Investigation of the passage of "fast" β - particles through gases

Abstract

1. The Wilson cloud method affords considerable scope for the investigation of the phenomena exhibited by electric particles traversing matter. its possibilities in this connection were abundantly shown by Wilson in his experiments, in which he applied the method to the investigation of such phenomena as ranges, primary and secondary ionisation, frequency of pro­duction of “branch” tracks, nuclear scattering, and the momentum relations in branch collisions. Since Wilson’s work the method has been used by several workers for the investigation of the passage of α-particles through matter, but has been little used for investigating the phenomena connected with the loss of energy by β-particles. In his experiments Wilson did not aim at a high degree of accuracy and this especially applies to the observations he made on “fast” β-particles. His observations were also confined to β-particles traversing air. In the present work the cloud method is applied to a further investigation of phenomena representing loss of energy by fast β-particles in oxygen, and methods are adopted to enable quantitative measurements to be made. The observations are also extended to hydrogen, which is of considerable importance for comparison wit theory. In the experiments a radioactive source of high-speed β-particles is used, β-particles with energy up to a million and a half volts being thus available. Though for several other methods such sources are too weak, they are of sufficient intensity for the purpose of investigation by the cloud method which deals with individual particles. The phenomena investigated here are the primary ionisation, the frequency of production of “branches” and the momentum relations in "branch” collisions. The primary ionisation gives information about the frequency of collisions in which the β-particle loses energy of the order of the ionisation potential for the outer electrons in the atoms traversed, whilst the frequency of branches refers to collisions in which the losses of energy are of the order of 5000 volts or greater.