The ionisation produced by an α-particle. Part II.—Connection between ionisation and absorption

In the course of some experiments described in a previous paper, the ionisation produced by an α-particle along its path in air (Bragg curve) was determined with Radium C as source, and under conditions which were most favourable for saturation. The general shape of this curve is well known. The ionisation due to a parallel pencil of α-particles from Radium C (range in air, 7·06 cm.) increases up to a maximum value at 6·5 cm. During the next 5 mm. of its path the ionisation decreases rapidly to zero. The following experiments were undertaken to throw light on the law of absorption of the α-particle, and to establish, if possible, a connection between its velocity and ionising power.