Angular correlation in positron annihilation

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 > psub(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 rsub(s) approximately = 2) agrees well both with the earlier theories given by Kahana and others, and also with recent accurate experimental observations. As rsub(s) increases into the alkali-metal region, the enhancement factor for P < psub(F) becomes relatively more and more constant, in contrast with the trend in the Kahana theory. In this density regime the experimental results vary widely, although most of them disagree with the present prediction. The additional effects due to core annihilation and lattice periodicity are studied and reviewed. (author)