RADIOACTIVITY
Geological application—Röntgen rays—Becquerel
rays—Mme. Curie and the discovery of Radium—The
gold-leaf electroscope—The nature of α-, β- and
γ-rays—Helium and its production from radium—The
heat output of radium—Transformation of the radium
atom—Disintegration theory—The half-life period of
radium and its generation from uranium—The uranium,
thorium and actinium families—Direct measurement of
the heat output—Radioactivity independent of external
influences—Pleochroic haloes—Measurement of radium in
rocks and minerals.
In the limited space at our disposal, only a brief outline of the
salient facts of the new science of radioactivity can be given. It is
necessary to give some account of these recent discoveries because
of their immediate application to the study of the earth itself.
In a science like geology, the function of which is to study the
consequences of energy transformations in the earth’s crust during the
long ages of her gyrations through space, the recognition of sources of
energy previously undreamed of is a stimulant to research which must
profoundly affect the interpretations of the phenomena we observe. For
our present purpose it will be necessary to review only those facts
which have a bearing on the question of the earth’s age, and of its
thermal history.
In 1895 Röntgen gave his first account of _A New Kind of
Radiation_. This extraordinary radiation, now familiar to us as
the X, or Röntgen rays, revealed itself by its remarkable property of
passing through objects which are entirely opaque to ordinary light.
It was found, for example, that a photographic plate was not protected
from its influence by being wrapped in black paper, but was affected
just as though it had been exposed.
This discovery awakened the deepest interest amongst all scientific
workers, and the idea arose that phosphorescence and the Röntgen
rays might in some way be vitally connected. Certain substances,
after being exposed to sunlight, are able to shine in the dark with
a phosphorescent light, and it was this radiation which was examined
from the new point of view. Working on these lines, with uranium salts,
Becquerel found, almost by accident, that they gave out rays which
were capable of penetrating black paper and making themselves evident
by their effect upon a photographic plate wrapped within it. This
observation in 1896 marks the commencement of the harvest of wonderful
discoveries which have revolutionised our conception of the structure
of matter.
[Illustration: FIG. 12.
Taken in the dark by its own rays.]
[Illustration: FIG. 13.
Taken in ordinary daylight.
PHOTOGRAPHS OF PITCHBLENDE.]
The emission of rays was found to be an inherent property of uranium
and entirely independent of the phosphorescence exhibited by certain
of its compounds. Physical and chemical changes made no difference;
the radioactive properties evidently belonged to the atom itself, for
they could be detected under whatever conditions uranium happened to
be present. Mme. Curie at once began an exhaustive examination of all
the other known elements, and came to the conclusion that thorium alone
possessed radioactive properties similar to those of uranium. In the
course of her researches, she found that uranium-bearing minerals such
as pitchblende (uraninite) were far more active in their emission of
rays than could be accounted for by the uranium present. Systematically
following up so significant a fact, she traced the activity to the
bismuth and barium separated from these minerals. In 1898 two new
elements were announced. One, associated with bismuth, was called
Polonium; the other, found in minute quantities with the barium, was
appropriately named Radium.
The spontaneous radiations of these elements, collectively known as the
Becquerel rays, were soon distinguished from the Röntgen rays by their
greater complexity. Their analysis is largely due to the scientific
genius of Rutherford, who has classed them into three well-marked
types, the Alpha or α-, Beta or β-, and Gamma or γ-rays. The instrument
chiefly employed in their detection is the gold-leaf electroscope,
which not only is simple in construction, but is capable of extreme
sensitiveness. Under ordinary conditions the air is a good insulator,
and a charged electroscope loses its charge only very slowly. However,
if radium is brought near the instrument, the discharge proceeds much
more rapidly, the effect of the Becquerel rays being to ionise the
air and so render it conductive to electricity. In the electroscope
this effect can be readily observed and accurately measured, for the
leaf falls at a regular rate, which depends on the intensity of the
radiation. The sensitiveness of this method so far surpasses that of
the most delicately adjusted balance that by its means a quantity of
radium can be detected which would need to be multiplied thousands of
millions of times before the balance would turn to its weight.
It is easy—by interposing between the electroscope and the radium a
gradually increasing thickness of suitable material such as thin sheet
aluminium—to establish the existence of three kinds of rays which
differ greatly in their power of penetrating matter. The γ-rays are, on
the average, about a hundred times more penetrating than the β-rays,
and these in turn are equally the superior of the α-rays. In each case
the penetration in different materials depends approximately on their
density. The three types can also be distinguished by their behaviour
in a strong magnetic field. Giesel showed that the β-rays were
deflected in such a way as to indicate that they consisted of particles
carrying a negative charge of electricity. The α-rays were much more
difficult to deflect, but Rutherford successfully turned them from
their path by applying a very intense field, and from the direction
of their deflection he concluded that they consisted of positively
charged particles. The γ-rays have, however, resisted all attempts made
to alter their course. Their real nature is not yet beyond dispute,
and from the great similarity between their properties and those of
the Röntgen rays they are generally considered to be, like the latter,
either electro-magnetic pulses in the ether, or uncharged corpuscles of
a kind not yet understood.
When an electric discharge is passed through a high-vacuum tube, rays
are emitted from the cathode which, on reaching the walls of the
tube, produce a yellowish-green phosphorescence. The cathode rays
are identical with β-rays in every essential respect, and both are
proved to be tiny, negatively charged particles, called electrons. The
velocities of both cathode and β-particles are inconceivably high;
in the latter case even approaching that of light. Now Röntgen rays,
whatever may be their true nature, are set up by the sudden stoppage of
electrons when they impinge upon the anti-cathode of a vacuum tube. In
the same way it is supposed that the β-rays, by their sudden expulsion,
give rise at the same moment to γ-rays. It is a significant fact in
this connection that the β- and γ-rays are always associated together,
whereas the α-ray may be quite independent.
The story of the brilliant researches which culminated in the
measurement of the mass of an electron, and of the charge it carries,
is one of the most fascinating in the annals of science. But here we
must be content with the bare statement that the electron has only
¹/₁₇₀₀ of the mass of a hydrogen atom. The application of similar
methods to the α-particle has demonstrated that it is of atomic
dimensions, and consists either of a hydrogen atom with a single ionic
charge, or of a helium atom with twice that charge. Other evidence
decides conclusively in favour of the second alternative.
Of all the elements, helium has probably the most interesting history.
It was first discovered in the sun during the eclipse of 1868, but it
was not until 1895 that it was detected in terrestrial materials. In
that year Sir W. Ramsay identified it in uranium- and thorium-bearing
minerals by means of its highly characteristic spectrum. Even then,
before radioactivity had been recognised, the association of helium
with these elements had forced itself upon his notice. In 1902 the
genetic relationship between helium and certain of the radioactive
elements was predicted by Rutherford and Soddy. During the following
year Ramsay and Soddy working in conjunction sought for, and actually
observed, its generation from radium. Using a radium preparation from
which helium was at first entirely absent they were able to prove,
with the aid of the spectroscope, that in the course of a few months
sufficient of the gas had been generated to be identified. This
important discovery has received abundant confirmation during the last
decade, and to-day we even know the number of helium atoms which a gram
of radium emits in a given time. Moreover, the production of helium has
been demonstrated from uranium, thorium and actinium, and from most of
the other radioactive elements which are characterised by the emission
of α-rays. The rate of generation of helium will be considered in more
detail in Chapter X, where it finds an important application.
An observation of fundamental importance was made by Curie and Laborde
in 1903 when they discovered that radium is able to maintain a
temperature above that of its environment. The source of this emission
of heat lies in the kinetic energy of the Becquerel rays. Projected
from radium, they are sooner or later completely absorbed by the
matter through which they penetrate. Simultaneously, their energy is
given up to the molecules with which they collide, and it therefore
reappears as heat. By far the greater proportion—over 85%—of the
energy is carried by the relatively heavy α-particles which move with
velocities nearly one-tenth that of light. Knowing their masses and
velocities, and the number liberated per hour from a gram of radium, it
is a simple dynamical problem to calculate their total kinetic energy.
Expressing this quantity in heat units, it is found to be equivalent
to an hourly production of 113 calories. (Ap. A, p. 177.) As we shall
see later, this figure is in close agreement with the results of direct
measurements.
The most remarkable feature about these phenomena is that they appear
to continue unceasingly. Year after year the spontaneous production of
helium goes on, accompanied by a steady evolution of heat. But what
of the atoms that remain? They evidently cannot continue to be the
same element after having given up part of their energy and expelled
from themselves the material particles of the α- and β-rays. A close
examination reveals the genesis of a new element—a gaseous substance
known as radium emanation. Its atom represents the residual fraction
of the radium atom after the separation of a single α-particle. The
emanation is highly unstable; so rapidly does it give up energy and
liberate helium that its original intense activity decreases by half
every four days. In its place arises another product, radium A, of
still more transitory existence. A long succession of similar
transmutations has been traced, each accompanied by a setting free of
energy.
In 1902 Rutherford and Soddy advanced the theory of the disintegration
of the radioactive atoms. The radium atom is essentially unstable and
ultimately breaks up, explosively emitting α- and β-particles. Each
atom has a certain expectation of life which distinguishes it from the
atoms of other radioactive substances; but such is the distribution
of instability that a constant proportion of the total number of
atoms present is always breaking up. Out of a million atoms of radium
a definite number disintegrates each second, and the proportion has
been proved to be invariable in every case it has been possible to
test. From element to element it is this proportion, and therefore the
average promise of life, which varies. An equilibrium between formation
and transformation is gradually attained, and once it is established
a given quantity of radium produces as many atoms of emanation as the
emanation produces of radium A, and so on. When in equilibrium the
longer-lived radio-elements are present in greater quantity than those
of shorter lives in order to compensate for their comparatively slow
rate of decay, and so to enable them to keep pace with their more
rapidly changing associates.
We may now calculate the average life period of radium. The volume of
emanation in equilibrium with one gram of radium has been measured and
found to be O·58 cubic millimetres—less than the volume of a pin’s
head. The rate of change into radium A also lends itself to direct
measurement, and from these two quantities it can be shown (see Ap. A,
p. 179) that in one year ¹/₂₆₀₀ of the radium originally in existence
must undergo transformation. The same result can be arrived at more
directly. We know the number of atoms in a gram of radium and also the
number of helium atoms expelled in the course of a year. The proportion
which has suffered disruption follows at once, and is found to be
¹/₂₅₀₀.
From evidence of this kind it is concluded that after about 1850 years
only half of any given quantity of radium then maintains its identity
as that element. Now it is clear that radium must have a progenitor by
which it is formed as fast as it decays, or otherwise there could now,
after the lapse of millions of years, be none in existence.
That progenitor is undoubtedly uranium—an element which disintegrates
so slowly that its half-life period is three million times as long
as that of radium. This association is revealed by the study of
radioactive minerals. For every gram of uranium there exists in
equilibrium with it, 3·4 × 10⁻⁷ grams of radium. The constancy of
this proportion points to a genetic connection and admits of no other
interpretation. The generation of radium has been experimentally
verified by Soddy, who observed it after a number of years in a uranium
preparation originally quite free from it. In practice this work is
complicated, and the desired effects are retarded, by the intermediate
production of another radio-element, ionium, which has an average life
many times longer than that of radium. The time taken to establish
equilibrium is therefore very long. The primary minerals from which the
equilibrium ratio of radium to uranium is deduced, have existed
for hundreds of millions of years and so far as they have remained
unaltered by percolating waters they afford a safe guide. But in
secondary minerals, like autunite, the age of which is often to
be reckoned only in thousands of years, the equilibrium stage has
not always been reached, and the ratio between the two elements is
consequently somewhat lower than that found in the older minerals.
In the accompanying diagram the complete family of uranium is
summarised together with the half-life period of each member and the
rays it emits. Another series of elements related in the same way
is derived from thorium which is a parent of even longer life than
uranium. As to the final products of transformation, which, being
stable, ought to survive and in the course of time to accumulate,
helium is the only one which has been directly observed. That lead is
the end product of the uranium series is most probable, as we shall see
in a later chapter. Of the stable element to which the thorium series
ultimately leads we are still in ignorance. Neither lead nor bismuth,
which suggest themselves by their appropriate atomic weights, satisfy
the conditions, for their presence in a mineral bears no relation to
the quantity of thorium (see p. 190).
The actinium group of radio-elements calls for little notice here.
Boltwood finds a fixed ratio between actinium and uranium in minerals,
and the former would therefore appear to be a descendant of uranium
just as radium is. Actinium, however, does not fall in the main line of
descent. There is some probability that it marks the beginning of a
collateral series, also springing from uranium. The proportion of
uranium which disintegrates in this direction rather than towards
radium can only be very small.[12] It is possible that uranium atoms, or
those of one of its products, may be of two different types, according
to their internal structure, and that on disruption each may give rise
to an independent chain of elements.
[Footnote 12: About 8%.]
The heat output of radium and its relation to the energy of the
α-particle has already been mentioned. The first direct measurement
of the heat generated was made by Curie and Laborde. Using suitably
constructed calorimeters, they were able to demonstrate a regular
evolution of about 100 calories in an hour from a gram of radium—heat
enough to raise the temperature of an equal mass of water from freezing
to boiling-point. The radium in these experiments was in radioactive
equilibrium with the emanation and radium A, B and C, so that
altogether four α-particles were disengaged for each atom of radium
which transformed. The approximate correctness of this first estimate
will be seen from the following table, in which the best results up to
date are quoted:
1903. Curie and Laborde 100 }
1904. Rutherford and Barnes 110 } gram calories
1904. Runge and Precht 105 } emitted per
1908. v. Schweidler and Hess 118 } hour per
1909. Duane 120 } gram
1911. Duane 117 } of radium
1912. St. Meyer and Hess 132 }
The correspondence of these results with the theoretical figure, 113
calories, which represents about 85% of the energy,[13] is eminently
satisfactory (see Ap. A, p. 177).
[Footnote 13: Atomic recoil 6%, β-rays 4%, γ-rays 5%, approximately.]
The disintegration theory affords a simple explanation of the origin
of this energy. From the new point of view the atom is no longer
indivisible, no longer the ultimate foundation of all matter; rather is
it regarded as a tiny universe of electrons, a whirling assemblage of
charged particles. These minute entities move with very high velocities
within the atom itself. In general the energy is locked up, but in the
radioactive elements the atomic structure is unstable and from time
to time some of the energy can escape. The intrinsic energy of the
atom itself is the real source of the heat liberated by radium, and
the apparent permanence of its output is simply an expression of the
immense supplies of energy which an atom can store within itself.
The essential features of radioactivity are two: the spontaneous
emission of Becquerel rays, and the atomic character of the change. The
radiation from a radioactive substance is proportional to the quantity
of the substance and to nothing else. Temperature, being a function of
molecular movement, is without influence upon atomic transformation.
Experiments made at temperatures ranging between that of liquid air and
1600° C. show that the same law of disintegration holds throughout.
Similarly, the effect of very high pressures indicates that these
inter-atomic processes are quite independent of any external influence.
Whether in solution or in chemical combination, whether taking part
in energetic chemical reactions or suffering bombardment by its own
radiations, the atom continues to obey the law which determines its
life period, unmoved by any experiences through which we can oblige
it to pass. The radioactive properties are simply superimposed upon
the chemical and other properties of the substance, and as far as
laboratory conditions are concerned, they are entirely spontaneous and
can in no way be affected.
Whether or not radioactivity is a general property of atomic
matter cannot yet be announced. Potassium and rubidium are feebly
radioactive—a thousand times less than uranium—and emit β-rays. It
has been thought that some other metals may share in a minor degree the
same attributes, but up to the present no reliable results have been
forthcoming.
In its geological applications radioactivity possesses a great
interest. The most important bearing of all lies in the part it
plays as a source of terrestrial heat. Radium is widespread amongst
the surface materials of the earth’s crust. Rocks and soils, and
natural waters all contain radium as a constituent, widely diffused
but always present. In the following two chapters the influence of
radio-thermal action in the earth and sun, and on speculations as to
their ages, will be dealt with. Before passing on to these cosmic
aspects of the subject, a brief account may be given of the way in
which minute quantities of radium can be detected and accurately
measured—quantities which in rocks average little more than a
billionth of a gram in each gram of the rock.
Radium itself is not directly dealt with; it is the emanation in
equilibrium with it which is actually measured. The emanation is
separated from a suitable amount of rock, mixed with air, and
introduced into an electroscope. After about three hours, the emanation
has generated radium A, B and C in their equilibrium amounts, and the
emission of three sets of α-particles then steadily proceeds. In the
electroscope, the rate of fall of the changed leaf depends directly on
the conductivity of the air or on the number of ions which are utilised
as carriers of electricity. Now each α-particle, in virtue of its
enormous energy, is capable of producing in air about 100,000 ions, and
the total number present is therefore determined almost wholly by the
number of α-particles. The β- and γ-rays certainly have an ionising
effect, but so small that in this connection it may be ignored. To be
quite sure that all the ions are taking an active part in the discharge
and collapse of the leaf, it is only necessary to observe the latter
before its divergence from the central support falls to less than, say,
10°. The rate of collapse can easily be gauged by observing the leaf
through a microscope with a divided scale attached to the eyepiece,
the time during which the leaf passes over a given number of divisions
being taken with a stop watch. The rate of fall is proportional to
the conductivity, which depends in turn on the number of α-particles
expelled, and therefore on the quantity of emanation present in
the electroscope; this quantity, provided that it was initially in
equilibrium with the radium in the original rock, determines the amount
of that element which was present. A constant can be determined once
for all by experiment with known materials, which directly connects the
rate of fall in scale divisions per hour with the equivalent amount of
radium or uranium in grams. Some idea of the delicacy of the method may
be gained from a particular example. In the apparatus devised by Prof.
Strutt and used both in his own investigations and those of the present
writer, a leakage of one scale division per hour corresponds to 9 ×
10⁻¹³ grams of radium.
To use this apparatus it is necessary to bring the rock into solution
before the emanation can be extracted. The solution is stored up for a
few weeks until the equilibrium amount has been generated and the flask
A containing it is then attached to the water condenser B (Fig. 14).
The emanation can be expelled by vigorous boiling, and greatly diluted
with air, it passes out into the gas-holder C. The stream condenses in
B and drops back. At the end of an hour the cooling water is run out of
B and the steam then drives all the air charged with emanation into C.
In order that none should be allowed to escape back, the connection at
D is closed. Meanwhile the electroscope F has been exhausted and the
emanation is made to pass into it through the tap at E, the gas-holder
being slowly filled with water. After three hours the measurements may
then be made.
[Illustration: FIG. 14.
Apparatus for estimating Radium by its Emanation.]
Joly has recently varied this method in order to save the labour
attending the process of getting a rock into clear solution. The
mixture of finely-powdered rock and fusion mixture is heated in an
electric tube-furnace, and the expelled gases containing the emanation
are drawn straight from the furnace. Carbon-dioxide is absorbed by
soda-lime, and the remaining gas is collected and finally passed
into the electroscope as before. The absence or presence of thorium
makes no appreciable difference in these measurements, for the
life of its emanation is many thousands of times less than that of
radium emanation. Joly has utilised this fact in his solution-method
for determining minute quantities of thorium in rocks and minerals—a
method similar in principle to that described above for radium.
The presence of radioactive elements in rocks sometimes reveals itself
in a most beautiful way. In mica, cordierite, hornblende, chlorite,
tourmaline, and a few other minerals, small circular spots known as
pleochroic haloes are sometimes seen. A tiny crystal can generally be
detected in the centre, usually of zircon, but sometimes of apatite,
epidote, rutile, or sphene. Until quite lately the nature and origin of
these intensely pleochroic spots was entirely unknown. Joly showed in
1907 that they are due to the radioactivity of tiny inclusions around
which they spread spherically outwards. The α-rays discharged from
the minute central crystals are able to ionise the biotite (or other
mineral) and this effect, to which the colouring action is due, spreads
just as far as the α-particle can penetrate. The range of the different
α-particles from the uranium family is given overleaf, both for air and
for biotite.
-----------------------+-----------------------
| RANGE IN MILLIMETRES.
+---------+-------------
SOURCE OF Α-PARTICLE. | |
| IN AIR. | IN BIOTITE.
-----------------------+---------+-------------
Radium C | 70·6 | 0·033
Radium A | 48·3 | 0·023
Ra. Emanation | 42·3 | 0·020
Radium F | 38·6 | 0·018
Radium | 35·4 | 0·017
Ionium | 28·0 | 0·013
Uranium 1 and 2 | 27·0 | 0·013
-----------------------+---------+-------------
Now Bragg has shown that the ionising effect of an α-particle is
greatest just before it comes to rest. We should therefore expect to
find a number of intensely coloured spherical shells existing around
the central crystal and corresponding in each case to one or other of
the range limits here given. In section, and magnified 700 diameters,
the appearance of the shells would be as in the following diagram.
[Illustration: FIG. 15.
Pleochroic Haloes in Biotite due to Uranium and its transformation
products. Magnified 700 diameters.]
The dimensions of the actual haloes are exactly in agreement with
the distances to which the different α-particles can travel, and
occasionally the successive spheres of ionisation and colouring are
beautifully developed. In the biotite of Co. Carlow granite, Joly has
found several very perfect examples, and by his courtesy four of his
photographs are reproduced in the Frontispiece. Not only is the proof
that the haloes are due to α-rays conclusive beyond question, but
the correctness of Bragg’s laws from which the range in biotite is
calculated are established in an unexpected way. Thorium haloes are
also found (as in Fig. 2, upper left-hand part of the field), and these
again have diameters in perfect accord with the demands of theory.
A point of interest in the history of igneous rocks is that the haloes
disappear when the rock is heated. The presence of well-formed haloes,
therefore, implies the maintenance for very long periods of fairly
uniform conditions. So slowly do these haloes form that it may become
possible, when they have been further investigated, to give a rough
minimum estimate of the age of the minerals in which they occur. The
radioactivity of a zircon is much more intense than that of the rock
enclosing it, but the smallness of the quantities involved is such
that two or three weeks may elapse between the expulsion of successive
α-particles or helium atoms. By the accumulative effects of millions
of these atoms a spherical halo, faint in youth, but assuming a deeper
tint with age, is gradually produced.