A positron annihilation investigation of defects in neutron irradiated copper

Abstract

The response of positron annihilation parameters to the defect structures produced by neutron irradiation in copper, and their annealing behaviour, have been studied on specimens irradiated at two different temperatures, 50°C and 250°C. Both lifetime and angular correlation measurements were made while some aspects of the irradiation damage substructure were also covered by transmission electron microscopy. Marked changes in the positron parameters were found after the two irradiations. After the 50°C irradiation the dominant TEM defect was a high concentration of small loops, after the 250°C irradiation it was a population of voids. The voids gave rise to a long-lived component (420 ± 75 psec) that disappeared during isochronal annealing to between 450T and 550T in agreement with TEM observations. Another trapped positron component (180 ± 7 psec) appearing after both irradiations annealed out between 275 ± 25°C and 475 ± 25°C. Although we could not entirely exclude that the defects giving rise to this trapping were the dislocation loops, the results suggested that the traps were a population of small submicroscopic vacancy clusters (microvoids), probably gas stabilised. This appears to be important since such clusters are usually not considered, although there are good physical arguments to support their existence. The question of positron trapping at dislocations and loops still appears to be an open one.