THERMALIZATION TIME OF POSITRONS IN METALS

Abstract

The thermalization rate of positrons in metals is computed as a function of the electron density parameter r, for the entire range of metallic density 2 < rg < 5.7. The calculation is based on the propagator technique of many-body perturbation theory. In this formalism the rate of energy loss is essentially determined by the imaginary part of the positron self-energy operator which we treat both in the random phase approximation and the Hubbard approximation. In general, we find that the time required for the positron to drop to an energy of 0.025 eV is not as short as is commonly believed, although at room temperature there can be little doubt that complete thermalization has occurred. For aluminium at 100 °K, however, the thermalization time is longer than the annihilation time and it is suggested that this effect should be detectable in an experiment similar to Stewart and Shand's recent positron effective mass experiment in sodium.