Transport theory for low-energy positron thermalization and annihilation in helium

Abstract

A transport theory that explicitly incorporates loss of flux due to annihilating collisions is developed and applied to low-energy positron diffusion and annihilation. The use of more complete momentum transfer and annihilation cross sections for helium has resulted in improved descriptions of the time dependence of $\ensuremath{\langle}{Z}_{\mathrm{eff}}\ensuremath{\rangle}$ for positrons injected into gaseous helium. Similarly, the variation of $\ensuremath{\langle}{Z}_{\mathrm{eff}}\ensuremath{\rangle}$ versus $E/{n}_{0}$ for experiments where the annihilation region is immersed in an electric field is in closer agreement with experimental data. Inclusion of loss of flux due to annihilation was found to have a very small effect on the derived $\ensuremath{\langle}{Z}_{\mathrm{eff}}(t)\ensuremath{\rangle}$ for helium.