MOMENTUM-TRANSFER CROSS-SECTION FOR ELECTRONS IN MERCURY-VAPOR DERIVED FROM DRIFT VELOCITY-MEASUREMENTS IN MERCURY-VAPOR GAS-MIXTURES

The Bradbury-Nielsen time-of-flight method has been used to measure electron drift velocities at 573 K in pure mercury vapour, a mixture of 46.80% helium-53.20% mercury vapour and a mixture of 9.37% nitrogen-90.63% mercury vapour. The E/N and pressure ranges used were 0.2 to 1.5 Td and 5.4 to 15.2 kPa for pure mercury vapour, 0.08 to 3.0 Td and 5.40 to 26.88 kPa for the mixture containing helium and 0.06 to 5.0 Td and 3.33 to 16.67 kPa for the mixture containing nitrogen. It is shown that the use of mixtures significantly reduces the dependence of the measured drift velocity on the pressure, due to the effect of mercury dimers, from that measured in pure mercury vapour. An iterative procedure to derive the momentum transfer cross section for electrons in mercury vapour over the range 0.04 to 4 eV with an uncertainty between +/- 5 and 10% is described. It is concluded that previously published momentum transfer cross sections for mercury vapour derived from drift velocity data are significantly in error, due to diffusion effects and the procedure used to correct for the influence of dimers. The present cross section is in good agreement with the semi-empirical calculations of Walker (personal communication).