It follows from these results that the emanating power of Th X is directly proportional to its activity, i.e. that the rate of production of emanating particles is always proportional to the number of α particles, projected from the Th X per second. The radiation from Th X thus accompanies the change of the Th X into the emanation. Since the emanation has chemical properties distinct from those of the Th X, and also a distinctive rate of decay, it cannot be regarded as a vapour of Th X, but it is a distinct chemical substance, produced by the changes occurring in Th X. On the view advanced in [section 136], the atom of the emanation consists of the part of the atom of Th X left behind after the expulsion of one or more α particles. The atoms of the emanation are unstable, and in turn expel α particles. This projection of α particles constitutes the radiation from the emanation, which serves as a measure of the amount of emanation present. Since the activity of the emanation falls to half value in one minute while that of Th X falls to half value in four days, the emanation consists of atoms which disintegrate at intervals nearly 6000 times shorter than those of the atoms of Th X.
Source of the Radium and Actinium Emanation.
155. No intermediate stage—Radium X—between radium and its emanation, corresponding to the Th X for thorium, has so far been observed. The emanation from radium is probably produced directly from that element. In this respect, the radium emanation holds the same position in regard to radium as Th X does to thorium, and its production from radium can be explained on exactly similar lines. It will be shown later in chapter X, that the emanation of actinium, like that of thorium, does not arise directly from the parent element but from an intermediate product actinium X, which is very analogous in physical and chemical properties to Th X.
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.