Electron drift and diffusion in counting gases.

An experimental system has been designed with a twofold objective. The system, which has a variable drift length, has been used to measure, for a range of counting gases, drift velocities and longitudinal diffusion coefficients employing the time-of-flight method, and lateral diffusion coefficients using the Townsend method. Several problems have been encountered during the work, these requiring continued modification to be carried out on the system. A detailed description is given of the system and its stages of development. Attempts to measure longitudinal diffusion coefficients were not successful and possible reasons are fully discussed. A modified Boltzmann-Holstein equation has been employed with published cross-section data to calculate numerically the electron energy distribution, and hence electron transport coefficients, in the gas mixtures studied, at non-ionising field values. The Boltzmann-Holstein equation is discussed from first principles and the method of numerical solution is explained. Good agreement has been found between calculations and measurements of drift velocities for methane and a mixture of neon/10% methane and of D/ for Ne/10% CH4. Predictions for argon/methane and argon/carbon dioxide mixture are less accurate; possible reasons are discussed.