ON THE INTERPRETATION OF POSITRON-ANNIHILATION DATA IN POWDERS AND FINE-GRAINED MATERIALS - A MONTE-CARLO SIMULATION

Monte-Carlo simulations of positron diffusion are carried out for powd ers consisting of spherical and ellipsoidal particles with and without defects. Following Bergersen et al. [1], elastic positron-phonon inte raction is considered to be dominant for scattering processes in posit ron diffusion. The central question is which fraction of the positrons would be able to reach the particle boundaries. Hence, we calculate t he Fraction of Positrons reaching particle Surface (FPS). The presence of defects in the particles can drastically reduce FPS depending on t he defect concentration and capture rate. We demonstrate that for smal l-grained materials the grain surface can influence the lifetime signa l significantly.