Positron diffusion and trapping at vacancies

The temperature dependence of positron trapping at vacancies in metals is discussed taking account of the velocity dependence of the capture cross section for a freely moving positron and also the role of positron scattering in limiting the arrival rate of particles to the vicinity of a vacancy trap. The Fermi golden rule and the analogy with low-energy neutron capture by nuclei strongly suggest a temperature-independent free particle trapping rate. Simple arguments for a classical itinerant positron sweeping out volume as it moves through the crystal show that scattering only affects the trapping rate if the mean free path is less than the capture radius. Recent estimates of the positron mean free path in metals suggest that this is not the case in most situations.