Thermalization and diffusion of positrons in solids

Abstract

Trapping of thermalized positrons in solids of temperature T by sinks, such as surfaces, vacancies, or voids, can change the mean kinetic positron energy E + in the medium so that (E+—3kBT/2)<0. The phenomenon of positron heating ( > ) or cooling ( < ) of the positron ensemble in a medium with absorbing boundaries arises when microscopic processes of positron scattering and transitions into sink states depend on the positron momentum in the thermal distribution. Positron heating and cooling always reduces the positron trapping rate at sinks, the more so the smaller the volume between absorbing boundaries. The influence of the initial distribution of kinetic positron energies on positron trapping is investigated, and implications for the study of defects in solids with positrons are analyzed.