Electric field dependence of transient electron longitudinal and transverse diffusion coefficients in rare-gas moderators

Abstract

The transient behaviour of the longitudinal and transverse diffusion coefficients of electrons in rare-gas moderators is studied with solutions of the Boltzmann equation. The method of solution employed is the discrete ordinate method employed in an earlier study of the transient behaviour of the energy, transverse diffusion coefficient and mobility for both zero electric field and finite field conditions. The effects on the transient behaviour of the Ramsauer-Townsend minima in the momentum transfer cross sections for the heavier moderators is studied in detail. The transient behaviour of the longitudinal diffusion coefficient is found to be different from the behaviour of the transverse diffusion coefficient. The transient longitudinal diffusion coefficients in krypton and xenon exhibit extremum values for a particular narrow range of E/n. The range of E/n for which this effect occurs coincides with the E/n values for which the lowest non-zero eigenvalue of the Fokker-Planck operator attains a minimum value. The effect does not occur in argon.