Positron annihilation in Pb2+ doped KCl single crystals

The angular distribution of annihilation photons (ADAP) has been measured in a pure KCl single crystal and in one doped with Pb²⁺ impurities to concentrations of 100 ppm, 550 ppm and 1050 ppm. In unannealed, impurity doped, crystals the intensity of the narrow component increases with increase of impurity concentration. The narrowing of the ADAP curves is explained by assuming trapping of positrons at impurity induced positive ion vacancies. On annealing, the result on the 550 ppm impurity doped crystal shows that there is a dissolution of impurity-vacancy complexes at nominal concentrations of impurity, and the result on the 1050 ppm crystal shows that there is precipitation of impurity for very high concentrations.