A study was made of irradiation-induced defects in undoped material. The irradiation was performed at 4K, using an incident energy of 2MeV. Defect formation and annealing was then studied as a function of fluences of between 1015 and 1019/cm2. Annealing experiments were performed between 90 and 800K, and temperature-dependent measurements were made between 20 and 600K. A principal annealing stage between 140 and 210K, which depended upon the fluence, was attributed to the annealing of Frenkel pairs in both sub-lattices. Above an annealing temperature of about 350K, a second vacancy-like defect became detectable by using positron techniques. This defect contained several vacancies, and their number increased with irradiation fluence. The annealing stage of these clusters occurred between 590 and 800K; depending upon the cluster size.

A.Polity, T.Engelbrecht: Physical Review B, 1997, 55[16], 10480-6