Positron trapping at grain boundaries

A simple model is developed for the trapping of positrons at grain boundaries. It is shown that there is a linear relationship between any linear annihilation parameter and the inverse grain size. An effective grain boundary width is defined, which depends on the positron diffusion length and on the strength of the grain boundary for positrons. The effect of detrapping on this effective width is also considered. The model is tested by using the experimental results available in the literature.