The measurement of the ratio of diffusion coefficient to mobility for electrons at low E/p

An accurate method is given for the calculation of the ratio of radial diffusion coefficient to mobility (D_er/μ_e) for electron swarms in the absence of electron ionization. The technique required the measurement of the radial distribution of electron current across the anode, and the analysis is based upon a modified form of the drift and diffusion equation. Experimental values are given for electrons crossing uniform electric fields in hydrogen with 5<or=E/p₀<or=30 V cm⁻¹ Torr⁻¹ at 0°C. The analysis of the radial distributions by the Huxley equation is shown to give values of D_e/μ_e which are not constant across the anode. The variations, which are as large as ±5% of the mean value, are explained.