The Ionisation produced by an α-particle.—Part I

Abstract

Using an electrical method, Prof. Rutherford and myself were recently able to determine accurately the number N of α-partieles which are expelled from a gramme of radium per second. The final value of N obtained as an average of a great number of observations was 3·4 × 10 10 α-particles per second from a gramme of radium itself, or four times this number if the radium is in equilibrium with its three α-ray products. In another paper the charge carried by an α-particle was measured by the same authors and found to correspond to 9·3 × 10 -10 E. S. unit. Since recent experiments have given conclusive evidence that an α-particle is identical with an helium atom carrying twice the ionic charge, it was necessary to take the ionic charge as 4·65 x 10 -10 E. S. unit. The values of N and e as found from the above experiments enable us to determine the number of ions which are produced by an α-particle along its whole path with a greater accuracy than hitherto. A determination of the number of ions produced by an α-particle emitted from radium itself was made in 1905 by Rutherford in the following way. The ionisation current due to a thin film of radium was measured at its minimum activity, and the total number of α-particles fired off from this film was calculated from the total charge which the α-particles carried with them. Taking the charge on an α-particle as equal to twice the ionic charge e , the number Z of ions produced by an α-particle from radium itself was found to be 1·72 × 10 5 . This number becomes 1·8 × 10 5 if for N and e the latest values, referred to above, are introduced.