The study of defects in crystals by positron annihilation

The study of defects in crystals through their interaction with thermalized positrons is usually based on the trapping model. The theoretical foundation of the trapping model, based on the theory of diffusion-limited reactions, is reviewed with particular emphasis on short-range interactions between positrons and trapping defects. For this case the analytical properties of the trapping model are developed in some detail, including the effects of the initial pair distribution on the trapping rate, the effects of detrapping, and the effects associated with more than one type of trap. The assumptions that are usually made in determinations of monovacancy formation enthalpies from positron annihilation experiments are critically discussed. The need for additional experiments capable of providing information on the temperature dependence of the various parameters involved is, stressed. As an illustration we treat in detail monovacancies with finite detrapping rate plus divacancies, both in thermal equilibrium. In the final section a few remarks are added on cases with long-range interactions between positrons and traps, such as electrically charged point-defects in insulators or semiconductors, and dislocations.