CHANGES IN RADIO-ACTIVE BODIES
Is Radio-activity a Permanent Property?
Is this power of emitting radiations a permanent property or is it
lost with the passage of time? The first investigations of the
activity of uranium and thorium showed no loss of intensity at the end
of several years, and radium also seemed to show no decrease in its
enormous activity. Polonium, however, was found to lose most of its
activity in a year, and later it appeared that some radio-active
substances lost most of their activity in the course of a few minutes
or hours.
Induced Activity
A phenomenon called induced or secondary radio-activity was also
observed. Thus a metal plate or wire exposed to the action of thorium
oxide for some hours became itself active. This induced activity was
not permanent but decreased to half its value in about eleven hours
and practically disappeared within a week. Similar phenomena were
observed when radium was substituted for thorium.
Discovery of Uranium X
In 1900 Crookes precipitated a solution of an active uranium salt with
ammonium carbonate. The precipitate was dissolved so far as possible
in an excess of the reagent, leaving an insoluble residue. This
residue was many hundred times more active, weight for weight, than
the original salt, and the solution containing the salt was
practically inactive. At the end of a year the uranium salt had
regained its activity while the residue had become inactive.
Another method of obtaining the same result is to dissolve
crystallized uranium nitrate in ether. Two layers of solution are
formed, one ether and the other water coming from the water of
crystallization. The aqueous layer is active, while the water layer is
inactive. Similarly, by adding barium chloride solution to a solution
of a salt of uranium and then precipitating the barium as sulphate,
the activity is transferred to this precipitate. These experiments
give proof of the formation and separation of a radio-active body by
ordinary chemical operations.
So, too, in the case of thorium salts a substance can be obtained by
means of ammonium hydroxide which is several thousand times more
active than an equal weight of the original salt. After standing a
month, the separated material has lost its activity and the thorium
salt has regained it. Here, again, there is the formation, separation,
and loss of a radio-active body.
Conclusions Drawn
Now, these are ordinary chemical processes for the separation of
distinct chemical individuals. The results, therefore, lead naturally
to the conclusions: (1) it would seem that uranium and thorium are
themselves inactive and the activity is due to some other substance
formed by these elements; (2) this active substance is produced by
some transformation in those elements, for on standing the activity
is regained. This latter conclusion is startling, for it indicates a
change in the atom which, up to the time of this discovery, was deemed
unchangeable under the influence of such physical and chemical changes
as were known to us.
Search for New Radio-active Bodies
The search for new radio-active bodies and the study of their
characteristics has been systematically and successfully carried on.
The bodies obtained in the above experiments were named uranium _X_
and thorium _X_, respectively. Further, it became clear from the
investigation of uranium minerals that radium, polonium, actinium, and
ionium originated from uranium. From thorium minerals a body was
separated called mesothorium, which was analogous to radium. Both
thorium and radium were found to give off a radio-active gas. The
first lost half of its activity in less than one minute. The second
was more stable and lost half of its activity in about four days. The
name radium emanation was given to the latter and it was found
chemically and physically to belong to the class of monatomic or noble
gases, such as helium, argon, neon, etc., which had been discovered by
Ramsay. In some cases the chemical action was determined and these new
bodies were found analogous to well-known elements, as radium to
barium, polonium to bismuth. The physical properties were investigated
and, where possible, spectra were mapped and atomic weights
determined.
It is clear, therefore, that these bodies are elemental in character
and as such are made up of distinct, similar atoms, just as the
commonly recognized elements are believed to be. In this way more than
thirty new elements have been added to the list. These new elements
are called radio-active elements, but it is an open question whether
all atoms do not possess this property in greater or less degree.
Certainly, it is possessed in varying degree by four of the old
elements widely separated in the Periodic System, namely, uranium,
thorium, rubidium, and potassium. The last two, while feebly active
themselves, do not form any secondary radio-active substance so far as
is known. Only two of the elements, then, can definitely be said to go
through these transformations. It is just possible that radio-activity
may be found to be a common property of all atoms and of all matter.
Methods of Investigation
It is important to know how these new bodies were discovered and
distinguished from one another. Two properties are relied upon. One is
the nature of the rays emitted and the other is the duration of the
activity. Of course, knowledge of the physical and chemical properties
is also of great importance whenever obtainable.
Nature of the Radiations
The nature of the radiation is a distinguishing characteristic, though
similarity here does not prove identity of substances. Some emit
[alpha] rays only, some emit [beta] rays, some emit two of the
possible rays, as for instance, [beta] and [gamma], and some emit all
three. The rays may also differ in the velocity with which they are
emitted by different radio-active substances. Thus, in the case of one
substance the [alpha] rays may have a slightly greater or less
penetrating power than those emitted by some other substance, and this
may be true also of the other rays.
Life Periods
The duration of the activity is called the life period. This is
absolutely fixed for each body and furnishes the most important mode
of differentiating among them. It measures the relative stability and
is the time which must elapse before their activity is lost and they,
changing into something else, entirely disappear. The measure usually
adopted is the half-value period. Two hypotheses are made use of:
1. That there is a constant production of fresh radio-active matter by
the radio-active body.
2. That the activity of the matter so formed decreases according to an
exponential law with the time from the moment of its formation.
These hypotheses agree with the experimental results. The decrease and
rise of activity, for example, of uranium and uranium _X_, and also of
thorium and thorium _X_, have been measured, plotted, and the
equations worked out.
Manifestly, a state of equilibrium will be reached when the rate of
loss of activity of the matter already produced is balanced by the
activity of the new matter produced. This equilibrium and the
knowledge of the rate of decrease in general will have little value if
this rate, like chemical changes, is subject to the influence of
chemical and physical conditions. The rate of decrease has been found
to be unaltered by any known chemical or physical agency. For
instance, neither the highest temperatures applicable nor the cold of
liquid air have any appreciable effect.
Equilibrium Series
In order to measure the disintegration of a radio-active body in units
of time so that the rate may be comparable with that of other
radio-active bodies, the relation between the amounts under
consideration must be a definite one. For this purpose equal weights
of the bodies are not taken, but use is made of the amounts which are
in equilibrium with a fixed amount of the parent substance.
One gram of radium has been settled upon as the standard for that
series and a unit known as the "curie" has been adopted to express the
equilibrium quantity of radium emanation. Thus, a curie of radium
emanation (or niton) is the weight (or, as this is a gas, the volume
at standard pressure and temperature) of the emanation in equilibrium
with one gram of radium. This, by calculation and experiment, is found
to be 0.63 cubic millimeter. When this amount has been produced by one
gram of radium, the formation and decay will exactly balance one
another. This is, therefore, one curie of emanation.
The measurement of the rate of decay is difficult but can be carried
out with great accuracy, even down to seconds, in the case of certain
short-lived bodies. Errors crept in at first from the failure to
completely separate the substances produced in the series, and
sometimes because of the simultaneous production of two substances.
As stated, the decay follows an exponential law. The time required for
the decay of activity to half-value does not mean, therefore, that
there will be total decay in twice that time. Thus the half-value
period for uranium _X_ is about 22 days. The period for complete decay
is about 160 days. This half-value period corresponds to the
half-value recovery period of uranium, which is also 22 days.
These were the earlier figures obtained for uranium _X_ and they
illustrate some of the difficulties surrounding such determinations.
It was found later that the body examined as uranium _X_ was really a
constant mixture and of course the decay and recovery periods were
also composite. It required later and very skilful work to separate
them into the bodies indicated in the disintegration series.
The half-value period for thorium _X_ is much shorter, namely, a
little over four days, and this is also the recovery period for
thorium _X_. The plotted decay and recovery curves will intersect at
this point.
The consecutive disintegration series, with the half-value periods,
for the uranium and thorium series as given by Soddy are seen in the
following tables. They are probably subject to some changes on further
and more accurate determination. The nature of the rays emitted is
also given.
[Illustration:
Uranium (8 x 10^9 years) 238.5 -> [alpha]
-> [alpha]
\/
Uranium X (35.5 days) (230.5) -> [beta]&[gamma]
-> ([beta])
\/
\/
\/
Ionium (5 x 10^4 to 10^6 years) (230.5) -> [alpha]
\/
Radium (2,500 years) 226.4 -> [alpha]
\/
Emanation (5.57 days) (222.4) -> [alpha]
\/
Radium A (4.3 minutes) (218.4) -> [alpha]
\/
Radium B (38.5 minutes) (214.4) -> ([beta])
\/
Radium C_{1} (28.1 minutes) { (214.4) -> [alpha]
{ -> [beta]&[gamma]
{ \/
Radium C_{2} (1.9 minutes) { (210.4) -> [beta]&[gamma]
\/
Radium D (24 years?) (210.4) -> ([beta])
\/
Radium E (7.25 days) (210.4) -> [beta]&[gamma]
\/
Radium F (Polonium 202 days) (210.4) -> [alpha]
\/
Radium G (probably lead) (206.8)
Actinium (?)
\/
Radio-Actinium (28.1 days) -> [alpha]
-> ([beta])
\/
Actinium X (15 days) -> [alpha]
\/
Emanation (5.6 seconds) -> [alpha]
\/
Actinium A (0.0029 second) -> [alpha]
\/
Actinium B (52.1 minutes) -> ([beta])
\/
Actinium C_{1} (3.10 mins.) { -> [alpha]
{ \/
Actinium C_{2} (?) { -> [alpha]
\/
Actinium D (7.4 minutes) -> [beta]&[gamma]
\/
Actinium E (unknown)
Thorium (4 x 10^{10} years?) 232.4 -> [alpha](?)
\/
Mesothorium_{1} (7.9 years)
\/
Mesothorium_{2} (8.9 hours) -> [beta]&[gamma]
\/
Radiothorium (2.91 years?) -> [alpha]
\/
Thorium X (5.35 days) -> [alpha]
\/
Emanation (76 seconds) -> [alpha]
\/
Thorium A (0.203 second) -> [alpha]
\/
Thorium B (15.3 hours) -> ([beta])
\/
Thorium C_{1} (79 minutes) { -> [alpha]
{ \/
Thorium C_{2} (?) { -> [alpha]
\/
Thorium D (4.5 minutes) -> [beta]&[gamma]
\/
Thorium E (unknown)
FIG. 6.--DISINTEGRATION SERIES FOR URANIUM, ACTINIUM, AND
THORIUM, AS GIVEN BY SODDY.]