The evolution of defect structures in copper, following (1) helium production by (n,α) reaction in boron-impregnated copper and (2) direct helium implantation, has been studied by detailed positron lifetime and line shape measurements and the results compared. In the as-irradiated Cu-B (Case 1 above), the shorter lifetime τ1 = 167 ± 2 ps of 97% intensity has been understood as due to positron trapping at small helium vacancy complexes, while in the as-irradiated state of copper (Case 2 above), τ1 = 180 ±10 ps of 92% intensity is explained as due to the combined effects of positron trapping at dislocation loops and substitutional helium-vacancy complexes. The longer lifetime τ2 = 450 ± 30 ps of 3% intensity in Cu-B and τ2 = 420 ± 30 ps of 8% intensity in copper are understood as due to helium-free voids formed during irradiation. The observed difference in the lifetime behavior in the initial stage of defect recovery in copper, as compared to that in Cu-B, is attributed to the effect of irradiation temperature on dislocation loop density. In the intermediate annealing interval, a stage has been observed in both Cu-B and copper, marked by the occurrence of a minimum in the variation of τ2, maximum in I2, and a shoulder in the line shape curve. These are explained in terms of the formation of bubble nucleus, controlled by a thermally activated migration of helium atoms to vacancy traps. A small shift in the bubble nucleation temperature is observed in α-irradiated copper as compared to that in Cu-B.
In the post nucleation growth stage, the behavior of bubble lifetime and its intensity are found to be similar for both Cu-B and copper, wherein τ2 increases towards a saturation value accompanied by a corresponding decrease in I2 towards a plateau. The observed positron lifetime for large bubbles has been analyzed by relating the annihilation rate to helium atom density and helium pressure in bubbles evaluated for both Cu-B and copper. These are found to be in fair agreement with the estimates of equilibrium pressures, supporting the mechanism of thermal vacancy condensation for bubble relaxation. Helium retention in bubbles is found to be stable in copper even close to its melting point.