Passage of slow β-particles through matter—production of branches

Abstract

Important information about the loss of energy by β-particles may be obtained by studying the secondary electrons with large energy which are occasionally produced and which appear in a Wilson cloud photograph as branches to the main β-particle tracks. In the present work the number of branches produced by β-particles with energy of about 20,000 volts in nitrogen and oxygen has been observed and also the energy relations which exist in branch collisions. For small angles of scattering of β-particles by electrons the formulæ derived on the new quantum theory and on the classical theory agree, but for large angles of scattering such as those concerned in the branch collisions investigated here there is an appreciable difference. This difference arises from the application in the case of the quantum theory calculations of Pauli’s exclusion principle to free electrons ( i. e ., aperiodic systems). The results of the present experiments support the quantum theory formula. Previous observations on the production of branches by slow β-particles have been made by C. T. R. Wilson. His observations on the frequency of branches do not apply to sufficiently large angles of scattering to test the difference between the classical and quantum theory formulæ and the results obtained are not very conclusive.