The Momentum Transfer Cross Section for Electrons in Helium

Abstract

SummaryMeasurements of the drift velocity, the ratio of diffusion coefficient to mobility, and the "magnetic drift velocity" for electrons in helium have been made at 293°K in the range 1· 8 X 10-19 < E/N < 3 X 10-17 V cm 2 • From an analysis of the drift velocity data, an energy-dependent momentum transfer cross section has been derived for which an error of less than ± 2 % is claimed over the central portion of the energy range. The cross section agrees with the theoretical cross section of La Bahn and Callaway to within 2% over the whole energy range. The agreement with the cross section derived by a number of procedures from the total elastic scattering cross section measured by Golden and Bandel is less satisfactory. The drift data are sufficiently accurate to enable a search to be made for the effects of fine structure in the cross section at low energy. The results do not support the existence of such structure.

A Note on End Effects in Electron Drift Tube Experiments

“…The transport coefficient that is subject to the least experimental error is the drift velocity, and Crompton et al (1970) have claimed that measurements by the Bradbury-Nielsen method of the electron drift velocity in helium have an absolute error of less than 1 %.…”

“…5·006 500 0·638 1·619 2·372 9·991 200 1·054 1·842 2·125 2·377 2·671 500 0·638 1·054 1·622 10·000 200 1·055 1·840 2·213 2·669 500 1·053 The collected data for hydrogen are shown in Table lea), where the drift velocities Ware listed as functions of E/ N and gas pressure p for tubes of three different drift lengths L, these lengths being the geometrical distances between the mid planes of the grids. The tube with L = 5·084 cm is the one described by Elford (1971; Fig. 1, system B without guard rings Rl, R2, and R3), that with L = 10·195 cm is system B of Elford (1971), and that with L = 50·000 cm is the tube employed by Crompton and Elford (1973).…”

“…The tube with L = 5·084 cm is the one described by Elford (1971; Fig. 1, system B without guard rings Rl, R2, and R3), that with L = 10·195 cm is system B of Elford (1971), and that with L = 50·000 cm is the tube employed by Crompton and Elford (1973). The data in Table l(a) do not include corrections for diffusive effects.…”

“…The drift tubes with L = 5·006, 9·991, and 10·000 cm are respectively those described by Crompton et al (1968), Crompton et al (1967), and Crompton et al (1970). The tube of Crompton et al (1967) was specifically designed to have a highly uniform electric field while that of Crompton et al (1970) is known to have a small, and it is believed insignificant, degree of field distortion in the vicinity of the grids. As for the hydrogen data, no allowance has been made for diffusive effects in the values of Table l (b), but an assumed correction of 3 (Ddll)jV leads to a maximum adjustment to the data of less than O· 1 %.…”

Classical collisional diffusion in the annular Penning trap

“…The background gas is helium because low energy collisions of electrons with helium are elastic and the collision cross section for momentum transfer varies slowly with energy. 7 ' 8 The energies of the confined electrons are usually below 1 eV and the cross section rises about 20% between 0 eV and I eV. The electron-neutral collision frequency is v, = rfle (v) v, where v is the electron velocity, a(v) is the collision cross section, and n 1 t, is the helium density, and is shown as a function of electron energy in Fig.…”