THE TRAPPING MODEL FOR POSITRONS DIFFUSING INSIDE THE GRAIN OF ARBITRARY SHAPE

We present a general solution of the trapping model for positrons whic h diffuse inside a grain of arbitrary shape towards its surface. The c onsideration takes into account both the diffusion and transition regi me. The explicit form relations for the mean positron lifetime and the positron lifetime spectrum were obtained. The theoretical relation fo r the mean positron lifetime was analysed thoroughly for a grain of el lipsoidal shape. We established that the shape of the grain influenced the positron annihilation characteristics.