Diffusion of positrons in helium gas

Abstract

The caution describing diffusion of positrons in helium gas is solved with the scattering and annihilation data previously obtained by Humberston (1973), and Campeanu and Humberston (1975) to obtain the positron annihilation rate under various conditions of temperature and electric field strength and for various initial velocity distributions. The results at zero electric field are in reasonably good agreement with the recent experimental results of Coleman et al., with the differences being attributed to inaccuracies in the theoretical annihilation data. There appears to be good agreement between the present equilibrium results for non-zero electric fields and the experimental results of Leung and Paul, and Lee et al., but the agreement is fortuitous since the experimental and theoretical results are almost certainly too low. With the present level of accuracy of the theoretical results it has not been possible to determine the detailed form of the initial velocity distribution of the positrons.