A study was made of positron lifetimes in heavily gamma-irradiated monocrystals which were subjected to pulsed thermal annealing at temperatures of up to 740C. At temperatures above 300C, where the samples were completely bleached, 2 exponential terms were sufficient to fit the lifetime curve. Whereas the shorter lifetime did not depend upon the annealing temperature, and had a value which was close to that for unirradiated samples, the other lifetime exhibited a monotonic increase with annealing temperature; with recovery stages at about 340 and 540C. This behavior paralleled the thermal recovery of the ionic conductivity of irradiated NaCl. It was suggested that the longer lifetime reflected the annihilation of positrons at cation vacancies that were thermally released from radiation-induced di-vacancies. The change in this lifetime with annealing temperature was analyzed in terms of a trapping model, and a value of 1.96eV was estimated for the di-vacancy formation enthalpy.

Positron Annihilation in Colourless Centres in Heavily Gamma-Irradiated NaCl. F.J.Navarro, L.C.Damonte, J.L.Alvarez Rivas, J.Díaz, J.L.Ferrero: Journal of Physics - Condensed Matter, 1995, 7[43], 8273-8