POSITRON-ANNIHILATION IN COLORLESS CENTERS IN HEAVILY GAMMA-IRRADIATED NACL

A detailed study of positron lifetimes in heavily gamma-irradiated NaC l single crystals, which undergo a pulsed thermal annealing treatment up to 740 degrees C, is presented. Above 300 degrees C, at which tempe rature the samples are completely bleached, two exponential terms suff ice to fit the lifetime curve. While the shortest lifetime tau(1) does not depend upon the annealing temperature and has a value close to th at 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 re covery of the ionic conductivity of irradiated NaCl. It is suggested t hat tau(2) reflects the annihilation of positrons at cation vacancies thermally released from radiation-induced divacancies. The tau(2) evol ution with the annealing temperature has been analysed in terms of a t rapping model and a value of 1.96 +/- 0.08 eV for the divacancy format ion enthalpy is obtained.