Annihilation Momentum Density of Positrons Trapped at Vacancy-Type Defects in Metals And Alloys

Abstract

Positron annihilation, especially the angular correlation of annihilation radiation (ACAR), is a powerful tool for investigating the electronic spectra of ordered as well as defected materials. The tendency of positrons to trap at vacancy-type defects should enable this technique to provide a signature of the local environment of such defects; an understanding of the changes in the defect structure at phase transitions is relevant also in the context of this conference. In order to achieve this goal, however, we need to develop a theoretical basis for calculating the two-photon annihilation momentum density ${{\rho }_{{{{2}_{\gamma }}}}}(\vec{p})$. With this motivation, we have recently formulated and implemented a theory of ${{\rho }_{{{{2}_{\gamma }}}}}(\vec{p})$ from vacancy-type defects in metals and alloys.[l] This article gives an outline of our approach together with a few of our results. Within the constraints of space, the discussion is mostly illustrative; the citation of literature is minimal.