Epithermal positron dynamics

Energetic positrons implanted into a solid rapidly lose energy, down to the order of an electron volt, through plasmon and single electron scattering. However, the later stages of thermalisation are slower, involving predominantly acoustic phonon scattering, with no discrete boundary between the epithermal and thermal equilibrium states. Both the initial stages of thermalisation (stopping) and the diffusion of thermal positrons have been extensively studied both theoretically and experimentally. Theoretically stopping and the thermalisation processes have been mainly studied using Monte Carlo techniques, whereas the thermal equilibrium state is usually described in terms of diffusion annihilation equations. Attempts to model epithermal positron transport in a similar manner have met with limited success. In this work the dynamics of near-thermal positrons are studied using a Fokker-Planck equation, with particle energy and depth below the surface as stochastic variables. The three components of the diffusion tensor are calculated from the stopping powers of positrons in the low energy limit, due to scattering with acoustic phonons and single electron scattering processes.