Angular correlation in positron annihilation

Abstract

The angular correlation of the two gamma quanta emitted when a thermalised positron annihilates with metallic conduction electrons is investigated by applying the newly developed theory in which an electron gas is considered as a system of interacting collective excitations. The conduction electrons are treated in a jellium model. The method leads in a natural way to the appearance of high-momentum components (i.e. pair momentum P>p_F) in the annihilation radiation. The amount of these components is significant ( approximately=10%) in a dilute electron gas (such as in alkali metals), but fairly irrelevant for higher densities. The dependence on momentum of the enhancement factor for a dense system (with r_s approximately=2) agrees well both with the earlier theories given by Kahana and others, and also with recent accurate experimental observations. As r_s increases into the alkali-metal region, the enhancement factor for PF becomes relatively more and more constant, in contrast with the trend in the Kahana theory.