A Brief Account of Radio-activity Part 2

3. By exposure to magnetic and electric fields, noting extent and direction of deflection.

4. By their relative absorption by solids and gases.

5. By the scintillations on a zinc sulphide screen.

Identification of the Rays

The alpha rays have been identified as similar to the so-called ca.n.a.l rays. These were first observed in the study of the _X_ rays. When an electrical discharge is pa.s.sed through a vacuum tube with a cathode having holes in it, luminous streams pa.s.s through the holes toward the side away from the anode and the general direction of the stream. They travel in straight lines and render certain substances phosph.o.r.escent.

These rays are slightly deflected by a magnetic field and in an opposite direction from that taken by the cathode rays in their deflection. The rays seem to be positive ions with ma.s.ses never less than that of the hydrogen atom. Their source is uncertain, but they may be derived from the electrodes.

The beta rays are identical in type with the cathode rays and are negative electrons.

The gamma rays are a.n.a.logous to the _X_ rays and are of the order of light. They are in general considerably more penetrating than _X_ rays. For example, the gamma rays sent out by 30 milligrams of radium can be detected by an electroscope after pa.s.sing through 30 centimeters of iron, a much greater thickness than can be penetrated by the ordinary _X_ rays.

CHAPTER III

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 pa.s.sage 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 subst.i.tuted 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 a.n.a.logous 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 cla.s.s of monatomic or n.o.ble 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 a.n.a.logous to well-known elements, as radium to barium, polonium to bis.m.u.th. 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 pota.s.sium. 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 distinguis.h.i.+ng characteristic, though similarity here does not prove ident.i.ty 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 quant.i.ty 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 ill.u.s.trate 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.

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