Radio-Activity

by Ernest Rutherford

Experiments with large amounts of Radium Emanation.

147. With very active specimens of radium a large amount of emanation can be obtained, and the electrical, photographic, and fluorescent effects are correspondingly intense. On account of the small activity of thorium and the rapid decay of its emanation the effects due to it are weak, and can be studied only for a few minutes after its production. The emanation from radium, on the other hand, in consequence of the slow decay of its activity, may be stored mixed with air in an ordinary gas-holder, and its photographic and electrical actions may be examined several days or even weeks after, quite apart from those of the radium from which it was obtained.

It is, in general, difficult to study the radiation due to the emanation alone, on account of the fact that the emanation is continually producing a secondary type of activity on the surface of the vessel in which the emanation is enclosed. This excited activity reaches a maximum value several hours after the introduction of the emanation, and, as long as it is kept in the vessel, this excited activity on the walls decays at the same rate as the emanation itself, i.e. it falls to half its initial value in about 4 days. If, however, the emanation is blown out, the excited activity remains behind on the surface, but rapidly loses its activity in the course of a few hours. After several hours the intensity of the residual radiation is very small.

These effects and their connection with the emanation are discussed more fully in [chapter VIII]. Giesel[^[241]] has recorded some interesting observations of the effect of the radium emanation on a screen of phosphorescent zinc sulphide. When a few centigrams of moist radium bromide were placed on a screen any slight motion of the air caused the luminosity to move to and fro on the screen. The direction of phosphorescence could be altered at will by a slow current of air. The effect was still further increased by placing the active material in a tube and blowing the air through it towards the screen. A screen of barium platinocyanide or of Balmain’s paint failed to give any visible light under the same conditions. The luminosity was not altered by a magnetic field, but it was affected by an electric field. If the screen were charged the luminosity was more marked when it was negative than when it was positive.

Giesel states that the luminosity was not equally distributed, but was concentrated in a peculiar ring-shaped manner over the surface of the screen. The concentration of luminosity on the negative, rather than on the positive, electrode is probably due to the excited activity, caused by the emanation, and not to the emanation itself, for this excited activity is concentrated chiefly on the negative electrode in an electric field (see [chapter VIII]).

An experiment to illustrate the phosphorescence produced in some substances by the rays from a large amount of emanation is described in [section 165].

148. Curie and Debierne[^[242]] have investigated the emanation from radium, and the excited activity produced by it. Some experiments were made on the amount of emanation given off from radium under very low pressures. The tube containing the emanation was exhausted to a good vacuum by a mercury pump. It was observed that a gas was given off from the radium which produced excited activity on the glass walls. This gas was extremely active, and rapidly affected a photographic plate through the glass. It caused fluorescence on the surface of the glass and rapidly blackened it, and was still active after standing ten days. When spectroscopically examined, this gas did not show any new lines, but generally those of the spectra of carbonic acid, hydrogen, and mercury. In the light of the results described in [section 124] the gas, given off by the radium, was probably the non-active gases hydrogen and oxygen, in which the active emanation was mixed in minute quantity. It will be shown later ([section 242]) that the energy radiated from the emanation is enormous compared with the amount of matter involved, and that the effects observed, in most cases, are produced by an almost infinitesimal amount of the emanation.

In further experiments, Curie and Debierne[^[243]] found that many substances were phosphorescent under the action of the emanation and the excited activity produced by it. In their experiments, two glass bulbs A and B ([Fig. 54]) were connected with a glass tube. The active material was placed in the bulb A and the substance to be examined in the other.

Fig. 54.

They found that, in general, substances that were phosphorescent in ordinary light became luminous. The sulphide of zinc was especially brilliant and became as luminous as if exposed to a strong light. After sufficient time had elapsed the luminosity reached a constant value. The phosphorescence is partly due to the excited activity produced by the emanation on its surface, and partly to the direct radiation from the emanation.

Phosphorescence was also produced in glass. Thuringian glass showed the most marked effects. The luminosity of the glass was found to be about the same in the two bulbs, but was more marked in the connecting tube. The effect in the two bulbs was the same even if connected by a very narrow tube.

Some experiments were also made with a series of phosphorescent plates placed in the vessel at varying distances apart. With the plates 1 mm. apart the effect was very feeble, but increased directly as the distance and was large for a distance of 3 cms.

These effects receive a general explanation on the views already put forward. When the radium is placed in the closed vessel, the emanation is given off at a constant rate and gradually diffuses throughout the enclosure. Since the time taken for diffusion of the emanation through tubes of ordinary size is small compared with the time required for the activity to be appreciably reduced, the emanation, and also the excited activity due to it, will be nearly equally distributed throughout the vessel.

The luminosity due to it should thus be equal at each end of the tube. Even with a capillary tube connecting the two bulbs, the gas continuously given off by the radium will always carry the emanation with it and cause a practically uniform distribution.

The gradual increase of the amount of emanation throughout the tube will be given by the equation

where N_t is the number of emanation particles present at the time t, N₀ the number present when radio-active equilibrium is reached, and λ is the radio-active constant of the emanation. The phosphorescent action, which is due partly to the radiations from the emanation and partly to the excited activity on the walls, should thus reach half the maximum value in four days and should practically reach its limit after three weeks’ interval.

The variation of luminosity with different distances between the screens is to be expected. The amount of excited activity deposited on the boundaries is proportional to the amount of emanation present. Since the emanation is equally distributed, the amount of excited activity deposited on the screens, due to the emanation between them, varies directly as the distance, provided the distance between the screens is small compared with their dimensions. Such a result would also follow if the phosphorescence were due to the radiation from the emanation itself, provided that the pressure of the gas was low enough to prevent absorption of the radiation from the emanation in the gas itself between the screens.