DEFECT RECOVERY IN ALPHA-FE E--IRRADIATED AT 300-K

Defect annealing recovery has been studied, by measuring positron lifetime spectra, in high-purity alpha-iron irradiated at 300 K with 3 MeV electrons to a fluency of 7 x 10(19) cm-2. Vacancy clusters containing 6-10 single vacancies were observed immediately after irradiation during which they were possibly forming (the so-called ''irradiation annealing''). With increasing temperature, the agglomerates continually grow in size at the expense of their concentration, giving rise to the formation of microvoids (> 15 vacancies). Also present were other types of defects, probably immobile vacancies trapped by impurity (e.g. carbon) atoms and dislocation/loops generated presumably from collapse of voids during the relatively high dose irradiation and/or the annealing. The immobile vacancies eventually became movable at around 350 K, supplying the growing clusters and thus leading to a stabilization in their concentration till around 500 K. Between 500 and 700 K, microvoids gradually evaporated, but the dislocation-associated defects were able to survive annealing at temperatures as high as 700 K. The void size and concentration and their evolution have been evaluated on the basis of both the to date theoretical and experimental studies. The temperature dependence was also observed of positron trapping into vacancy agglomerates of various sizes.