist of positively charged
bodies projected with great velocity. Since the rays from radium are
made up in part of α rays from the emanation stored in the radium, and
from the excited activity which it produces, the α rays from each of
these products must consist of positively charged bodies; for it has
been shown that _all_ the α rays from radium are deviated in a strong
magnetic field.
The kinetic energy of each projected particle is enormous, compared with
its mass. The kinetic energy of each α particle is
1 1 _m_
--- _mV²_ = -- --- _V²e_ = 5·9 × 10⁻⁶ ergs.
2 2 _e_
Taking the velocity of a rifle bullet as 10⁵ cms. per second, it is seen
that, mass for mass, the energy of motion of the α rays is 6 × 10⁸ times
as great as that of the rifle bullet. In this projection of bodies
atomic in size with great velocity probably lies the principal cause of
the heating effects produced by radium (chapter XII).
=95. Atomic disintegration.= The radio-activity of the radio-elements is
an atomic and not a molecular property. The rate of emission of the
radiations depends only on the amount of the element present and is
independent of its combination with inactive substances. In addition, it
will be shown later that the rate of emission is not affected by wide
variations of temperature, or by the application of any known chemical
or physical forces. Since the power of radiating is a property of the
radio-atoms, and the radiations consist for the most part of positively
and negatively charged masses projected with great velocity, it is
necessary to suppose that the atoms of the radio-elements are undergoing
disintegration, in the course of which parts of the atom escape from the
atomic system. It seems very improbable that the α and β particles can
suddenly acquire their enormous velocity of projection by the action of
forces existing inside or outside the atom. For example, the α particle
would have to travel from rest between two points differing in potential
by 5·2 million volts in order to acquire the kinetic energy with which
it escapes. Thus it seems probable that these particles are not set
suddenly in motion, but that they escape from an atomic system in which
they were already in rapid oscillatory or orbital motion. On this view,
the energy is not communicated to the projected particles, but exists
beforehand in the atoms from which they escape. The idea that the atom
is a complicated structure consisting of charged parts in rapid
oscillatory or orbital motion has been developed by J. J. Thomson,
Larmor and Lorentz. Since the α particle is atomic in size, it is
natural to suppose that the atoms of the radio-active elements consist
not only of the electrons in motion, but also of positively charged
particles whose mass is about the same as that of the hydrogen or helium
atom.
It will be shown later that only a minute fraction of the atoms of the
radio-element need break up per second in order to account for the
radiations even of an enormously active element like radium. The
question of the possible causes which lead to this atomic disintegration
and the consequences which follow from it will be discussed later in
chapter XIII.
=96. Experiments with a zinc sulphide screen.= A screen of Sidot’s
hexagonal blend (phosphorescent crystalline zinc sulphide) lights up
brightly under the action of the α rays of radium and polonium. If the
surface of the screen is examined with a magnifying glass, the light
from the screen is found not to be uniformly distributed but to consist
of a number of scintillating points of light. No two flashes succeed one
another at the same point, but they are scattered over the surface,
coming and going rapidly without any movement of translation. This
remarkable action of the radium and polonium rays on a zinc sulphide
screen was discovered by Sir William Crookes[152], and independently by
Elster and Geitel[153], who observed it with the rays given out from a
wire which has been charged negatively either in the open air or in a
vessel containing the emanation of thorium.
In order to show the scintillations of radium on the screen, Sir William
Crookes has devised a simple apparatus which he has called the
“Spinthariscope.” A small piece of metal, which has been dipped in a
radium solution, is placed several millimetres away from a small zinc
sulphide screen. This screen is fixed at one end of a short brass tube
and is looked at through a lens fixed at the other end of the tube.
Viewed in this way, the surface of the screen is seen as a dark
background, dotted with brilliant points of light which come and go with
great rapidity. The number of points of light per unit area to be seen
at one time falls off rapidly as the distance from the radium increases,
and, at several centimetres distance, only an occasional one is seen.
The experiment is extremely beautiful, and brings vividly before the
observer the idea that the radium is shooting out a stream of
projectiles, the impact of each of which on the screen is marked by a
flash of light.
The scintillating points of light on the screen are the result of the
impact of the α particles on its surface. If the radium is covered with
a layer of foil of sufficient thickness to absorb all the α rays the
scintillations cease. There is still a phosphorescence to be observed on
the screen due to the β and γ rays, but this luminosity is not marked by
scintillations to any appreciable extent. Sir William Crookes showed
that the number of scintillations was about the same in vacuo as in air
at atmospheric pressure. If the screen was kept at a constant
temperature, but the radium cooled down to the temperature of liquid
air, no appreciable difference in the number of scintillations was
observed. If, however, the screen was gradually cooled to the
temperature of liquid air, the scintillations diminished in number and
finally ceased altogether. This is due to the fact that the screen loses
to a large extent its power of phosphorescence at such a low
temperature.
Not only are scintillations produced by radium, actinium, and polonium,
but also by the emanations and other radio-active products which emit α
rays. In addition, F. H. Glew[154] has found that they can be observed
from the metal uranium, thorium compounds and various varieties of
pitchblende. In order to show the scintillations produced by
pitchblende, a flat surface was ground, and a transparent screen, whose
lower surface was coated with zinc sulphide, placed upon it. Glew has
designed a modified and very simple form of spinthariscope. A
transparent screen, coated on one side with a thin layer of zinc
sulphide, is placed in contact with the active material, and the
scintillations observed by a lens in the usual way.
Since there is no absorption in the air, the luminosity is a maximum.
The relative transparency of different substances placed between the
active material and the screen may, in this way, be directly studied.
The production of scintillations appears to be a general property of the
α rays from all radio-active substances. The scintillations are best
shown with a zinc sulphide screen; but are also observed with willemite
(zinc silicate), powdered diamond, and potassium platinocyanide (Glew,
_loc. cit._). If a screen of barium platinocyanide is exposed to the α
rays from radium, the scintillations are difficult to observe, and the
luminosity is far more persistent than for a zinc sulphide screen
exposed under the same conditions. The duration of the phosphorescence
in this case probably accounts for the absence of visible
scintillations.
There can be no doubt that the scintillations result from the continuous
bombardment of the sensitive screen by the α particles. Each of these
particles moves with enormous velocity, and has a considerable energy of
motion. On account of the ease with which these particles are stopped,
most of this energy is given up at the surface of the screen, and a
portion of the energy is in some way transformed into light. Zinc
sulphide is very sensitive to mechanical shocks. Luminosity is observed
if a penknife is drawn across the screen, or if a current of air is
directed on to the screen. The disturbance effected by the impact of the
α particle extends over a distance very large compared with the size of
the impinging particle, so that the spots of light produced have an
appreciable area. Recently Becquerel[155] has made an examination of the
scintillations produced by different substances, and has concluded that
the scintillations are due to irregular cleavages in the crystals
composing the screen, produced by the action of the α rays.
Scintillations can be mechanically produced by crushing a crystal.
Tommasina[156] found that a zinc sulphide screen removed from the action
of the radium rays for several days, showed the scintillations again
when an electrified rod was brought near it.
The number of scintillations produced in zinc sulphide depends upon the
presence of a slight amount of impurity and on its crystalline state. It
can be shown that even with the most sensitive zinc sulphide screens,
the number of scintillations is probably only a small fraction of the
total number of α particles which fall upon it. It would appear that the
crystals are in some way altered by the bombardment of the α particles,
and that some of the crystals occasionally break up with emission of
light[157].
Although the scintillations from a particle of pure radium bromide are
very numerous, they are not too numerous to be counted. Close to the
radium, the luminosity is very bright, but by using a high power
microscope the luminosity can still be shown to consist of
scintillations. Since the number of scintillations probably bears no
close relation to the number of α particles emitted, a determination of
the number of scintillations would have no special physical
significance. The relation between the number of α particles and the
number of scintillations would probably be variable, depending greatly
on the exact chemical composition of the sensitive substance and also
upon its crystalline state.
=97. Absorption of the α rays by matter=. The α rays from the different
radio-active substances can be distinguished from one another by the
relative amounts of their absorption by gases or by thin screens of
solid substances. When examined under the same conditions, the α rays
from the active substances can be arranged in a definite order with
reference to the amount of absorption in a given thickness of matter.
In order to test the amount of absorption of the α rays for different
thicknesses of matter, an apparatus similar to that shown in Fig. 17, p.
98, was employed[158]. A thin layer of the active material was spread
uniformly over an area of about 30 sq. cms., and the saturation current
observed between two plates 3·5 cms. apart. With a thin layer[159] of
active material, the ionization between the plates is due almost
entirely to the α rays. The ionization due to the β and γ rays is
generally less than 1% of the total.
The following table shows the variation of the saturation current
between the plates due to the α rays from radium and polonium, with
successive layers of aluminium foil interposed, each ·00034 cm. in
thickness. In order to get rid of the ionization due to the β rays from
radium, the radium chloride employed was dissolved in water and
evaporated. This renders the active compound, for the time, nearly free
from β rays.
The initial current with 1 layer of aluminium over the active material
is taken as 100. It will be observed that the current due
_Polonium._ _Radium._
Layers of Current Ratio of Layers of Current Ratio of
aluminium decrease aluminium decrease
for each for each
layer layer
0 100 0 100
·41 ·48
1 41 1 48
·31 ·48
2 12·6 2 23
·17 ·60
3 2·1 3 13·6
·067 ·47
4 ·14 4 6·4
·39
5 0 5 2·5
·36
6 ·9
7 0
to the radium rays decreases very nearly by half its value for each
additional thickness until the current is reduced to about 6% of the
maximum. It then decays more rapidly to zero. Thus, for radium, over a
wide range, the current decreases approximately according to an
exponential law with the thickness of the screen, or
$$ \frac {i} {i₀} = e^{λ d} $$,
where _i_ is the current for a thickness _d_, and _i₀_ the initial
current. In the case of polonium, the decrease is far more rapid than
would be indicated by the exponential law. By the first layer, the
current is reduced to the ratio ·41. The addition of the third layer
cuts the current down to a ratio of ·17. For most of the active bodies,
the current diminishes slightly faster than the exponential law would
lead one to expect, especially when the radiation is nearly all
absorbed.
=98.= The increase of absorption of the α rays of polonium with the
thickness of matter traversed has been very clearly shown in some
experiments made by Mme Curie. The apparatus employed is shown in Fig.
34.
[Illustration: Fig. 34.]
The saturation current was measured between two parallel plates _PP´_ 3
cms. apart. The polonium _A_ was placed in the metal box _CC_, and the
rays from it, after passing through an opening in the lower plate _P´_,
covered with a layer of thin foil _T_, ionized the gas between the
plates. For a certain distance _AT_, of 4 cms. or more, no appreciable
current was observed between _P_ and _P´_. As the distance _AT_ was
diminished, the current increased in a very sudden manner, so that for a
small variation of the distance _AT_ there was a large increase of
current. With still further decrease of distance the current increases
in a more regular manner. The results are shown in the following table,
where the screen _T_ consisted of one and two layers of aluminium foil
respectively. The current due to the rays, without the aluminium screen,
is in each case taken as 100.
Distance AT in cms. 3·5 2·5 1·9 1·45 0·5
For 100 rays 0 0 5 10 25
transmitted by one
layer
For 100 rays 0 0 0 0 0·7
transmitted by two
layers
The metallic screen thus cuts off a greater proportion of the rays the
greater the distance of air which the radiations traverse. The effects
are still more marked if the plates _PP´_ are close together. Results
similar but not so marked are found if radium is substituted for the
polonium.
It follows from these experiments that the ionization per unit volume,
due to a large plate uniformly covered with the radio-active matter,
falls off rapidly with the distance from the plate. At a distance of 10
cms. the α rays from uranium, thorium, or radium have been completely
absorbed in the gas, and the small ionization then observed in the gas
is due to the more penetrating β and γ rays. The relative amount of the
ionization observed at a distance from the source will increase with the
thickness of the layer of active matter, but will reach a maximum for a
layer of a certain thickness. The greater proportion of the ionization,
due to unscreened active matter, is thus entirely confined to a shell of
air surrounding it not more than 10 cms. in depth.
[Illustration: Fig. 35.]
=99.= The α rays from different compounds of the same active element,
although differing in amount, have about the same average penetrating
power. Experiments on this point have been made by the writer[160] and
by Owens[161]. Thus in comparing the relative power of penetration of
the α rays from the different radio-elements, it is only necessary to
determine the penetrating power for one compound of each of the
radio-elements. Rutherford and Miss Brooks[162] have determined the
amount of absorption of the α rays from the different active substances
in their passage through successive layers of aluminium foil ·00034 cm.
thick. The curves of absorption are given in Fig. 35. For the purpose of
comparison in each case, the initial current with the bare active
compound was taken as 100. A very thin layer of the active substance was
used, and, in the case of thorium and radium, the emanations given off
were removed by a slow current of air through the testing vessel. A
potential difference of 300 volts was applied between the plates, which
was sufficient to give the maximum current in each case.
Curves for the minerals organite and thorite were very nearly the same
as for thoria.
For comparison, the absorption curves of the excited radiations of
thorium and radium are given, as well as the curve for the
radio-elements uranium, thorium, radium, and polonium. The α radiations
may be arranged in the following order, as regards their power of
penetration, beginning with the most penetrating.
Thorium}
Radium } excited radiation.
Thorium.
Radium.
Polonium.
Uranium.
The same order is observed for all the absorbing substances examined,
viz., aluminium, Dutch metal, tinfoil, paper, and air and other gases.
The differences in the absorption of the α rays from the active bodies
are thus considerable, and must be ascribed either to a difference of
mass or of velocity of the α particles or to a variation in both these
quantities.
Since the α rays differ either in mass or velocity, it follows that they
cannot be ascribed to any single radio-active impurity common to all
radio-active bodies.
=100. Absorption of the α rays by gases=. The α rays from the different
radio-active substances are quickly absorbed in their passage through a
few centimetres of air at atmospheric pressure and temperature. In
consequence of this, the ionization of the air, due to the α rays, is
greatest near the surface of the radiating body and falls off very
rapidly with the distance (see section 98).
[Illustration: Fig. 36.]
A simple method of determining the absorption in gases is shown in Fig.
36. The maximum current is measured between two parallel plates _A_ and
_B_ kept at a _fixed_ distance of 2 cms. apart, and then moved by means
of a screw to different distances from the radio-active surface. The
radiation from this active surface passed through a circular opening in
the plate _A_, covered with thin aluminium foil, and was stopped by the
upper plate. For observations on other gases besides air, and for
examining the effect at different pressures, the apparatus is enclosed
in an air-tight cylinder.
If the radius of the active surface is large compared with the distance
of the plate _A_ from it, the intensity of the radiation is
approximately uniform over the opening in the plate _A_, and falls off
with the distance _x_ traversed according to an exponential law. Thus
$$ \frac {I} {I₀} = e^{–λ x} $$,
where λ is the “absorption constant” of the radiation for the gas under
consideration[163]. Let
_x_ = distance of lower plate from active material,
_l_ = distance between the two fixed plates.
The energy of the radiation at the lower plate is then
$$ I₀ e^{–λ x} $$,
and at the upper plate
$$ I₀ e^{–λ (l + x)} $$ .
The total number of ions produced between the parallel plates _A_ and
_B_ is therefore proportional to
$$ e^{–λ x} − e^{–λ (l + x)} = e^{–λ x} (1 -
e^{–λ l}) $$ .
Since the factor
$$ 1 = e^{–λ l} $$
is a constant, the saturation current between _A_ and _B_ varies as
$$ e^{–λ x} $$,
_i.e._ it decreases according to an exponential law with the distance
traversed.
[Illustration: Fig. 37.]
The variation of the current between _A_ and _B_ with the distance from
a thin layer of uranium oxide is shown in Fig. 37 for different gases.
The initial measurements were taken at a distance of about 3·5 mms. from
the active surface. The actual values of this initial current were
different for the different gases, but, for the purposes of comparison,
the value is in each case taken as unity.
It will be seen that the current falls off with the distance
approximately in a geometrical progression, a result which is in
agreement with the simple theory given above. The distance through which
the rays pass before they are absorbed is given below for different
gases.
Gas Distance in
mms. to absorb
half of
radiation
Carbonic acid 3
Air 4·3
Coal-gas 7·5
Hydrogen 16
The results for hydrogen are only approximate, as the absorption is
small over the distance examined.
The absorption is least in hydrogen and greatest in carbonic acid, and
follows the same order as the densities of the gases. In the case of air
and carbonic acid, the absorption is proportional to the density, but
this rule is widely departed from in the case of hydrogen. Results for
the relative absorption by air of the α rays from the different active
bodies are shown in Fig. 38.
[Illustration: Fig. 38.]
The initial observation was made about 2 mms. from the active surface,
and the initial current is in each case taken as 100. The current, as in
the case of uranium, falls off at first approximately in geometrical
progression with the distance. The thickness of air, through which the
radiation passes before the intensity is reduced to half value, is given
below.
Distance in mms.
Uranium 4·3
Radium 7·5
Thorium 10
Excited radiation 16·5
from Thorium and
Radium
The order of absorption by air of the radiations from the active
substances is the same as the order of absorption by the metals and
solid substances examined.
=101. Connection between absorption and density.= Since in all cases the
radiations first diminish approximately according to an exponential law
with the distance traversed, the intensity _I_ after passing through a
thickness _x_ is given by
$$ I = I₀ e^{–λ x} $$
where λ is the absorption constant and _I₀_ the initial intensity.
The following table shows the value of λ with different radiations for
air and aluminium.
Radiation λ for λ for air
aluminium
Excited radiation 830 ·42
Thorium 1250 ·69
Radium 1600 ·90
Uranium 2750 1·6
Taking the density of air at 20° C. and 760 mms. as 0·00120 compared
with water as unity, the following table shows the value of λ divided by
density for the different radiations.
Radiation Aluminium Air
Excited radiation 320 350
Thorium 480 550
Radium 620 740
Uranium 1060 1300
Comparing aluminium and air, the absorption is thus roughly proportional
to the density for all the radiations. The divergence, however, between
the absorption-density numbers is large when two metals like tin and
aluminium are compared. The value of λ for tin is not much greater than
for aluminium, although the density is nearly three times as great.
If the absorption is proportional to the density, the absorption in a
gas should vary directly as the pressure, and this is found to be the
case. Some results on this subject have been given by the writer (_loc.
cit._) for uranium rays between pressures of ¼ and 1 atmosphere. Owens
(_loc. cit._) examined the absorption of the α radiation in air from
thoria between the pressures of 0·5 to 3 atmospheres and found that the
absorption varied directly as the pressure.
The variation of absorption with density for the projected positive
particles is thus very similar to the law for the projected negative
particles and for cathode rays. The absorption, in both cases, depends
mainly on the density, but is not in all cases directly proportional to
it. Since the absorption of the α rays in gases is probably mainly due
to the exhaustion of the energy of the rays by the production of ions in
the gas, it seems probable that the absorption in metals is due to a
similar cause.
=102. Relation between ionization and absorption in gases.= It has been
shown (section 45) that if the α rays are completely absorbed in a gas,
the _total_ ionization produced is about the same for all the gases
examined. Since the rays are unequally absorbed in different gases,
there should be a direct connection between the relative ionization and
the relative absorption. This is seen to be the case if the results of
Strutt (section 45) are compared with the relative absorption constants
(section 100).
Gas Relative Relative
absorption ionization
Air 1 1
Hydrogen ·27 ·226
Carbon dioxide 1·43 1·53
Considering the difficulty of obtaining accurate determinations of the
absorption, the relative ionization in a gas is seen to be directly
proportional to the relative absorption within the limits of
experimental error. This result shows that the energy absorbed in
producing an ion is about the same in air, hydrogen, and carbon dioxide.
=103. Mechanism of the absorption of α rays by matter=. The experiments,
already described, show that the ionization of the gas, due to the α
rays from a large plane surface of radio-active matter, falls off in
most cases approximately according to an exponential law, until most of
the rays are absorbed, whereupon the ionization decreases at a much
faster rate. In the case of polonium, the ionization falls off more
rapidly than is to be expected on the simple exponential law.
The ionization produced in the gas is due to the collision of the
rapidly moving α particles with the molecules of the gas in their path.
On account of its large mass, the α particle is a far more efficient
ionizer than the β particle moving at the same speed. It can be deduced
from the results of experiment that each projected α particle is able to
produce about 100,000 ions in passing through a few centimetres of the
gas before its velocity is reduced to the limiting value, below which it
no longer ionizes the gas in its path.
Energy is required to ionize the gas, and this energy can only be
obtained at the expense of the kinetic energy of the projected α
particle. Thus it is to be expected that the α particle should gradually
lose its velocity and energy of motion in its passage through the gas.
Since the rate of absorption of the α rays in gases is deduced from
measurements of the ionization of the gas at different distances from
the source of radiation, a knowledge of the law of variation of the
ionizing power of the projected α particle with its speed is required in
order to interpret the results. The experimental data on this question
are, however, too incomplete to be applied directly to a solution of
this question. Townsend[164] has shown that a moving electron produces
ions in the gas after a certain limiting velocity is reached. The number
of ions produced per centimetre of its path through the gas then rises
to a maximum, and for still higher speeds continuously decreases. For
example, Townsend found that the number of ions produced by an electron
moving in an electric field was small at first for weak fields, but
increased with the strength of the electric field to a maximum
corresponding to the production of 20 ions per cm. of path in air at a
pressure of 1 mm. of mercury. Durack[165] found that the electrons,
generated in a vacuum tube, moving with a velocity of about 5 × 10⁹
cms. per second produced a pair of ions every 5 cms. of path at 1 mm.
pressure. In a later paper, Durack showed that for the electrons from
radium, which are projected with a velocity greater than half the
velocity of light, a pair of ions was produced every 10 cms. of path.
The high speed electron from radium is thus a very inefficient ionizer
and produces only about ¹⁄₁₀₀ of the ionization per unit path observed
by Townsend for the slow moving electron.
=104.= In the case of the α particle, no direct measurements have been
made upon the variation of the ionization with the velocity of the
particle, so that the law of absorption of the rays cannot be deduced
directly. An indirect attack upon the question has, however, been made
recently by Bragg and Kleeman[166] who have formulated a simple theory
to account for the experimental results which they have obtained upon
the absorption of the α rays. The α particles from each simple type of
radio-active matter are supposed to be projected with the same velocity,
and to pass through a definite distance a in air at atmospheric pressure
and temperature before they are all absorbed. As a first approximation
the ionization per unit path is supposed to be the same over the whole
length traversed before absorption, and to cease fairly suddenly at a
definite distance from the source of radiation. This is in agreement
with the observed fact that the ionization between parallel plates
increases very rapidly when it approaches nearer than a certain distance
to the radiant source. The range _a_ depends upon the initial energy of
motion of the α particle and will thus be different for different kinds
of radio-active matter. If a thick layer of radio-active matter is
employed, only the α particles from the surface have a range _a_. Those
which reach the surface from a depth _d_ have their range diminished by
an amount ρ_d_, where ρ is the density of the radio-active matter
compared with air. This is merely an expression of the fact that the
absorption of the α rays is proportional to the thickness and density of
matter traversed. The rays from a thick layer of active matter will thus
be complex, and will consist of particles of different velocity whose
ranges have all values between 0 and _a_.
Suppose that a narrow pencil of α rays is emitted from a thick layer of
radio-active material, and confined by metal stops as in Fig. 39.
[Illustration: Fig. 39.]
The pencil of rays passes into an ionization vessel _AB_ through a fine
wire gauze _A_. The amount of ionization is to be determined between _A_
and _B_ for different distances _h_ from the source of the rays _R_ to
the plate _A_.
All the particles coming from a depth _x_ of the material given by _h_ =
_a_ − ρ_x_ will enter the ionization vessel. The number of ions produced
in a depth _dh_ of the ionization vessel is equal to _nxdh_, _i.e._ to
_a_ − _h_
_n_ --------- _dh_,
ρ
where _n_ is a constant.
If the depth of the ionization vessel be _b_, the total number of ions
produced in the vessel is
$$ \int_h^{h+b} n \frac {a-h} {\rho} dh = \frac {nb} {\rho} (a − h -
\frac {b} {2}) $$ .
This supposes that the stream of particles passes completely across the
vessel. If not, the expression becomes
$$ \int_h^a n \frac {a − h} {\rho} dh = \frac {n (a − h)^2} {2\rho} $$ .
If the ionization in the vessel _AB_ is measured, and a curve plotted
showing its relation to _h_, the curve in the former case should be a
straight line whose slope is _nb_/ρ and in the latter a parabola.
Thus if a thin layer of radio-active material is employed and a shallow
ionization vessel, the ionization would be represented by a curve such
as _APM_ (Fig. 40), where the ordinates represent distances from the
source of radiation, and the abscissae the ionization current between
the plates _AB_.
[Illustration: Fig. 40.]
In this case, _PM_ is the range of the α particles from the lowest layer
of the radio-active matter. The current should be constant for all
distances less than _PM_.
For a thick layer of radio-active matter, the curve should be a straight
line such as _APB_.
Curves of the above character should only be obtained when definite
cones of rays are employed, and where the ionization vessel is shallow
and includes the whole cone of rays. In such a case the inverse square
law need not be taken into account.
In the experiments previously recorded (sections 99 and 100), the
ionization was measured between parallel plates several centimetres
apart for a large area of radio-active material. Such an arrangement was
necessary at the time at which the experiments were made, as only weak
radio-active material was available. Measurable electrical effects could
not then be obtained with narrow cones of rays and shallow ionization
vessels, but this disadvantage is removed by the advent of pure radium
bromide as a source of radiation.
The interesting experiments described by Bragg and Kleeman show that the
theoretical curves are approximately realized in practice. The chief
difficulty experienced in the analysis of the experimental results was
due to the fact that radium is a complex radio-active substance and
contains four radio-active products each of which gives rise to α rays
which have different ranges. The general character of the results
obtained from radium are shown graphically in Fig. 41, curves _A_, _B_,
_C_, _D_.
[Illustration: Fig. 41.]
The ordinates represent the distance between the radium and the gauze of
the testing vessel; the abscissae the current in the ionization vessel
in arbitrary units. Five milligrams of radium bromide were used, and the
depth of the ionization vessel was about 5 mms. Curve _A_ is for a cone
of rays of angle 20°. The initial current at a distance of 7 cms. is due
to the β and γ rays and natural leak. This curve is initially parabolic,
and then is made up of two straight lines. Curve _B_ is for a smaller
cone, and shows the straight line character of the curve to within a
short distance of the radium. Curve _C_ was obtained under the same
condition as curve _A_, but with a layer of gold beater’s skin placed
over the radium. The effect of this is to reduce all the ordinates of
curve _A_ by the same quantity. This is to be expected on the simple
theory already considered. Curve _D_ was obtained when the radium was
heated so as to get rid of the emanation and its products. The α
particles of greatest range are quite absent and the curve is simpler in
character.
[Illustration: Fig. 42.]
The complex character of the radium curves are more clearly brought out
by a careful examination of a portion of the curve at distances between
2 and 5 cms. from the radium, using an ionization vessel of depth only 2
mms. The results are shown in Fig. 42, where the curve is seen to
consist approximately of four straight lines of different slopes
represented by _PQ_, _QR_, _RS_, _ST_.
Such a result is to be expected, for it will be shown later that four
distinct α ray products exist in radium when in radio-active
equilibrium. Each of these products of radium emits an equal number of α
particles per second, but the range of each is different. If _a₁_ is the
range of one stream, _a₂_ of another, the ionization in the vessel _AB_,
when two streams enter the vessel, should be
_nb_ _nb_
---- (_a₁_-_h_-_b_/2) + ----- (_a₂_ − _h_ − _b_/2),
ρ ρ
_i.e._
_nb_
---- (_a₁_ + _a₂_ − 2_h_ − _b_) .
ρ
Thus the slope of the curve should in this case be 2_nb_/ρ, while if
only one stream enters, it should be _nb_/ρ. When three reach it, the
slope should be 3_nb_/ρ and for four 4_nb_/ρ. These results are realized
fairly closely in practice. The curve (Fig. 42) consists of four parts,
whose slopes are in the proportion 16, 34, 45, 65, _i.e._ very nearly in
the ratio 1, 2, 3, 4.
Experiments were also made with very thin layers of radium bromide,
when, as we have seen (Fig. 40) a very different shape of curve is to be
expected. An example of the results is shown in Fig. 43, curves I., II.
and III. Curve I. is obtained from radium bromide which has been heated
to drive off the emanation, and curves II. and III. from the same
substance several days later, when the emanation was again accumulating.
The portion _PQ_, which is absent in the first curve, is probably due to
the “excited” activity produced by the emanation. By careful examination
of the successive changes in the curves after the radium has been heated
to drive off the emanation, it is possible to tell the range of the α
rays from each of the different products, and this has been done to some
extent by Bragg and Kleeman.
It will be seen later that the results here obtained support in a novel
way the theory of radio-active changes which has been advanced from data
of quite a different character.
The inward slope of the curve in Fig. 43 due to the radium indicates
that the α particles become more efficient ionizers as their velocity
decreases. This is in agreement with observations on the β rays. In some
cases Bragg also observed that the α particles are the most efficient
ionizers just before they lose their power of ionizing the gas.
[Illustration: Fig. 43.]
Thus we may conclude from these experiments that the α particles from a
simple radio-active substance traverse a definite distance in air, at a
definite pressure and temperature, and that the ionization ends fairly
abruptly. If the rays traverse a sheet of metal, the effective range of
ionization is diminished by a distance corresponding to ρ_d_, where ρ is
the density of the material compared with air and _d_ its thickness. The
α rays from a thick layer of a simple radio-active substance consist of
α particles of different velocities, which have ranges in air lying
between 0 and the maximum range. The ionization of the particles per
unit path is greatest near the end of its range, and decreases somewhat
as we approach the radiant source. A complex source of rays like radium
gives out four types of rays, each of which has a different but distinct
range.
From this theory it is possible to calculate approximately the decrease
of current to be observed when sheets of metal foil are placed over a
large area of radio-active substance. This is the method that has been
employed to obtain the curves of Figs. 35 and 38.
Suppose a very thin layer of simple radio-active matter is employed (for
example a bismuth plate covered with radio-tellurium or a metal plate
made active by exposure to the presence of the thorium or radium
emanations) and that the ionization vessel is of sufficient depth to
absorb the α rays completely.
Let _d_ be the thickness of the metal plate, ρ its density compared with
air. Consider a point _P_ close to the upper side of the plate. The
range of the particles moving from a point, when the path makes an angle
θ with the normal at _P_, is _a_ − ρ_d_ sec θ, where _a_ is the range in
air. The rays coming from points such that the paths make an angle with
the normal greater than
$$ \cos^{−1} \frac {\rho d} {a} $$
will thus be absorbed in the plate. By integrating over the circular
area under the point _P_, it is easy to show that the total ionization
in the vessel is proportional to
$$ \int₀^{\cos^{−1} \frac {\rho d} {a}} 2 \pi \sin \theta \cos \theta (a −
\rho d \sec \theta) d\theta = \frac {\pi (a − \rho d)^2} {a} $$ .
The curves showing the relation between current and distance of metal
traversed should thus be parabolic with respect to _d_. This is
approximately the case for a simple substance like radio-tellurium. The
curve for a thick layer of radium would be difficult to calculate on
account of the complexity of the rays, but we know from experiment that
it is approximately exponential. An account of some recent
investigations made to determine the range of velocity over which the α
particle is able to ionize the gas is given in Appendix A. The results
there given strongly support the theory of absorption of the α rays
discussed above.
PART IV.
The γ or very penetrating Rays.
=105.= In addition to the α and β rays, the three active substances,
uranium, thorium, and radium, all give out a radiation of an
extraordinarily penetrating character. These γ rays are considerably
more penetrating than the X rays produced in a “hard” vacuum tube. Their
presence can readily be observed for an active substance like radium,
but is difficult to detect for uranium and thorium unless a large
quantity of active material is used. Villard[167], using the
photographic method, first drew attention to the fact that radium gave
out these very penetrating rays, and found that they were non-deviable
by a magnetic field. This result was confirmed by Becquerel[168].
Using a few milligrams of radium bromide, the γ rays can be detected in
a dark room by the luminosity they excite in the mineral willemite or a
screen of platinocyanide of barium. The α and β rays are completely
absorbed by placing a thickness of 1 centimetre of lead over the radium,
and the rays which then pass through the lead consist entirely of γ
rays. The very great penetrating power of these rays is easily observed
by noting the slight diminution of the luminosity of the screen when
plates of metal several centimetres thick are placed between the radium
and the screen. These rays also produce ionization in gases and are best
investigated by the electrical method. The presence of the γ rays from
30 mgrs. of radium bromide can be observed in an electroscope after
passing through 30 cms. of iron.
=106. Absorption of the γ rays=. In an examination of the active
substances by the electrical method, the writer[169] found that both
uranium and thorium gave out γ rays in amount roughly proportional to
their activity. An electroscope of the type shown in Fig. 12 was
employed. This was placed on a large lead plate ·65 cm. thick, the
active substance being placed in a closed vessel beneath.
The discharge due to the natural ionization of the air in the
electroscope was first observed. The additional ionization due to the
active substance must be that produced by rays which have passed through
the lead plate and the walls of the electroscope. The following table
shows that the discharge due to these rays decreases approximately
according to an exponential law with the thickness of lead traversed.
Thickness of lead Rate of discharge
·62 cms. 100
„ + ·64 cms. 67
„ + 2·86 „ 23
„ + 5·08 „ 8
Using 100 grs. of uranium and thorium, the discharge due to the rays
through 1 cm. of lead was quite appreciable, and readily measured. The
results showed that the amount of γ rays was about the same for equal
weights of thorium and uranium oxides. The penetrating power was also
about the same as for the radium rays.
[Illustration: Fig. 44.]
The writer showed that the absorption of the γ rays from radium was
approximately proportional to the density of the substance traversed. A
more detailed examination of the absorption of these rays in various
substances has been recently made by McClelland[170]. The curve (Fig.
44) shows the decrease of the ionization current in a testing vessel due
to the β and γ rays with successive layers of lead. It is seen that the
β rays are almost completely stopped by 4 mms. of lead; the ionization
is then due entirely to the γ rays.
In order to leave no doubt that all the β rays were absorbed, the radium
was covered with a thickness of 8 mms. of lead, and measurements of the
coefficient of absorption λ were made for additional thicknesses. The
average value of λ was calculated from the usual equation
$$ \frac {I} {I₀} = e^{–λ d} $$,
where _d_ is the thickness of matter traversed. The following table
shows the value of λ, (I) for the first 2·5 mms. of matter traversed
(after initially passing through 8 mms. of lead), (II) for the thickness
2·5 to 5 mms., (III) for 5 to 10 mms., (IV) 10 to 15 mms.
TABLE A.
Substance I II III IV
Platinum 1·167
Mercury ·726 ·661 ·538 ·493
Lead ·641 ·563 ·480 ·440
Zinc ·282 ·266 ·248 ·266
Aluminium ·104 ·104 ·104 ·104
Glass ·087 ·087 ·087 ·087
Water ·034 ·034 ·034 ·034
In the above table, the absorption in aluminium, glass and water was too
small to determine with accuracy the variation of λ with distance
traversed. It will be observed that, for the denser substances, the
coefficient of absorption decreases with the distance through which the
rays have passed. This indicates that the rays are heterogeneous. The
variation of λ is more marked in heavy substances.
Table B gives the values of λ divided by density for the above numbers.
If the absorption were directly proportional to the density, the
quotient would be the same in all cases.
TABLE B.
λ _divided by density_.
Substance I II III IV
Platinum ·054
Mercury ·053 ·048 ·039 ·036
Lead ·056 ·049 ·042 ·037
Zinc ·039 ·037 ·034 ·033
Aluminium ·038 ·038 ·038 ·038
Glass ·034 ·034 ·034 ·034
Water ·034 ·034 ·034 ·034
The numbers in column I vary considerably, but the agreement becomes
closer in the succeeding columns, until in column IV the absorption is
very nearly proportional to the density.
It is seen that the absorption of all three types of rays from
radio-active substances is approximately proportional to the density of
the substance traversed—a relation first observed by Lenard for the
cathode rays. This law of absorption thus holds for both positively and
negatively electrified particles projected from the radio-active
substances, and also for the electromagnetic pulses which are believed
to constitute the γ rays; although the absorption of the α rays, for
example, is 10,000 times greater than for the γ rays. We have seen in
section 84 that the value of the absorption constant λ for lead is 122
for the β rays from uranium. The value for the γ rays from radium varies
between ·64 and ·44, showing that the γ rays are more than 200 times as
penetrating as the β rays.
=107. Nature of the rays.= In addition to their great penetrating power,
the γ rays differ from the α and β rays in not being deflected to an
appreciable degree by a magnetic or electric field. In a strong magnetic
field, it can be shown, using the photographic method, that there is an
abrupt discontinuity between the β and γ rays, for the former are bent
completely away from the latter. This indicates that, as regards the
action of a magnetic field, there is no gradual transition of magnetic
properties between the β and γ rays. Paschen[171] has examined the γ
rays in a very intense magnetic field, and, from the absence of
deflection of these rays, has calculated that, if they consist of
electrified particles carrying an ionic charge, and projected with a
velocity approaching that of light, their apparent mass must be at least
45 times greater than that of the hydrogen atom.
It now remains for us to consider whether the γ rays are corpuscular in
character, or whether they are a type of electromagnetic pulse in the
ether similar to Röntgen rays. They resemble Röntgen rays in their great
penetrating power and in their absence of deflection in a magnetic
field. Earlier experiments seemed to indicate an important difference
between the action of γ and X rays. It is well known that ordinary X
rays produce much greater ionization in gases such as sulphuretted
hydrogen and hydrochloric acid gas, than in air, although the
differences in density are not large. For example, exposed to X rays,
sulphuretted hydrogen has six times the conductivity of air, while with
γ rays the conductivity only slightly exceeds that of air. The results
obtained by Strutt, in this connection, have already been given in
section 45. It is there shown that the relative conductivity of gases
exposed to γ rays (and also to α and β rays) is, in most cases, nearly
proportional to their relative densities; but, under X rays, the
relative conductivity for some gases and vapours is very much greater
than for the γ rays. It must be remembered, however, that the results
obtained by Strutt were for “soft X rays,” whose penetrating power was
very much less than that of the γ rays. In order to see if the relative
conductivity of gases produced by X rays depended upon their penetrating
power, A. S. Eve[172] made some experiments with a very “hard” X ray
bulb, which gave an unusually penetrating type of rays.
The results of the measurements are shown in the table below, where the
conductivity for each type of rays is expressed relative to air as
unity. The results obtained for “soft” X rays by Strutt and by Eve for γ
rays are added for comparison.
It is seen that the hard rays show a much closer agreement than the soft
rays with the density law found for the γ rays. The high values
previously obtained for the vapours of chloroform and carbon
tetrachloride are greatly reduced, and are very nearly the same as for
the γ rays. On the other hand, the vapour of methyl iodide is an
exception, and still shows a high conductivity. The γ rays were,
however, forty times as penetrating as the hard X rays, and it is
probable that the value of methyl iodide would be reduced with still
more penetrating X rays.
_Relative conductivities of gases._
Gas Relative “Soft” “Hard” γ rays
Density X rays X rays
Hydrogen ·07 ·11 ·42 ·19
Air 1·0 1·0 1·0 1·0
Sulphuretted 1·2 6 ·9 1·23
Hydrogen
Chloroform 4·3 32 4·6 4·8
Methyl Iodide 5·0 72 13·5 5·6
Carbon 5·3 45 4·9 5·2
Tetrachloride
The hard X rays were found to give far more secondary radiation than the
γ rays, but this effect is probably also a function of the penetrating
power of the primary rays. It will be seen later (section 112) that γ
rays give rise to a secondary radiation of the β ray type. This has also
been observed for the X rays.
Considering the experimental evidence as a whole, there is undoubtedly a
very marked similarity between the properties of γ and X rays. The view
that the γ rays are a type of very penetrating X rays, also receives
support from theoretical considerations. We have seen (section 52) that
the X rays are believed to be electromagnetic pulses, akin in some
respects to short light waves, which are set up by the sudden stoppage
of the cathode ray particles. Conversely, it is also to be expected that
X rays will be produced at the sudden starting, as well as at the sudden
stopping, of electrons. Since most of the β particles from radium are
ejected from the radium atom with velocities much greater than the
cathode particles in a vacuum tube, X rays of a very penetrating
character will arise. But the strongest argument in support of this view
is derived from an examination of the origin and connection of the β and
γ rays from radio-active substances. It will be shown later that the α
ray activity observed in radium arises from several disintegration
products, stored up in the radium, while the β and γ rays arise only
from one of these products named radium _C_. It is found, too, that the
activity measured by the γ rays is always proportional to the activity
measured by the β rays, although by separation of the products the
activity of the latter may be made to undergo great variations in value.
Thus the intensity of the γ rays is always proportional to the rate of
expulsion of β particles, and this result indicates that there is a
close connection between the β and γ rays. Such a result is to be
expected if the β particle is the parent of the γ ray, for the expulsion
of each electron from radium will give rise to a narrow spherical pulse
travelling from the point of disturbance with the velocity of light.
=108.= There is another possible hypothesis in regard to the nature of
these rays. It has been shown (sections 48 and 82) that the apparent
mass of an electron increases as the speed of light is approached;
theoretically it should be very great when the velocity of the electron
is exceedingly close to the velocity of light. In such a case, a moving
electron would be difficult to deflect by a magnetic or electric field.
The view that the γ rays are electrons carrying a negative charge and
moving with a velocity nearly equal to that of light has recently been
advocated by Paschen[173]. He concluded from experiment that the γ rays
like the β rays carried a negative charge. We have seen (section 85)
that Seitz also observed that a small negative charge was communicated
to bodies on which the γ rays impinged, but the magnitude of this charge
was much smaller than that observed by Paschen. I do not think that much
weight can be attached to observations that a small positive or negative
charge is communicated to bodies on which the γ rays fall, for it will
be shown later that a strong secondary radiation, consisting in part of
electrons, is set up during the passage of the γ rays through matter. It
is not improbable that the small charge observed is not a direct result
of the charge carried by the γ rays, but is an indirect effect due to
the secondary radiations emitted from the surface of bodies. There is no
doubt that a thick lead vessel, completely enclosing a quantity of
radium, acquires a small positive charge, but this result would follow
whether the γ rays carry a charge or not, since the secondary radiations
from the lead surface consist of projected particles which carry with
them a negative charge.
On this corpuscular theory of the nature of the γ rays, each electron
must have a large apparent mass, or otherwise it would be appreciably
deflected by an intense magnetic field. The energy of motion of the
electron must, in consequence, be very great, and, if the number of the
electrons constituting the γ rays is of the same order of magnitude as
the number of the β particles, a large heating effect is to be expected
when the γ rays are stopped in matter. Paschen[174] made some
experiments on the heat emission of radium due to the γ rays; he
concluded that the γ rays were responsible for more than half of the
total heat emission of radium and carried away energy at the rate of
over 100 gram calories per hour per gram of radium. This result was not
confirmed by later experiments of Rutherford and Barnes[175], who found
that the heating effect of the γ rays could not be more than a few per
cent. of the total heat emission of radium. These results will be
considered later in chapter XII.
The weight of evidence, both experimental and theoretical, at present
supports the view that the γ rays are of the same nature as the X rays
but of a more penetrating type. The theory that the X rays consist of
non-periodic pulses in the ether, set up when the motion of electrons is
arrested, has found most favour, although it is difficult to provide
experimental tests to decide definitely the question. The strongest
evidence in support of the wave nature of the X rays is derived from the
experiments of Barkla[176], who found that the amount of secondary
radiation set up by the X rays on striking a metallic surface depended
on the orientation of the X ray bulb. The rays thus showed evidence of a
one-sidedness or polarization which is only to be expected if the rays
consist of a wave motion in the ether.
PART V.
Secondary Rays.
=109. Production of secondary rays.= It has long been known that Röntgen
rays, when they impinge on solid obstacles, produce secondary rays of
much less penetrating power than the incident rays. This was first shown
by Perrin and has been investigated in detail by Sagnac, Langevin,
Townsend and others. Thus it is not surprising that similar phenomena
should be observed for the radiation from radio-active substances. By
means of the photographic method, Becquerel[177] has made a close study
of the secondary radiations produced by radio-active substances. In his
earliest observations, he noticed that radiographs of metallic objects
were always surrounded by a diffuse border. This effect is due to the
secondary rays set up by the incident rays at the surface of the screen.
The secondary rays produced by the α rays are very feeble. They are best
shown by polonium, which gives out only α rays, so that the results are
not complicated by the action of the β rays. Strong secondary rays are
set up at the point of impact of the β or cathodic rays. Becquerel found
that the magnitude of this action depended greatly on the velocity of
the rays. The rays of lowest velocity gave the most intense secondary
action, while the penetrating rays gave, in comparison, scarcely any
secondary effect. In consequence of the presence of this secondary
radiation, the photographic impression of a screen pierced with holes is
not clear and distinct. In each case there is a double photographic
impression, due to the primary rays and the secondary rays set up by
them.
These secondary rays are deviable by a magnetic field, and in turn
produce tertiary rays and so on. The secondary rays are in all cases
more readily deviated and absorbed than the primary rays, from which
they arise. The very penetrating γ rays give rise to secondary rays,
which cause intense action on the photographic plate. When some radium
was placed in a cavity inside a deep lead block, rectangular in shape,
besides the impression due to the direct rays through the lead,
Becquerel observed that there was also a strong impression due to the
secondary rays emitted from the surface of the lead. The action of these
secondary rays on the plate is so strong that the effect on the plate
is, in many cases, increased by adding a metal screen between the active
material and the plate.
The comparative photographic action of the primary and secondary rays
cannot be taken as a relative measure of the intensity of their
radiations. For example, only a small portion of the energy of the β
rays is in general absorbed in the sensitive film. Since the secondary
rays are far more easily absorbed than the primary rays, a far greater
proportion of their energy is expended in producing photographic action
than in the case of the primary rays. It is thus not surprising that the
secondary rays set up by the β and γ rays may in some cases produce a
photographic impression comparable with, if not greater than, the effect
of the incident rays.
On account of these secondary rays, radiographs produced by the β rays
of radium in general show a diffuse border round the shadow of the
object. For this reason radiographs of this kind lack the sharpness of
outline of X ray photographs.
=110. Secondary radiation produced by α rays=. Mme Curie[178] has shown
by the electric method that the α rays of polonium produce secondary
rays. The method adopted was to compare the ionization current between
two parallel plates, when two screens of different material, placed over
the polonium, were interchanged.
These results show that the α rays of polonium are modified in passing
through matter, and that the amount of secondary rays set up varies with
screens of different material. Mme Curie, using the same method, was
unable to observe any such effect for the β rays of radium. The
production of secondary rays by the β rays of radium is, however,
readily shown by the photographic method. We have already seen (section
93) that very low velocity electrons accompany the α rays from radium or
radio-tellurium spread on a metal plate. These electrons are probably
liberated when the α rays escape from or impinge upon matter, and the
number emitted depends upon the kind of matter used as a screen. The
differences shown in the above table when the screens were interchanged
are explained simply in this way.
Screens employed Thickness Current
in mms. observed
Aluminium 0·01
Cardboard 0·005 17·9
Cardboard 0·005
Aluminium 0·01 6·7
Aluminium 0·01
Tin 0·005 150
Tin 0·005
Aluminium 0·01 126
Tin 0·005
Cardboard 0·005 13·9
Cardboard 0·005
Tin 0·005 4·4
[Illustration: Fig. 45.]
=111. Secondary rays produced by= β =and= γ =rays=. An examination of
the amount and character of the secondary radiation emitted by various
substances, when exposed to the β and γ rays of radium, has recently
been made by A. S. Eve[179]. The general experimental method employed is
shown in Fig. 45.
The electroscope (Fig. 45) was placed behind a lead screen 4·5 cms.
thick, which stopped all the β rays and absorbed the greater proportion
of the γ rays from the radium tube placed at _R_. On bringing near a
plate of matter _M_, the primary rays fell upon it and some of the
secondary rays, emitted in all directions, passed into the side of the
electroscope, which was covered with aluminium foil of thickness ·05 mm.
Before the plate _M_ was placed in position the rate of discharge of the
electroscope was due to the natural leak and the γ rays from _R_, and
the secondary radiation from the air. On bringing the radiator _M_ into
position, the rate of discharge was much increased, and the difference
between the rate of movement of the gold-leaf in the two cases was taken
as a measure of the amount of secondary rays from _M_. The absorption of
the secondary rays was tested by placing an aluminium plate ·85 mm.
thick before the face of the electroscope.
The secondary rays were found to be fairly homogeneous, for the
intensity fell off according to an exponential law with the distance
traversed. The value of the absorption constant λ was determined from
the usual equation
$$ \frac {I} {I₀} = e^{–λ d} $$,
where _d_ is the thickness of the screen. The table given below shows
the results obtained when thick plates of different substances of the
same dimensions were placed in a definite position at _M_. The secondary
radiation from fluids was obtained by a slight alteration of the
experimental arrangements.
Thirty milligrammes of radium bromide were used, and the results are
expressed in terms of the number of scale divisions passed over per
second by the gold-leaf.
It will be noticed that the amount of secondary radiation follows in
most cases the same order as the densities, and is greatest for mercury.
The value of (secondary radiation)/density is not a constant, but varies
considerably, being greatest for light substances. The absorption
constant of the secondary rays from different radiators is not very
different, with the exception of substances such as granite, brick, and
cement, which give out secondary rays of nearly twice the penetrating
power of other substances.
β _and_ γ _rays_.
Radiator Density Secondary Sec. Aluminium
Radiation Rad. / ·085 cm.
Density λ
Mercury 13·6 147 10·8
Lead 11·4 141 12·4 18·5
Copper 8·8 79 9·0 20
Brass 8·4 81 9·6 21
Iron 7·8 75 9·6 20
(wrought)
Tin 7·4 73 9·9 20·3
Zinc 7·0 79 11·3
Granite 2·7 54 20·0 12·4
Slate 2·6 53 20·4 12·1
Aluminium 2·6 42 16·1 24
Glass 2·5 44 17·6 24
Cement 2·4 47 19·6 13·5
Brick 2·2 49 22·3 13·0
Ebonite 1·1 32 29·1 26
Water 1·0 24 24·0 21
Ice ·92 26 28·2
Paraffin ·9 17 18·8 21
solid
„ liquid ·85 16 18·8
Mahogany ·56 21·4 38·2 23
Paper ·4? 21·0 52 22
Millboard ·4? 19·4 48 20·5
Papier-mâché ... 21·9
Basswood ·36 20·7 57 22
Pine ·35 21·8 62 21
X ray screen 75·2 23·6
The secondary radiation not only comes from the surface of the radiator
but from a considerable depth. The amount of secondary rays increases
with the thickness of the radiator, and, in the case of glass and
aluminium, reaches a practical maximum for a plate about 3 mms. thick.
In the above table, the secondary radiation arises from both the β rays
and γ rays together. When the β rays were cut off by a layer of lead 6·3
mms. thick, placed between the radium and the radiator, the effect on
the electroscope was reduced to less than 20 per cent. of its former
value, showing that the β rays supplied more than 80 per cent. of the
secondary radiation. The following table shows the relative amount of
secondary rays from different substances when exposed to β and γ rays
together and to γ rays alone. The amount from lead in each case is taken
as a standard and equal to 100. The amount of secondary radiation found
by Townsend from soft X rays is added for comparison.
_Secondary Radiations._
β and γ Röntgen
Radiator γ rays
rays
Lead 100 100 100
Copper 57 61 291
Brass 58 59 263
Zinc 57 ... 282
Aluminium 30 30 25
Glass 31 35 31
Paraffin 12 20 125
It will be observed that the relative amounts are about the same for the
γ rays alone as for the β and γ rays together. On the other hand, the
amount of secondary radiation set up by X rays is very different, lead
for example giving much less than brass or copper. The secondary rays
from the γ rays alone are slightly less penetrating than for the β and γ
rays together, but are far more penetrating than the secondary radiation
from the X rays examined by Townsend.
The amount of secondary radiation set up by the β and γ rays is mainly
independent of the state of the surface of the radiator. About the same
amount is obtained from iron as from iron filings; from liquid as from
solid paraffin; and from ice as from water[180].
Becquerel has shown that the secondary rays set up by the β rays are
deflected by a magnet and consist of negatively charged particles
(electrons). It has been pointed out in section 52 that the cathode
rays are diffusely reflected from the metal on which they fall. These
secondary rays consist in part of electrons moving with about the
same velocity as the primary, and in part of some electrons with a much
slower speed. The secondary rays set up by the β rays of radium have on
an average less penetrating power than the primary rays, and
consequently less velocity than the primary rays. It must be remembered
that the β rays from radium are very complex, and consist of electrons
projected with a considerable range of velocities. The secondary rays
are, on an average, certainly more penetrating than the most easily
absorbed β rays emitted from radium, and probably move with a velocity
of about half that of light.
It is still uncertain whether the secondary rays are produced by the
action of the primary rays on matter, or whether they consist of a
portion of the primary rays whose direction of motion has been deflected
in their passage through matter, so that they emerge again with
diminished velocity from the surface.
=112. Magnetic deflection of secondary rays from γ rays=. It has been
seen that the secondary rays set up by the γ rays alone are very similar
in character to those caused by the β rays. This result was still
further confirmed by Eve, who showed that the secondary rays produced by
the γ rays are readily deflected by a magnetic field. The experimental
arrangement is shown in Fig. 46.
[Illustration: Fig. 46.]
A small electroscope was mounted on one side of a lead platform 1·2 cms.
thick, which rested on a lead cylinder 10 cms. high and 10 cms. in
diameter. The radium was placed at the bottom of a hole reaching to the
centre of the cylinder.
On applying a strong magnetic field, at right angles to the plane of the
paper, so as to bend the secondary rays from the platform towards the
electroscope, the rate of discharge was much increased. On reversing the
field, the effect was much diminished. Since the γ rays are not
themselves deflected by a magnetic field, this result shows that the
secondary radiation is quite different in character from the primary
rays, and consists of electrons projected with a velocity (deduced from
the penetrating power) of about half the velocity of light. We have
already pointed out that the emission of electrons from a substance
traversed by the rays will account sufficiently well for the charge
observed by Paschen, without the necessity of assuming that the γ rays
carry a negative charge of electricity.
The secondary radiation set up by Röntgen rays, like that due to the β
and γ rays, consists in part of electrons projected with considerable
velocity. These three types of rays seem about equally efficient in
causing the expulsion of electrons from the substance through which they
pass. We have seen that the X and γ rays are, in all probability,
electromagnetic pulses set up by the sudden starting or stopping of
electrons, and, since these rays in turn cause the removal of electrons,
the process appears to be reversible. Since the β rays pass through some
thickness of matter before their energy of motion is arrested, theory
would lead us to expect that a type of soft X rays should be generated
in the absorbing matter.
PART VI.
=113. Comparison of the ionization produced by the α and β rays=. With
unscreened active material the ionization produced between two parallel
plates, placed as in Fig. 17, is mainly due to the α rays. On account of
the slight penetrating power of the α rays, the current due to them
practically reaches a maximum with a small thickness of radio-active
material. The following saturation currents were observed[181] for
different thicknesses of uranium oxide between parallel plates
sufficiently far apart for all the α rays to be absorbed in the gas
between them.
_Surface of uranium oxide 38 sq. cms._
Weight of uranium Saturation current
oxide in grammes per in amperes per sq.
sq. cm. of surface cm. of surface
·0036 1·7 × 10⁻¹³
·0096 3·2 × 10⁻¹³
·0189 4·0 × 10⁻¹³
·0350 4·4 × 10⁻¹³
·0955 4·7 × 10⁻¹³
The current reached about half its maximum value for a weight of oxide
·0055 gr. per sq. cm. If the α rays are cut off by a metallic screen,
the ionization is then mainly due to the β rays, since the ionization
produced by the γ rays is small in comparison. For the β rays from
uranium oxide it has been shown (section 86) that the current reaches
half its maximum value for a thickness of 0·11 gr. per sq. cm.
Meyer and Schweidler[182] have found that the radiation from a water
solution of uranium nitrate is very nearly proportional to the amount of
uranium present in the solution.
On account of the difference in the penetrating power of the α and β
rays, the ratio of the ionization currents produced by them depends on
the thickness of the radio-active layer under examination. The following
comparative values of the current due to the α and β rays were obtained
for very thin layers of active matter[183]. A weight of ⅒ gramme of
fine powder, consisting of uranium oxide, thorium oxide, or radium
chloride of activity 2000, was spread as uniformly as possible over an
area of 80 sq. cms. The saturation current was observed between parallel
plates 5·7 cms. apart. This distance was sufficient to absorb most of
the α rays from the active substances. A layer of aluminium ·009 cm.
thick absorbed all the α rays.
Current Current Ratio of
due to α due to β currents
rays rays β/α
Uranium 1 1 ·0074
Thorium 1 ·27 ·0020
Radium 2000 1350 ·0033
In the above table the saturation current due to the α and β rays of
uranium is, in each case, taken as unity. The third column gives the
ratio of the currents observed for equal weights of substance. The
results are only approximate in character, for the ionization due to a
given weight of substance depends on its fineness of division. In all
cases, the current due to the β rays is small compared with that due to
the α rays, being greatest for uranium and least for thorium. As the
thickness of layer increases, the ratio of currents β/α steadily
increases to a constant value.
=114. Comparison of the energy radiated by the α and β rays=. It has not
yet been found possible to measure directly the energy of the α and β
rays. A comparison of the energy radiated in the two forms of rays can,
however, be made indirectly by two distinct methods.
If it be assumed that the same amount of energy is required to produce
an ion by either the α or the β ray, and that the same proportion of the
total energy is used up in producing ions, an approximate estimate can
be made of the ratio of the energy radiated by the α and β rays by
measuring the ratio of the total number of ions produced by them. If λ
is the coefficient of absorption of the β rays in air, the rate of
production of ions per unit volume at a distance x from the source is
$$ q₀ e^{–λ x} $$
where _q₀_ is the rate of ionization at the source.
The total number of ions produced by complete absorption of the rays is
$$ \int₀^{\infty} q₀ e^{–λ x} dx = \frac {q₀} {λ} $$,
Now λ is difficult to measure experimentally for air, but an approximate
estimate can be made of its value from the known fact that the
absorption of β rays is approximately proportional to the density of any
given substance.
For β rays from uranium the value of λ for aluminium is about 14, and λ
divided by the density is 5·4. Taking the density of air as ·0012, we
find that for air
λ = ·0065.
The total number of ions produced in air is thus 154_q₀_ when the rays
are completely absorbed.
Now from the above table the ionization due to the β rays is ·0074 of
that produced by α rays, when the β rays passed through a distance of
5·7 cms. of air.
Thus we have approximately
Total number of ions produced by β rays ·0074
---------------------------------- = ----- × 154 = 0·20.
Total number of ions produced by α rays 5·7
Therefore about ⅙ of the total energy radiated into air by a thin
layer of uranium is carried by the β rays or electrons. The ratio for
thorium is about ¹⁄₂₂ and for radium about ¹⁄₁₄, assuming the rays to
have about the same average value of λ.
This calculation takes into account only the energy which is radiated
out into the surrounding gas; but on account of the ease with which the
α rays are absorbed, even with a thin layer, the greater proportion of
the radiation is _absorbed by the radio-active substance itself_. This
is seen to be the case when it is recalled that the α radiation of
thorium or radium is reduced to half value after passing through a
thickness of about 0·0005 cm. of aluminium. Taking into consideration
the great density of the radio-active substances, it is probable that
most of the radiation which escapes into the air is due to a thin skin
of the powder not much more than ·0001 cm. in thickness.
* * * * *
An estimate, however, of the relative rate of emission of energy by the
α and β rays from a thick layer of material can be made in the following
way:—For simplicity suppose a thick layer of radio-active substance
spread uniformly over a large plane area. There seems to be no doubt
that the radiations are emitted uniformly from each portion of the mass;
consequently, the radiation, which produces the ionizing action in the
gas above the radio-active layer, is the sum total of all the radiation
which reaches the surface of the layer.
* * * * *
Let λ₁ be the average coefficient of absorption of the α rays in the
radio-active _substance itself_ and σ the specific gravity of the
substance. Let _E₁_ be the _total_ energy radiated per sec. per unit
mass of the substance when the absorption of the rays in the substance
itself is disregarded. The energy per sec. radiated to the upper surface
by a thickness _dx_ of a layer of unit area at a distance _x_ from the
surface is given by
$$ \frac {1} {2} E_1 \sigma e^{–λ_1 x} dx $$ .
The total energy _W₁_ per unit area radiated to the surface per sec. by
a thickness _d_ is given by
$$ W_1 = \frac {1}{2} \int₀^d E_1 \sigma e^{–λ_1 x} dx $$
$$ = \frac {E_1 \sigma} {2 λ_1} (1 − e^{–λ_1 d}) $$
$$ = \frac {E_1 \sigma} {2 λ_1} $$
if λ₁_d_ is large.
* * * * *
In a similar way it may be shown that the energy _W₂_ of the β rays
reaching the surface is given by
$$ W_2 = \frac {E_2 \sigma} {2 λ_2} $$
where _E₂_ and λ₂ are the values for the β rays corresponding to _E₁_
and λ₁ for the α rays. Thus it follows that
_E₁_ λ₁_W₁_
---- = ------
_E₂_ λ₂_W₂_
λ₁ and λ₂ are difficult to determine directly for the radio-active
substance itself, but it is probable that the ratio λ₁/λ₂ is not very
different from the ratio for the absorption coefficients for another
substance like aluminium. This follows from the general result that the
absorption of both α and β rays is proportional to the density of the
substance; for it has already been shown in the case of the β rays from
uranium that the absorption of the rays in the radio-active material is
about the same as for non-radio-active matter of the same density.
With a thick layer of uranium oxide spread over an area of 22 sq. cms.,
it was found that the saturation current between parallel plates 6·1
cms. apart, due to the α rays, was 12·7 times as great as the current
due to the β rays. Since the α rays were entirely absorbed between the
plates and the total ionization produced by the β rays is 154 times the
value at the surface of the plates,
_W₁_ total number of ions due to α rays
----- = ----------------------------------
_W₂_ total number of ions due to β rays
12·7 × 6·1
= ------ = 0·5 approximately.
154
Now the value of λ₁ for aluminium is 2740 and of λ₂ for the same metal
14, thus
_E₁_ λ₁_W₁_
----- = ---------- = 100 approximately
_E₂_ λ₂_W₂_
This shows that the energy radiated from a thick layer of material by
the β rays is only about 1 per cent. of the energy radiated in the form
of α rays.
This estimate is confirmed by calculations based on independent data.
Let _m₁_, _m₂_ be the masses of the α and β particles respectively and
_v₁_, _v₂_ their velocities.
$$ \frac {Energy of one \alpha particle} {Energy of one \beta particle}
= \frac {m_1 v_1^2} {m_2 v_2^2} $$
$$ = \frac {\frac {m_1} {e} v_1^2} {\frac {m^2} {e} v_2^2} $$ .
Now it has been shown that for the α rays of radium
_v₁_ = 2·5 × 10⁹,
_e_
---- = 6 × 10³.
_m₁_
The velocity of the β rays of radium varies between wide limits. Taking
for an average value
_v₂_ = 1·5 × 10¹⁰,
_e_
---- = 1·8 × 10⁷,
_m₂_
it follows that the energy of the α particle from radium is almost 83
times the energy of the β particle. If equal numbers of α and β
particles are projected per second, the total energy radiated in the
form of α rays is about 83 times the amount in the form of β rays.
Evidence will be given later (section 253) to show that the number of α
particles projected is probably four times the number of β particles; so
that a still greater proportion of the energy is emitted in the form of
α rays. These results thus lead to the conclusion that, from the point
of view of the energy emitted, the α rays are far more important than
the β rays. This conclusion is supported by other evidence which is
discussed in chapters XII and XIII, where it will be shown that the α
rays play by far the most important part in the changes occurring in
radio-active bodies, and that the β rays only appear in the latter
stages of the radio-active processes. From data based on the relative
absorption and ionization of the β and γ rays in air, it can be shown
that the γ rays carry off about the same amount of energy as the β rays.
These conclusions are confirmed by direct measurement of the heating
effect of radium, which is discussed in detail in chapter XII.
Footnote 111:
In an examination of uranium the writer (_Phil. Mag._ p. 116, Jan.
1899) found that the rays from uranium consist of two kinds, differing
greatly in penetrating power, which were called the α and β rays.
Later, it was found that similar types of rays were emitted by thorium
and radium. On the discovery that very penetrating rays were given out
by uranium and thorium as well as by radium, the term γ was applied to
them by the writer. The word “ray” has been retained in this work,
although it is now settled that the α and β rays consist of particles
projected with great velocity. The term is thus used in the same sense
as by Newton, who applied it in the _Principia_ to the stream of
corpuscles which he believed to be responsible for the phenomenon of
light. In some recent papers, the α and β rays have been called the α
and β “emanations.” This nomenclature cannot fail to lead to
confusion, since the term “radio-active emanation” has already been
generally adopted in radio-activity as applying to the material
substance which gradually _diffuses_ from thorium and radium
compounds, and itself emits rays.
Footnote 112:
This method of illustration is due to Mme Curie, _Thèse présentée à la
Faculté des Sciences de Paris_, 1903.
Footnote 113:
Giesel, _Annal. d. Phys._ 69, p. 834, 1899.
Footnote 114:
Meyer and Schweidler, _Phys. Zeit._ 1, pp. 90, 113, 1899.
Footnote 115:
Becquerel, _C. R._ 129, pp. 997, 1205, 1899.
Footnote 116:
Curie, _C. R._ 130, p. 73, 1900.
Footnote 117:
Rutherford, _Phil. Mag._ January, 1899.
Footnote 118:
Rutherford and Grier, _Phil. Mag._ September, 1902.
Footnote 119:
Becquerel, _C. R._ 130, pp. 206, 372, 810, 979. 1900.
Footnote 120:
M. and Mme Curie, _C. R._ 130, p. 647, 1900.
Footnote 121:
The activity of the radium preparation was not stated in the paper.
Footnote 122:
Dorn, _Phys. Zeit._ 4, No. 18, p. 507, 1903.
Footnote 123:
Strutt, _Phil. Mag._ Nov. 1903.
Footnote 124:
Wien, _Phys. Zeit._ 4, No. 23, p. 624, 1903.
Footnote 125:
Dorn, _C. R._ 130, p. 1129, 1900.
Footnote 126:
Becquerel, _C. R._ 130, p. 809, 1900.
Footnote 127:
Kaufmann, _Phys. Zeit._ 4, No. 1 b, p. 54, 1902.
Footnote 128:
Abraham, _Phys. Zeit._ 4, No. 1 b, p. 57, 1902.
Footnote 129:
Kaufmann, _Nachrichten d. Ges. d. Wiss. zu Gött._, Nov. 8, 1901.
Footnote 130:
Simon, _Annal. d. Phys._ p. 589, 1899.
Footnote 131:
Kaufmann, _Phys. Zeit._ 4, No. 1 b, p. 54, 1902.
Footnote 132:
Paschen, _Annal. d. Phys._ 14, p. 389, 1904.
Footnote 133:
Meyer and Schweidler, _Phys. Zeit._ pp. 90, 113, 209, 1900.
Footnote 134:
Lenard, _Annal. d. Phys._ 56, p. 275, 1895.
Footnote 135:
Strutt, _Nature_, p. 539, 1900.
Footnote 136:
Seitz, _Phys. Zeit._ 5, No. 14, p. 395, 1904.
Footnote 137:
It is presumed that the results were corrected, if necessary, for the
discharging action due to the ionized gas, although no direct mention
of this is made in the paper by Seitz.
Footnote 138:
Strutt, _Phil. Trans._ A, p. 507, 1901.
Footnote 139:
Crookes, _Proc. Roy. Soc._ 1902. _Chem. News_, 85, p. 109, 1902.
Footnote 140:
Mme Curie, _C. R._ 130, p. 76, 1900.
Footnote 141:
Rutherford, _Phil. Mag._ Feb. 1903. _Phys. Zeit._ 4, p. 235, 1902.
Footnote 142:
Becquerel, _C. R._ 136, p. 199, 1903.
Footnote 143:
Becquerel, _C. R._ 136, p. 431, 1903.
Footnote 144:
Des Coudres, _Phys. Zeit._ 4, No. 17, p. 483, 1903.
Footnote 145:
Becquerel, _C. R._ 136, p. 1517, 1903.
Footnote 146:
Bragg, _Phil. Mag._ Dec. 1904; Bragg and Kleeman, _Phil. Mag._ Dec.
1904.
Footnote 147:
Further experimental results bearing on this important question are
given in an Appendix to this book.
Footnote 148:
Bakerian Lecture, _Phil. Trans._ A, p. 169, 1904.
Footnote 149:
Strutt, _Phil. Mag._ Aug. 1904.
Footnote 150:
J. J. Thomson, _Proc. Camb. Phil._ Soc. 13, Pt. I. p. 39, 1905.
_Nature_, Dec. 15, 1904.
Footnote 151:
Rutherford, _Nature_, March 2, 1905. J. J. Thomson, _Nature_, March 9,
1905.
Footnote 152:
Crookes, _Proc. Roy. Soc._ 81, p. 405, 1903.
Footnote 153:
Elster and Geitel, _Phys. Zeit._ No. 15, p. 437, 1903.
Footnote 154:
Glew, _Arch. Röntgen Ray_, June 1904.
Footnote 155:
Becquerel, _C. R._ 137, Oct. 27, 1903.
Footnote 156:
Tommasina, _C. R._ 137, Nov. 9, 1903.
Footnote 157:
An interesting side-light is thrown on this question by the
experiments described in Appendix A of this book.
Footnote 158:
Rutherford and Miss Brooks, _Phil. Mag._ July 1902.
Footnote 159:
In order to obtain a thin layer, the compound to be tested is ground
to a fine powder and then sifted through a fine gauge uniformly over
the area, so that the plate is only partially covered.
Footnote 160:
Rutherford, _Phil. Mag._ Jan. 1899.
Footnote 161:
Owens, _Phil. Mag._ Oct. 1899.
Footnote 162:
Rutherford and Miss Brooks, _Phil. Mag._ July, 1900.
Footnote 163:
Since the ionization at any point above the plate is the resultant
effect of the α particles coming from all points of the large
radio-active layer, λ is not the same as the coefficient of absorption
of the rays from a point source. It will however be proportional to
it. For this reason λ is called the “absorption constant.”
Footnote 164:
Townsend, _Phil. Mag._ Feb. 1901.
Footnote 165:
Durack, _Phil. Mag._ July 1902, May 1903.
Footnote 166:
Bragg and Bragg and Kleeman, _Phil. Mag._ Dec. 1904.
Footnote 167:
Villard, _C. R._ 130, pp. 1010, 1178, 1900.
Footnote 168:
Becquerel, _C. R._ 130, p. 1154, 1900.
Footnote 169:
Rutherford, _Phys. Zeit._ 3, p. 517, 1902.
Footnote 170:
McClelland, _Phil. Mag._ July 1904.
Footnote 171:
Paschen, _Phys. Zeit._ 5, No. 18, p. 563, 1904.
Footnote 172:
A. S. Eve, _Phil. Mag._ Nov. 1904.
Footnote 173:
Paschen, _Annal. d. Physik_, 14, p. 114, 1904; 14, 2, p. 389, 1904.
_Phys. Zeit._ 5, No. 18, p. 563, 1904.
Footnote 174:
Paschen, _Phys. Zeit._ 5, No. 18, p. 563, 1904.
Footnote 175:
Rutherford and Barnes, _Phil. Mag._ May 1905. _Nature_, p. 151, Dec.
15, 1904.
Footnote 176:
Barkla, _Nature_, March 17, 1904.
Footnote 177:
Becquerel, _C.R._ 132, pp. 371, 734, 1286. 1901.
Footnote 178:
Mme Curie, _Thèse présentée à la Faculté des Sciences_, Paris 1903, p.
85.
Footnote 179:
A. S. Eve, _Phil. Mag._ Dec. 1904.
Footnote 180:
In a recent paper (_Phil. Mag._ Feb. 1905), McClelland has, in the
main, confirmed the experimental results obtained by Eve. An
electrometer was used instead of an electroscope. He finds, in
addition, that the amount of secondary radiation depends on the angle
of incidence of the primary rays, and is greatest for an angle of 45°.
In a letter to _Nature_ (Feb. 23, p. 390, 1905), he states that more
recent experiments have shown that the amount of secondary radiation
from different substances is a function of their atomic weights rather
than of their densities. In every case examined, the amount of
secondary radiation increases with the atomic weight, but is not
proportional to it.
Footnote 181:
Rutherford and McClung, _Phil. Trans._ A. p. 25, 1901.
Footnote 182:
Meyer and Schweidler, _Wien Ber._ 113, July, 1904.
Footnote 183:
Rutherford and Grier, _Phil. Mag._ Sept. 1902.
CHAPTER V.
PROPERTIES OF THE RADIATIONS.
=115.= Besides their power of acting on a photographic plate, and of
ionizing gases, the radiations from active bodies are able to produce
marked chemical and physical actions in various substances. Most of
these effects are due either to the α or β rays. The γ rays produce
little effect in comparison. Since the β rays are similar in all
respects to high velocity cathode rays, it is to be expected that they
will produce effects similar in character to those produced by the
cathode rays in a vacuum tube.
Phosphorescent action.
Becquerel[184] has studied the action of radium rays in producing
phosphorescence in various bodies. The substance to be tested was placed
above the radium in the form of powder on a very thin mica plate.
Examination was made of the sulphides of calcium and strontium, ruby,
diamond, varieties of spar, phosphorus and hexagonal blende. Substances
like the ruby and spar, which phosphoresce under luminous rays, did not
phosphoresce under the radium rays. On the other hand, those which were
made luminous by ultra-violet light were also luminous under the action
of radium rays. The radium rays show distinct differences from X rays.
For example, a diamond which was very luminous with radium rays was
unaffected by X rays. The double sulphate of uranium and potassium is
more luminous than hexagonal blende under X rays, but the reverse is
true for radium rays; under the influence of these rays, sulphide of
calcium gave a blue luminosity but was hardly affected by X rays.
The following table shows the relative phosphorescence excited in
various bodies.
Substance Without screen. Across screen
Intensity of black paper
Hexagonal blende 13·36 ·04
Platino-cyanide of 1·99 ·05
barium
Diamond 1·14 ·01
Double sulphate of 1·00 ·31
Uranium and
Potassium
Calcium Fluoride ·30 ·02
In the last column the intensity without the screen is in each case
taken as unity. The great diminution of intensity after the rays have
passed through black paper shows that most of the phosphorescence
developed without the screen is, in the majority of cases, due to the α
rays.
Bary[185] has made a very complete examination of the class of
substances which become luminous under radium rays. He found that the
great majority of substances belong to the alkali metals and alkaline
earths. All these substances were also phosphorescent under the action
of X rays.
Crystalline zinc sulphide (Sidot’s blende) phosphoresces very brightly
under the influence of the rays from radium and other very active
substances. This was observed by Curie and Debierne in their study of
the radium emanation and the excited activity produced by it. It has
also been largely used by Giesel as an optical means of detecting the
presence of emanations from very active substances. It is an especially
sensitive means of detecting the presence of α rays, when it exhibits
the “scintillating” property already discussed in section 96. In order
to show the luminosity due to the α rays, the screen should be held
close to the active substance, as the rays are absorbed in their passage
through a few centimetres of air. Zinc sulphide is also luminous under
the action of the β rays, but the phosphorescence is far more persistent
than when produced by the α rays.
Very beautiful luminous effects are produced by large crystals of the
platinocyanides exposed to the radium rays. Those containing lithium
give a brilliant pink colour. The calcium and barium salts fluoresce
with a deep green light, and the sodium compound with a lemon yellow.
The mineral willemite (zinc silicate) was recently found by Kunz to be
an even more sensitive means of detecting the presence of the radiations
than platinocyanide of barium. It fluoresces showing a beautiful
greenish colour, and a piece of mineral exposed to the action of the
rays appears quite translucent. The crystals of the platinocyanides of
barium and lithium are especially suited for showing the action of the γ
rays, and, in this respect, are superior to willemite.
A very striking effect is shown by the mineral kunzite—a new variety of
spodumene discovered by Kunz[186]. This is a transparent gem like
crystal, often of very large size, which glows with a beautiful reddish
colour under the action of the β or γ rays, but does not appear to be
sensitive to the α rays. The luminosity extends throughout the crystal,
but is not so marked as in the platinocyanides or willemite. The mineral
sparteite[187], a form of calcite containing a few per cent. of
manganese, has been found by Ambrecht to fluoresce with a very deep
orange light under the β and γ rays. The colour appears to depend on the
intensity of the rays, and is deeper close to the radium than at some
distance away.
If kunzite and sparteite are exposed to the action of the cathode rays
in a vacuum tube, the colour is different from that produced by the
radium rays. The former appears a deep yellow, instead of the deep red
observed with the radium rays.
The different actions of the radium rays on these fluorescent substances
can be illustrated very simply and beautifully by the following
experiment. A small U tube is filled with fragments of the fluorescent
substance arranged in layers. The U tube is immersed in liquid air and
the emanation from about 30 mgrs. of radium bromide is condensed in the
tube. On closing the tube and removing it from the liquid air, the
emanation distributes itself uniformly in the tube. The shades of colour
produced in the different substances are clearly seen.
It is observed that all the crystals increase in luminosity for several
hours, on account of the excited activity produced by the emanation.
This effect is especially observed in kunzite, which at first hardly
responds to the rays, since the β and γ rays, which causes it to
fluoresce, are not given out by the emanation itself but by one of its
later products. The intensity of the β and γ rays is, in consequence,
small at first but rises to a maximum after several hours; the
luminosity observed varies in a corresponding manner.
Sir William Crookes[188] has made an examination of the effect of
continued exposure of a diamond to the radium rays. An “off-colour”
diamond, of a pale yellow colour, was placed inside a tube with radium
bromide. After 78 days’ exposure, the diamond had darkened and become
bluish green in tint; when heated at 50° in a mixture of potassium
chlorate for ten days, the diamond lost its dull surface colour and was
bright and transparent, and its tint had changed to a pale bluish green.
The rays have thus a double action on the diamond; the less penetrating
β rays produce a superficial darkening due to the change of the surface
into graphite, while the more penetrating β rays and the γ rays produce
a change of colour throughout its mass. The diamond phosphoresced
brightly during the whole course of its exposure to the rays. Crookes
also observed that the diamond still retained enough activity to affect
a photographic plate 35 days after removal, although, during the period
of 10 days, it was heated in a mixture sufficiently powerful to remove
the outer skin of graphite. This residual activity may possibly be due
to a slow transformation product of the emanation which is deposited on
the surface of bodies (see chapter XI).
Marckwald observed that the α rays from radio-tellurium produced marked
phosphorescence on some kinds of diamonds. An account of the various
luminous effects produced on different gems by exposure to the radium
and actinium rays has been given by Kunz and Baskerville[189].
Both zinc sulphide and platinocyanide of barium diminish in luminosity
after exposure for some time to the action of the rays. To regenerate a
screen of the latter, exposure to solar light is necessary. A similar
phenomenon has been observed by Villard for a screen exposed to Röntgen
rays. Giesel made a screen of platinocyanide of radio-active barium.
The screen, very luminous at first, gradually turned brown in colour,
and at the same time the crystals became dichroic. In this condition the
luminosity was much less, although the active substance had increased in
activity after preparation. Many of the substances which are luminous
under the rays from active substances lose this property to a large
extent at low temperatures[190].
=116. Luminosity of radium compounds.= All radium compounds are
spontaneously luminous. This luminosity is especially brilliant in the
dry haloid salts, and persists for long intervals of time. In damp air
the salts lose a large amount of their luminosity, but they recover it
on drying. With very active radium chloride, the Curies have observed
that the light changes in colour and intensity with time. The original
luminosity is recovered if the salt is dissolved and dried. Many
inactive preparations of radiferous barium are strongly luminous. The
writer has seen a preparation of impure radium bromide which gave out a
light sufficient to read by in a dark room. The luminosity of radium
persists over a wide range of temperature and is as bright at the
temperature of liquid air as at ordinary temperatures. A slight
luminosity is observed in a solution of radium, and if crystals are
being formed in the solution, they can be clearly distinguished in the
liquid by their greater luminosity.
=117. Spectrum of the phosphorescent light of radium and actinium.=
Compounds of radium, with a large admixture of barium, are usually
strongly self-luminous. This luminosity decreases with increasing
purity, and pure radium bromide is only very feebly self-luminous. A
spectroscopic examination of the slight phosphorescent light of pure
radium bromide has been made by Sir William and Lady Huggins[191]. On
viewing the light with a direct vision spectroscope, there were faint
indications of a variation of luminosity at different points along the
spectrum. In order to get a photograph of the spectrum within a
reasonable time, they made use of a quartz spectroscope of special
design which had been previously employed in a spectroscopic examination
of faint celestial objects. After three days’ exposure with a slit of
¹⁄₄₅₀ of an inch in width, a negative was obtained which showed a number
of bright lines. The magnified spectrum is shown in Fig. 46 A. The lines
of this spectrum were found to agree not only in position but also in
relative intensity with the band spectrum of nitrogen. The band spectrum
of nitrogen and also the spark spectrum[192] of radium are shown in the
same figure.
Some time afterwards Sir William Crookes and Prof. Dewar showed that
this spectrum of nitrogen was not obtained if the radium was contained
in a highly exhausted tube. Thus it appears that the spectrum is due to
the action of the radium rays either on occluded nitrogen or the
nitrogen in the atmosphere surrounding the radium.
It is very remarkable that a phosphorescent light, like that of radium
bromide, should show a bright line spectrum of nitrogen. It shows that
radium at ordinary temperatures is able to set up radiations which are
produced only by the electric discharge under special conditions.
Sir William and Lady Huggins were led to examine the spectrum of the
natural phosphorescent light of radium with the hope that some
indications might be obtained thereby of the processes occurring in the
radium atom. Since the main radiation from radium consists of positively
charged atoms projected with great velocity, radiations must be set up
both in the expelled body and in the system from which it escapes.
[Illustration: Fig. 46a.]
Giesel[193] observed that the spectrum of the phosphorescent light of
actinium consists of three bright lines. Measurements of the wave length
were made by Hartmann[194]. The luminosity was very slight and a long
exposure was required. The lines observed were in the red, blue and
green. The wave length λ and velocity are shown below.
Line Intensity λ
1 10 4885·4 ± 0·1
Ångström units
2 6 5300 ± 6 „
3 1 5909 ± 10 „
The line 4885 was very broad; the other two lines were so feeble that it
was difficult to determine their wave length with accuracy. Hartmann
suggests that these lines may be found in the spectrum of the new stars.
The lines observed have no connection with radium or its emanation[195].
=118. Thermo-luminescence.= E. Wiedemann and Schmidt[196] have shown
that certain bodies after exposure to the cathode rays or the
electric spark become luminous when they are heated to a temperature
much below that required to cause incandescence. This property of
thermo-luminescence is most strikingly exhibited in certain cases
where two salts, one of which is much in excess of the other, are
precipitated together. It is to be expected that such bodies would
also acquire the property when exposed to the β or cathodic rays of
radium. This has been found to be the case by Wiedemann[197].
Becquerel showed that fluor-spar, exposed to the radium rays, was
luminous when heated. The glass tubes in which radium is kept are
rapidly blackened. On heating the tube, a strong luminosity is
observed, and the coloration to a large extent disappears. The
peculiarity of many of these bodies lies in the fact that the
property of becoming luminous when heated is retained for a long
interval of time after the body is removed from the influence of the
exciting cause. It appears probable that the rays cause chemical
changes in these bodies, which are permanent until heat is applied.
A portion of the chemical energy is then released in the form of
visible light.
Physical actions.
=119. Some electric effects.= Radium rays have the same effect as
ultra-violet light and Röntgen rays in increasing the facility with
which a spark passes between electrodes. Elster and Geitel[198] showed
that if two electrodes were separated by a distance such that the spark
just refused to pass, on bringing near a specimen of radium the spark at
once passes. This effect is best shown with short sparks from a small
induction coil. The Curies have observed that radium completely
enveloped by a lead screen 1 cm. thick produces a similar action. The
effect in that case is due to the γ rays alone. This action of the rays
can be very simply illustrated by connecting two spark-gaps with the
induction coil in parallel. The spark-gap of one circuit is adjusted so
that the discharge just refuses to pass across it, but passes by the
other. When some radium is brought near the silent spark-gap, the spark
at once passes and ceases in the other[199].
Hemptinne[200] found that the electrodeless discharge in a vacuum tube
began at a higher pressure when a strong preparation of radium was
brought near the tube. In one experiment the discharge without the rays
began at 51 mms. but with the radium rays at 68 mms. The colour of the
discharge was also altered.
Himstedt[201] found that the resistance of selenium was diminished by
the action of radium rays in the same way as by ordinary light.
F. Henning[202] examined the electrical resistance of a barium chloride
solution containing radium of activity 1000, but could observe no
appreciable difference between it and a similar pure solution of barium
chloride. This experiment shows that the action of the rays from the
radium does not produce any appreciable change in the conductivity of
the barium solution.
Kohlrausch and Henning[203] have recently made a detailed examination of
the conductivity of pure radium bromide solutions, and have obtained
results very similar to those for the corresponding barium solutions.
Kohlrausch[204] found that the conductivity of water exposed to the
radiations from radium increased more rapidly than water which had not
been exposed. This increase of conductivity may have been due to an
increase of the conductivity of the water itself, or to an increased
rate of solution of the glass of the containing vessel.
Specimens of strongly active material have been employed to obtain the
potential at any point of the atmosphere. The ionization due to the
active substance is so intense that the body to which it is attached
rapidly takes up the potential of the air surrounding the active
substance. In this respect it is more convenient and rapid in its action
than the ordinary taper or water dropper, but on account of the
disturbance of the electric field by the strong ionization produced, it
is probably not so accurate a method as that of the water dropper.
=120. Effect on liquid and solid dielectrics.= P. Curie[205] made the
very important observation that liquid dielectrics became partial
conductors under the influence of radium rays. In these experiments the
radium, contained in a glass tube, was placed in an inner thin cylinder
of copper. This was surrounded by a concentric copper cylinder, and the
liquid to be examined filled the space between. A strong electric field
was applied, and the current through the liquid measured by means of an
electrometer.
The following numbers illustrate the results obtained:
Substance Conductivity in
megohms per 1 cm.³
Carbon bisulphide 20 × 10⁻¹⁴
Petroleum ether 15 „
Amyline 14 „
Carbon chloride 8 „
Benzene 4 „
Liquid air 1·3 „
Vaseline oil 1·6 „
Liquid air, vaseline oil, petroleum ether, amyline, are normally nearly
perfect insulators. The conductivity of amyline and petroleum ether due
to the rays at −17° C. was only ⅒ of its value at 0° C. There is thus
a marked action of temperature on the conductivity. For very active
material the current was proportional to the voltage. With material of
only ¹⁄₅₀₀ of the activity, it was found that Ohm’s law was not obeyed.
The following numbers were obtained:
Volts Current
50 109
100 185
200 255
400 335
For an increase of voltage of 8 times, the current only increases about
3 times. The current in the liquid thus tends to become “saturated” as
does the ordinary ionization current through a gas. These results have
an important bearing on the ionization theory, and show that the
radiation probably produces ions in the liquid as well as in the gas. It
was also found that X rays increased the conductivity to about the same
extent as the radium rays.
Becquerel[206] has recently shown that solid paraffin exposed to the β
and γ rays of radium acquires the property of conducting electricity to
a slight extent. After removal of the radium the conductivity diminishes
with time according to the same law as for an ionized gas. These results
show that a solid as well as a liquid and gaseous dielectric is ionized
under the influence of radium rays.
=121. Effect of temperature on the radiations.= Becquerel[207], by the
electric method, determined the activity of uranium at the temperature
of liquid air, and found that it did not differ more than 1 per cent.
from the activity at ordinary temperatures. In his experiments, the α
rays from the uranium were absorbed before reaching the testing vessel,
and the electric current measured was due to the β rays alone. P.
Curie[208] found that the luminosity of radium and its power of exciting
fluorescence in bodies were retained at the temperature of liquid air.
Observations by the electric method showed that the activity of radium
was unaltered at the temperature of liquid air. If a radium compound is
heated in an open vessel, it is found that the activity, measured by the
α rays, falls to about 25 per cent. of its original value. This is
however not due to a change in the radio-activity, but to the release of
the radio-active emanation, which is stored in the radium. No alteration
is observed if the radium is heated in a closed vessel from which none
of the radio-active products are able to escape.
=122. Motion of radium in an electric field.= Joly[209] found that a
disc, one side of which is coated with a few milligrams of radium
bromide, exhibits, when an electrified body is brought near it, motions
very different to those observed in the case of an inactive substance.
The electrified body, whether positive or negative, repels the suspended
body if brought up to it on the side coated with radium, but attracts it
if presented to the naked side.
This effect is very simply shown by constructing a small apparatus like
a radiometer. Two covered glasses are attached to the end of a glass
fibre about 6 cms. long, the surfaces lying in the same plane. The
apparatus is free to rotate on a pivot. The two vanes are coated on
alternate faces with radium bromide, and the whole apparatus contained
within a glass receiver. If an electrified rod of ebonite or sealing wax
is brought up close to the receiver, a rotation is communicated to the
vane which increases as the pressure of the air is lowered to 5 or 6
cms. of mercury. By placing the apparatus between parallel plates
connected with the terminals of a Wimshurst machine, a steady rotation
is communicated to the vanes. The rotation is always in such a direction
that the radium coated surface is repelled from the electrified body.
This action was examined still further by attaching the vanes to the
glass beam of a Coulomb’s balance. A metal sphere, which could be
charged from without, was fixed facing the side coated with radium. A
repulsion was always observed except when the charge was very strong and
the vane near the sphere. If, however, the two vanes were connected by a
light wire and a similar sphere placed exactly opposite the other, an
attraction was observed if one sphere was charged, but a repulsion if
both were charged. These effects were observed whether the vanes were of
aluminium or glass.
Joly found that the effect could not be explained by any direct action
due to the movement of the ions in an electric field. The recoil, due to
the expulsion of α particles from one side of the vane, is far too small
to account for the movement observed.
This effect can, I think, be simply accounted for by taking into
consideration the difference in conductivity of the gas on the two sides
of the radium coated vane. If a small vane, coated uniformly with radium
on both sides, and mounted on an insulating support, be brought near a
charged body kept at a constant potential, it acts like a water dropper
and rapidly acquires very nearly the average potential which existed at
that point before the vane was brought up. The mechanical force acting
on the vane will, in consequence, be small. If, however, the vane is
only coated with radium on the side near the charged body, the
ionization and consequently the conductivity of the gas is much greater
between the vane and the charged body than on the opposite side.
Suppose, for simplicity, the body is charged to a positive potential. On
account of the greater conductivity of the gas on the side facing the
charged body, it will rapidly acquire a positive charge, and the
potential of the vane will reach a higher value than existed at that
place before the vane was introduced. This will result in a repulsion of
the vane. This also accounts for the attraction observed in the
experiment with the Coulomb’s balance already referred to. Suppose that
one sphere is positively charged and the other earthed, and the two
vanes metallically connected together. The vane next to the charged body
will become charged positively, but this charge will be dissipated
rapidly on account of the ionization of the gas close to the opposite
vane, and, in most conditions, this loss of charge will be so rapid that
the potential of the vane is unable to reach the value which would exist
at that place in the field, if the vane were removed. There will, in
consequence, be an attracting force acting on the vane towards the
sphere.
The repulsion observed by Joly is thus only an indirect result of the
ionization in the gas produced by the radium, and should be shown under
conditions where similar unequal distribution of ionization is produced
by any other sources.
Since radium gives out heat at a fairly rapid rate, a radiometer in
which the vanes were coated on one side with radium instead of
lampblack, should rotate at low pressure of the gas, even if no source
of light is brought near it. This should evidently be the case, since
the face coated with radium should reach a slightly higher temperature
than the other. This experiment has been tried, but the effect seems too
small to produce rotation of the vanes.
Chemical actions.
=123.= Rays from active radium preparations change oxygen into
ozone[210]. Its presence can be detected by the smell or by the action
on iodide of potassium paper. This effect is due to the α and β rays
from the radium, and not to the luminous rays from it. Since energy is
required to produce ozone from oxygen, this must be derived from the
energy of the radiations.
The Curies found that radium compounds rapidly produced coloration in
glass. For moderately active material the colour is violet, for more
active material it is yellow. Long continued action blackens the glass,
although the glass may have no lead in its composition. This coloration
gradually extends through the glass, and is dependent to some extent on
the kind of glass used.
Giesel[211] found that he could obtain as much coloration in rock-salt
and fluor-spar by radium rays, as by exposure to the action of cathode
rays in a vacuum tube. The coloration, however, extended much deeper
than that produced by the cathode rays. This is to be expected, since
the radium rays have a higher velocity, and consequently greater
penetrating power, than the cathode rays produced in an ordinary vacuum
tube. Goldstein observed that the coloration is far more intense and
rapid when the salts are melted or heated to a red heat. Melted
potassium sulphate, under the action of a very active preparation of
radium, was rapidly coloured a strong greenish blue which gradually
changed into a dark green. Salomonsen and Dreyer[212] found that plates
of quartz were coloured by exposure to radium rays. When examined
minutely, plates cut perpendicular to the optic axis showed the presence
of lines and striae, parallel to the binary axes. Adjacent portions of
the striated system differed considerably in intensity of coloration and
clearly revealed the heterogeneity of structures of the crystal.
The cause of these colorations by cathode and radium rays has been the
subject of much discussion. Elster and Geitel[213] observed that a
specimen of potassium sulphate, coloured green by radium rays, showed a
strong photo-electric action, _i.e._ it rapidly lost a negative charge
of electricity when exposed to the action of ultra-violet light. All
substances coloured by cathode rays show a strong photo-electric action,
and, since the metals sodium and potassium themselves show
photo-electric action to a very remarkable degree, Elster and Geitel
have suggested that the colorations are caused by a solid solution of
the metal in the salt.
Although the coloration due to radium rays extends deeper than that due
to the cathode rays, when exposed to light the colour fades away at
about the same rate in the two cases.
Becquerel[214] found that white phosphorus is changed into the red
variety by the action of radium rays. This action was shown to be due
mainly to the β rays. The secondary radiation set up by the primary rays
also produced a marked effect. Radium rays, like ordinary light rays,
also caused a precipitate of calomel in the presence of oxalic acid.
Hardy and Miss Willcock[215] found that a solution of iodoform in
chloroform turned purple after exposure for 5 minutes to the rays from 5
milligrams of radium bromide. This action is due to the liberation of
iodine. By testing the effect of screens of different thicknesses, over
the radium, this action was found to be mainly due to the β rays from
the radium. Röntgen rays produce a similar coloration.
Hardy[216] also observed an action of the radium rays on the coagulation
of globulin. Two solutions of globulin from ox serum were used, one made
electro-positive by adding acetic acid, and the other electro-negative
by adding ammonia. When the globulin was exposed close to the radium in
naked drops, the opalescence of the electro-positive solution rapidly
diminished, showing that the solution became more complete. The
electro-negative solution was rapidly turned to a jelly and became
opaque. These actions were found to be due to the α rays of radium
alone.
This is further evidence in favour of the view that the α rays consist
of projected positively charged bodies of atomic dimensions, for a
similar coagulation effect is produced by the metallic ions of liquid
electrolytes, and has been shown by W. C. D. Whetham[217] to be due to
the electric charges carried by the ions.
=124. Gases evolved from radium.= Curie and Debierne[218] observed that
radium preparations placed in a vacuum tube continually lowered the
vacuum. The gas evolved was always accompanied by the emanation, but no
new lines were observed in its spectrum. Giesel[219] has observed a
similar evolution of gas from solutions of radium bromide. Giesel
forwarded some active material to Runge and Bödlander, in order that
they might test the gas spectroscopically. From 1 gram of a 5 per cent.
radium preparation they obtained 3·5 c.c. of gas in 16 days. This gas
was found, however, to be mainly hydrogen, with 12 per cent. of oxygen.
In later experiments Ramsay and Soddy[220] found that 50 milligrams of
radium bromide evolved gases at the rate of about 0·5 c.c. per day. This
is a rate of evolution about twice that observed by Runge and Bödlander.
On analysing the gases about 28·9 per cent. consisted of oxygen, and the
rest hydrogen. The slight excess of hydrogen over that attained in the
decomposition of water, they consider to be due to the action of oxygen
on the grease of the stop-cocks. The radio-active emanation from radium
has a strong oxidizing action and rapidly produces carbon dioxide, if
carbonaceous matter is present. The production of gas is probably due to
the action of the radiations in decomposing water. The amount of energy
required to produce the rate of decomposition observed by Ramsay and
Soddy—about 10 c.c. per day for 1 gram of radium bromide—corresponds to
about 30 gram-calories per day. This amount of energy is about two per
cent. of the total energy emitted in the form of heat.
Ramsay and Soddy (_loc. cit._) have also observed the presence of helium
in the gases evolved by solution of radium bromide. This important
result is considered in detail in section 267.
Physiological actions.
=125.= Walkhoff first observed that radium rays produce burns of much
the same character as those caused by Röntgen rays. Experiments in this
direction have been made by Giesel, Curie and Becquerel, and others,
with very similar results. There is at first a painful irritation, then
inflammation sets in, which lasts from 10 to 20 days. This effect is
produced by all preparations of radium, and appears to be due mainly to
the α and β rays.
Care has to be taken in handling radium on account of the painful
inflammation set up by the rays. If a finger is held for some minutes at
the base of a capsule containing a radium preparation, the skin becomes
inflamed for about 15 days and then peels off. The painful feeling does
not disappear for two months.
Danysz[221] found that this action is mainly confined to the skin, and
does not extend to the underlying tissue. Caterpillars subjected to the
action of the rays lost their power of motion in several days and
finally died.
Radium rays have been found beneficial in certain cases of cancer. The
effect is apparently similar to that produced by Röntgen rays, but the
use of radium possesses the great advantage that the radiating source
can be enclosed in a fine tube and introduced at the particular point at
which the action of the rays is required. The rays have also been found
to hinder or stop the development of microbes[222].
It would be out of place here to give an account of the numerous
experiments that have been made by physicists and physiologists on the
action of the rays of radium and of other radio-active substances on
different organisms, such as caterpillars, mice and guinea-pigs. In some
cases, the experiments have been carried out by placing the organisms in
an atmosphere impregnated with the radium emanation. The effect of an
exposure under such conditions for several days or weeks has been found
generally harmful and in many cases fatal. The literature in this new
department of study is already large and is increasing rapidly.
Another interesting action of the radium rays has been observed by
Giesel. On bringing up a radium preparation to the closed eye, in a dark
room, a sensation of diffuse light is observed. This effect has been
examined by Himstedt and Nagel[223] who have shown that it is due to a
fluorescence produced by the rays in the eye itself. The blind are able
to perceive this luminosity if the retina is intact, but not if the
retina is diseased. Hardy and Anderson[224] have examined this effect in
some detail. The sensation of light is produced both by the β and γ
rays. The eyelid practically absorbs all the β rays, so that the
luminosity observed with a closed eye is due to the γ rays alone. The
lens and retina of the eye are strongly phosphorescent under the action
of the β and γ rays. Hardy and Anderson consider that the luminosity
observed in a dark room with the open eye (the phosphorescent light of
the radium itself being stopped by black paper) is to a large extent due
to the phosphorescence set up in the eyeball. The γ rays, for the most
part, produce the sensation of light when they strike the retina.
Tommasina stated that the air exhaled by man contained a larger
proportion of ions than ordinary air, and, in consequence, caused an
increased rate of discharge of an electroscope. The experiment was
repeated by Elster and Geitel but with negative results. On the other
hand, they found that the breath of Dr Giesel, of Braunschweig, who had
been engaged continuously in the chemical separation of the radio-active
bodies, caused a rapid loss of charge of an electroscope. This increased
rate of discharge was probably mainly due to the radium emanation, with
which his system had become impregnated by inhaling the emanation-laden
air of the laboratory.
Footnote 184:
Becquerel, _C. R._ 129, p. 912, 1899.
Footnote 185:
Bary, _C. R._ 130, p. 776, 1900.
Footnote 186:
Kunz and Baskerville, _Amer. Journ. Science_ XVI. p. 335, 1903.
Footnote 187:
See _Nature_, p. 523, March 31, 1904.
Footnote 188:
Crookes, _Proc. Roy. Soc._ 74, p. 47, 1904.
Footnote 189:
Kunz and Baskerville, _Science_ XVIII, p. 769, Dec. 18, 1903.
Footnote 190:
Beilby in a recent communication to the Royal Society (Feb. 9 and 23,
1905) has examined in some detail the production of phosphorescence by
the β and γ rays of radium and has put forward a theory to account for
the different actions observed.
Footnote 191:
Huggins, _Proc. Roy. Soc._ 72, pp. 196 and 409, 1903.
Footnote 192:
The spark spectrum of the radium bromide showed the _H_ and _K_ lines
of calcium and also faintly some of the strong lines of barium. The
characteristic lines of radium of wave-lengths 3814·59, 3649·7, 4340·6
and 2708·6, as shown by Demarçay and others are clearly shown in the
figure. The strong line of wave-length about 2814 is due to radium.
Footnote 193:
Giesel, _Ber. d. D. Chem. Ges._ 37, p. 1696, 1904.
Footnote 194:
Hartmann, _Phys. Zeit._ 5, No. 18, p. 570, 1904.
Footnote 195:
In a recent paper, Giesel (_Ber. d. D. Chem. Ges._ No. 3, p. 775,
1905) has shown that the bright lines are due to didymium, which is
present as an impurity. Exposure of didymium to the radium rays also
causes the appearance of the lines.
Footnote 196:
Wiedemann and Schmidt, _Wied. Annal._ 59, p. 604, 1895.
Footnote 197:
Wiedemann, _Phys. Zeit._ 2, p. 269, 1901.
Footnote 198:
Elster and Geitel, _Annal. d. Phys._ 69, p. 673, 1899.
Footnote 199:
Willows and Peck (_Phil. Mag._ March, 1905) found that under some
conditions, especially for long sparks, the rays of radium hindered
the passage of the spark.
Footnote 200:
Hemptinne, _C. R._ 133, p. 934, 1901.
Footnote 201:
Himstedt, _Phys. Zeit._ p. 476, 1900.
Footnote 202:
Henning, _Annal. d. Phys._ p. 562, 1902.
Footnote 203:
Kohlrausch and Henning, _Verh. Deutsch. Phys. Ges._ 6, p. 144, 1904.
Footnote 204:
Kohlrausch, _Verh. Deutsch. Phys. Ges._ 5, p. 261, 1904.
Footnote 205:
P. Curie, _C. R._ 134, p. 420, 1902.
Footnote 206:
Becquerel, _C. R._ 136, p. 1173, 1903.
Footnote 207:
Becquerel, _C. R._ 133, p. 199, 1901.
Footnote 208:
P. Curie, Société de Physique, March 2, 1900.
Footnote 209:
Joly, _Phil. Mag._ March, 1904.
Footnote 210:
S. and P. Curie, _C. R._ 129, p. 823, 1899.
Footnote 211:
Giesel, _Verhandlg. d. D. Phys. Ges._ Jan. 5, 1900.
Footnote 212:
Salomonsen and Dreyer, _C. R._ 139, p. 533, 1904.
Footnote 213:
Elster and Geitel, _Phys. Zeit._ p. 113, No. 3, 1902.
Footnote 214:
Becquerel, _C. R._ 133, p. 709, 1901.
Footnote 215:
Hardy and Miss Willcock, _Proc. Roy. Soc._ 72, p. 200, 1903.
Footnote 216:
Hardy, _Proc. Physiolog. Soc._ May 16, 1903.
Footnote 217:
Whetham, _Phil. Mag._ Nov. 1899; _Theory of Solution_, Camb. 1902, p.
396.
Footnote 218:
Curie and Debierne, _C. R._ 132, p. 768, 1901.
Footnote 219:
Giesel, _Ber. D. d. Chem. Ges._ 35, p. 3605, 1902.
Footnote 220:
Ramsay and Soddy, _Proc. Roy. Soc._ 72, p. 204, 1903.
Footnote 221:
Danysz, _C. R._ 136, p. 461, 1903.
Footnote 222:
Aschkinass and Caspari, _Arch. d. Ges. Physiologie_, 86, p. 603, 1901.
Footnote 223:
Himstedt and Nagel, _Drude’s Annal._ 4, p. 537, 1901.
Footnote 224:
Hardy and Anderson, _Proc. Roy. Soc._ 72, p. 393, 1903.