Positron annihilation in colourless centres in heavily gamma-irradiated NaCl

Abstract

A detailed study of positron lifetimes in heavily gamma-irradiated NaCl single crystals, which undergo a pulsed thermal annealing treatment up to 740 degrees C, is presented. Above 300 degrees C, at which temperature the samples are completely bleached, two exponential terms suffice to fit the lifetime curve. While the shortest lifetime tau 1 does not depend upon the annealing temperature and has a value close to that found in unirradiated samples, the other lifetime tau 2 exhibits a monotonic increase with annealing temperature and two recovery stages at about 340 and 540 degrees C. This pattern parallels the thermal recovery of the ionic conductivity of irradiated NaCl. It is suggested that tau 2 reflects the annihilation of positrons at cation vacancies thermally released from radiation-induced divacancies. The tau 2 evolution with the annealing temperature has been analysed in terms of a trapping model and a value of 1.96+or-0.08 eV for the divacancy formation enthalpy is obtained.