It follows from these experiments that the ionization per unit volume, due to a large plate uniformly covered with the radio-active matter, falls off rapidly with the distance from the plate. At a distance of 10 cms. the α rays from uranium, thorium, or radium have been completely absorbed in the gas, and the small ionization then observed in the gas is due to the more penetrating β and γ rays. The relative amount of the ionization observed at a distance from the source will increase with the thickness of the layer of active matter, but will reach a maximum for a layer of a certain thickness. The greater proportion of the ionization, due to unscreened active matter, is thus entirely confined to a shell of air surrounding it not more than 10 cms. in depth.
Fig. 35.
99. The α rays from different compounds of the same active element, although differing in amount, have about the same average penetrating power. Experiments on this point have been made by the writer[[160]] and by Owens[[161]]. Thus in comparing the relative power of penetration of the α rays from the different radio-elements, it is only necessary to determine the penetrating power for one compound of each of the radio-elements. Rutherford and Miss Brooks[[162]] have determined the amount of absorption of the α rays from the different active substances in their passage through successive layers of aluminium foil ·00034 cm. thick. The curves of absorption are given in [Fig. 35]. For the purpose of comparison in each case, the initial current with the bare active compound was taken as 100. A very thin layer of the active substance was used, and, in the case of thorium and radium, the emanations given off were removed by a slow current of air through the testing vessel. A potential difference of 300 volts was applied between the plates, which was sufficient to give the maximum current in each case.
Curves for the minerals organite and thorite were very nearly the same as for thoria.
For comparison, the absorption curves of the excited radiations of thorium and radium are given, as well as the curve for the radio-elements uranium, thorium, radium, and polonium. The α radiations may be arranged in the following order, as regards their power of penetration, beginning with the most penetrating.
Thorium}
Radium } excited radiation.
Thorium.
Radium.