Grain-boundary positron annihilation in polycrystals

A study was made of the effect of thermal cycling of single crystals and polycrystalline specimens in the range 300-80-300 K on the temporal behavior of the annihilation characteristic N(0) corresponding to a peak in the angular distribution of annihilation photons. The specimens used in the study were Zn and Si single crystals and polycrystalline Zn, Si, and Nb annealed in vacuum or in an argon atmosphere. In the case of single crystals, the N(0) curve did not change with time after thermal cycling, whereas in the case of polycrystalline specimens the N(0) characteristic increased sharply and exhibited irregular oscillations with time. The results of the study thus support the presence of an effect associated with grain-boundary positron annihilation.