It has been suggested by J. J. Thomson[[359]] that the rate of disintegration of radium may be influenced by its own radiations. This, at first sight, appears very probable, for a small mass of a pure radium compound is subjected to an intense bombardment by the radiations arising from it, and the radiations are of such a character that they might be expected to produce a breaking up of the atoms of matter which they traverse. If this be the case, the radio-activity of a given quantity of radium should be a function of its concentration, and should be greater in the solid state than when disseminated through a large mass of matter.
The writer has made an experiment to examine this question. Two glass tubes were taken, in one of which was placed a few milligrams of pure radium bromide in a state of radio-active equilibrium, and in the other a solution of barium chloride. The two tubes were connected near the top by a short cross tube, and the open ends sealed off. The activity of the radium in the solid state was tested immediately after its introduction by placing it in a definite position near an electroscope made of thin metal of the type shown in [Fig. 12]. The increased rate of discharge of the electroscope due to the β and γ rays from the radium was observed. When a lead plate 6 mms. in thickness was placed between the radium and the electroscope, the rate of discharge observed was due to the γ rays alone. By slightly tilting the apparatus, the barium solution flowed into the radium tube and dissolved the radium. The tube was well shaken, so as to distribute the radium uniformly throughout the solution. No appreciable change of the activity measured by the γ rays was observed over the period of one month. The activity measured by the β and γ rays was somewhat reduced, but this was not due to a decrease of the radio-activity, but to an increased absorption of the β rays in their passage through the solution. The volume of the solution was at least 1000 times greater than that of the solid radium bromide, and, in consequence, the radium was subjected to the action of a much weaker radiation. I think we may conclude from this experiment that the radiations emitted by radium have little if any influence in causing the disintegration of the radium atoms.
Voller[[360]] recently published some experiments which appeared to show that the life of radium varied enormously with its concentration. In his experiments, solutions of radium bromide of known strengths were evaporated down in a platinum vessel 1·2 sq. cms. in area, and their activity tested from time to time. The activity of the radium, so deposited, at first showed the normal rise to be expected on account of the production of the emanation, but after reaching a maximum, it rapidly decayed. For a weight of 10-6 mgrs. of radium bromide, the activity for example, practically disappeared in 26 days after reaching its maximum. The time taken for the activity to disappear increased rapidly with the amount of radium present. In another set of experiments, he states that the activity observed on the vessel was not proportional to the amount of radium present. For example, the activity only increased 24 times for a millionfold increase of the radium present, from 10-9 mgrs. to 10-3 mgrs.
These results, however, have not been confirmed by later experiments made by Eve. He found that, over the range examined, the activity was directly proportional to the amount of radium present, within the limits of experimental error. The following table illustrates the results obtained. The radium was evaporated down in platinum vessels 4·9 sq. cms. in area.
| Weight of radium in milligrams | Activity in arbitrary units |
|---|---|
| 10-4 | 1000 |
| 10-5 | 106 |
| 10-6 | 11·8 |
| 10-7 | 1·25 |
For an increase of one-thousandfold of the quantity of radium, the activity increased 800 times, while Voller states that the activity, in his experiments, only increased 3 to 4 times.
In the experiments of Eve, the activity was measured by observing the increased rate of discharge of a gold-leaf electroscope when the platinum vessel containing the active deposit was placed inside the electroscope. The activity of 10-8 mgrs. was too small to be measured with accuracy in the electroscope employed, while 10-3 mgrs. gave too rapid a rate of discharge. On the other hand, the method of measurement employed by Voller was unsuitable for the measurement of very weak radio-activity.
Eve also found that a small quantity of radium kept in a closed vessel did not lose its activity with time. A silvered glass vessel contained a gold-leaf system, such as is shown in [Fig. 12]. A solution containing 10-6 mgrs. of radium bromide was evaporated over the bottom of the vessel of area 76 sq. cms. The activity, after reaching a maximum, has remained constant over the 100 days during which observations have so far been made.
These experiments of Eve, as far as they go, show that the activity of radium is proportional to the amount of radium present, and that radium, kept in a closed vessel, shows no signs of decreasing in activity. On the other hand, I think there is no doubt that a very small quantity of radium deposited on a plate and left in the open air does lose its activity fairly rapidly. This loss of activity has nothing whatever to do with the shortness of life of the radium itself, but is due to the escape of the radium from the plate into the surrounding gas. Suppose, for example, that a solution containing 10-9 mgrs. of radium bromide is evaporated in a vessel of one sq. cm. in area. This amount of radium is far too small to form even a layer of molecular thickness. It seems likely that, during the process of evaporation, the radium would tend to collect in small masses and be deposited on the surface of the vessel. These would very readily be removed by slow currents of air and so escape from the plate. The disappearance of such minute amounts of radium is to be expected, and would probably occur with all kinds of matter present in such minute amount. Such an effect has nothing to do with an alteration of the life of radium and must not be confused with it.
The result that the total radiation from a given quantity of radium depends only on the quantity of radium and not on the degree of its concentration is of great importance, for it allows us to determine with accuracy the content of radium in minerals and in soils in which the radium exists in a very diffused state.