Polonium.
Uranium.
The same order is observed for all the absorbing substances examined, viz., aluminium, Dutch metal, tinfoil, paper, and air and other gases. The differences in the absorption of the α rays from the active bodies are thus considerable, and must be ascribed either to a difference of mass or of velocity of the α particles or to a variation in both these quantities.
Since the α rays differ either in mass or velocity, it follows that they cannot be ascribed to any single radio-active impurity common to all radio-active bodies.
100. Absorption of the α rays by gases. The α rays from the different radio-active substances are quickly absorbed in their passage through a few centimetres of air at atmospheric pressure and temperature. In consequence of this, the ionization of the air, due to the α rays, is greatest near the surface of the radiating body and falls off very rapidly with the distance (see [section 98]).
Fig. 36.
A simple method of determining the absorption in gases is shown in [Fig. 36]. The maximum current is measured between two parallel plates A and B kept at a fixed distance of 2 cms. apart, and then moved by means of a screw to different distances from the radio-active surface. The radiation from this active surface passed through a circular opening in the plate A, covered with thin aluminium foil, and was stopped by the upper plate. For observations on other gases besides air, and for examining the effect at different pressures, the apparatus is enclosed in an air-tight cylinder.
If the radius of the active surface is large compared with the distance of the plate A from it, the intensity of the radiation is approximately uniform over the opening in the plate A, and falls off with the distance x traversed according to an exponential law. Thus
