Radiations from the Emanations.
156. Special methods are necessary to examine the nature of the radiation from the emanations, for the radiations arise from the volume of the gas in which the emanations are distributed. Some experiments to examine the radiations from the thorium emanation were made by the writer in the following way.
Fig. 55.
A highly emanating thorium compound wrapped in paper was placed inside a lead box B about 1 cm. deep, shown in Fig. 55. An opening was cut in the top of the box, over which a very thin sheet of mica was waxed. The emanation rapidly diffused through the paper into the vessel, and after ten minutes reached a state of radio-active equilibrium. The penetrating power of the radiation from the emanation which passed through the thin mica window was examined by the electrical method in the usual way by adding screens of thin aluminium foil. The results are expressed in the following table:
Thickness of mica window ·0015 cm.
Thickness of aluminium foil ·00034 cm.
| Layers of foil | Current |
| 0 | 100 |
| 1 | 59 |
| 2 | 30 |
| 3 | 10 |
| 4 | 3·2 |
The greater proportion of the conductivity is thus due to α rays, as in the case of the radio-active elements. The amount of absorption of these α rays by aluminium foil is about the same as that of the rays from the active bodies. No direct comparison can be made, for the α rays from the emanation show the characteristic property of increased rate of absorption with thickness of matter traversed. Before testing, the rays have been largely absorbed by the mica window, and the penetrating power has consequently decreased.
No alteration in the radiation from the emanation was observed on placing an insulated wire inside the emanation vessel, and charging it to a high positive or negative potential. When a stream of air through the vessel carried away the emanation as fast as it was produced, the intensity of the radiation fell to a small fraction of its former value.
No evidence of any β rays in the radiations was found in these experiments, although a very small effect would have been detected. After standing some hours, however, β rays began to appear. These were due to the excited activity deposited on the walls of the vessel from the emanation, and not directly to the emanation itself.
The radium emanation, like that of thorium, only gives rise to α rays. This was tested in the following way[[249]]:
A large amount of emanation was introduced into a cylinder made of sheet copper ·005 cm. thick, which absorbed all the α rays but allowed the β and γ rays, if present, to pass through with but little loss. The external radiation from the cylinder was determined at intervals, commencing about two minutes after the introduction of the emanation. The amount observed at first was extremely small, but increased rapidly and practically reached a maximum in three or four hours. Thus the radium emanation only gives out α rays, the β rays appearing as the excited activity is produced on the walls of the vessel. On sweeping out the emanation by a current of air, there was no immediately appreciable decrease of the radiation. This is another proof that the emanation does not emit any β rays. In a similar way it can be shown that the emanation does not give out γ rays; these rays always make their appearance at the same time as the β rays.
The method of examination of the radiations from the emanations has been given in some detail, as the results are of considerable importance in the discussion, which will be given later in chapters [X] and [XI], of the connection between the changes occurring in radio-active products and the radiations they emit. There is no doubt that the emanations, apart from the excited activity to which they give rise, only give out α rays, consisting most probably of positively charged bodies projected with great velocity.