The total ionisation due to the absorption in air of slow cathode rays

Abstract

The object of the experiments here described is to measure the average ionisation produced by the absorption in air of an electron with definite initial energy. From this the average expenditure of energy associated with the formation of a pair of ions can be estimated. The initial energies considered ranged between 200 and 1000 electron-volts. Experiments on ionisation by electronic impact have generally been concerned, either with a determination of the ionisation potential of the gas, or with the ionisation per unit path due to an electron having a definite energy. The ionisation potential has been measured by determining the minimum energy a stream of electrons must have in order to ionise, even occasionally, a normal atom. It represents the energy expended by the ionising electron if no kinetic energy be transferred to either of the ions formed. If at the impact an atomic electron were ejected with appreciable kinetic energy, the energy expended by the ionising electron would be correspondingly increased above the ionisation potential. Also electrons may dissipate their energy by processes other than ionisation, notably by excitation and by dissociation of diatomic molecules. For these reasons the average expenditure of electronic energy per pair of ions should exceed the ionisation potential. The excess of this average energy would indicate the extent to which processes other than ionisation contribute to the dissipation of the initial kinetic energy of the electrons. The purpose of the present experiments is to obtain further information on this phase of the ionisation problem.