Recent research on radioactivity

by Pierre Curie

We have seen that a salt of radium that has been freshly prepared possesses an activity which increases with the time, and becomes about five times as great as the initial activity. It seems that the emanation emitted by the radium can escape only with difficulty from the solid salt, and that it accumulates there and is transformed at once into induced radiation. An equilibrium is established when the spontaneous loss becomes sufficient to compensate the production.

When solid salt of radium is heated to redness, all the emanation which it had accumulated escapes. When the salt returns to the temperature of the room it emits Becquerel rays to a much less extent. However, the radiation recovers little by little its original value, which is reached after one or two months. The salt which had been heated to redness no longer possesses the power of emitting the emanation, but this property may be restored to it by dissolving it and drying it at a slightly elevated temperature.

Diffusion of the Emanation from Radium.—Danne and I have studied the law of the diffusion of the emanation of radium. A large glass reservoir filled with excited air communicates with the atmosphere by means of a capillary tube. The Becquerel radiation emitted by the walls of the reservoir is measured as a function of the time, and from this is deduced the law of the escape of the emanation through the capillary tube. It is found that the rapidity of the escape of the emanation is proportional to the quantity of it in the reservoir. It varies proportionally to the cross section of the capillary tube, and inversely as its length. These laws are the same as for a gas mixed with air under the same conditions. The coefficient of the diffusion of the emanation into air is equal to 0.100 at 10°. The coefficient is, therefore, of the same order of magnitude as that of the diffusion of carbon dioxide into the air, which is equal to 0.15 at the same temperature.

Radioactivity Induced by Thorium and the Emanation of Thorium.—Thorium emits an emanation and gives rise to the phenomena of induced radioactivity. These properties have been made the subject of numerous researches by Rutherford. The action of thorium is, otherwise, considerably less intense than that of radium.

The emanation of thorium disappears spontaneously according to a simple exponential law, but its disappearance is much more rapid than that of the emanation of radium. The quantity of the emanation from thorium diminishes by one-half in about one minute and ten seconds, while in the case of radium the quantity falls to one half in four days. This considerable difference causes a profound difference in the aspect of the phenomena.

In a closed space of not too great dimensions the emanation from radium spreads almost uniformly into all parts. But under the same conditions the emanation of thorium is found to accumulate in the vicinity of the thorium, because it disappears spontaneously before it has time to diffuse any considerable distance into the air.

The radiant activity of a substance can be measured by placing it upon the lower plate of a condenser formed of two horizontal plates, and measuring the conductivity communicated to the air between the plates. If this measurement be made with oxide of thorium, it is found that the conductivity of the air is greatly decreased when a current of air is sent between the plates. The oxide of thorium emits, indeed, an emanation that accumulates upon the substance and by its radiation helps to ionize the air between the plates. A current of air carries away the emanation as rapidly as it is set free, and the only thing left to cause ionization is the Becquerel radiation coming directly from the thorium.

If the same experiment be repeated with a salt of radium it is found that the current of air produces only a feeble effect. With uranium and polonium, which do not emit the emanation, the current of air has no effect. On the contrary, in the case of actinium, the action of the current of air is to suppress four-fifths of the conductivity of the air. It may be concluded that for thorium, and especially for actinium, the radiation of the emanation is very important in comparison with the radiation of the radioactive substance itself.

When one wishes to excite a solid to saturation with the emanation from thorium, it is necessary to cause the emanation to act for a long time, and hence new supplies of it must be brought continually to the surface of the body that is to be excited. To do this a current of air is passed through a solution of a salt of thorium and, then this current, of air charged with the emanation, is passed over the body. The solid, when excited by the emanation from thorium, loses its activity spontaneously according to an exponential law. The radiation falls to one-half every eleven hours. Hence, contrary to what takes place with the emanations, the activity induced by thorium upon solid substances disappears more slowly than that induced by radium.

Radioactivity Induced by Actinium and the Emanation from Actinium.—Actinium emits an emanation which gives a very intense radiation. This emanation disappears spontaneously with extreme rapidity, and diminishes by one-half in about one second. In air at the atmospheric pressure the emanation emitted by actinium cannot propagate itself to a greater distance than 7 or 8 mm. from the active substance, and excites bodies only when they are placed very near the source. On the contrary, in a vacuum the diffusion is rapid, and a body placed 10 cm. away from the actinium can be excited. The radioactivity induced by the actinium upon solids disappears according to an exponential law. It diminishes to one-half in about thirty-six minutes.