Stage II recovery in proton-irradiated aluminium studied by positrons

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Abstract
Defect recovery of low-temperature proton-irradiated aluminium has been studied by the positron lifetime technique. The positron lifetime parameters are observed to change significantly for isochronal annealing in stage I (30-45 K), stage II (45-190 K) and stage III (190-300 K). Stage I is the stage where interstitial atoms become mobile leading to annihilation of vacancies and interstitial atoms. In stage III the loss of vacancies at sinks and the growth of vacancy clusters, due to freely migrating vacancies left over from stages I and II, is observed. In stage II mobile interstitials form three-dimensional clusters that again rearrange into interstitial loops. Unexpectedly, the authors also observe remarkable changes in the positron lifetime parameters in this temperature region, which is explained by positron trapping at interstitial-type defects. The positron lifetime at these interstitial clusters is less than 230 ps, well below the positron lifetime in a monovacancy, but close to lifetimes deduced from recent studies on deformed aluminium. Comparison with diffuse X-ray scattering results show that the positron lifetime technique is sensitive to both the growth and the structural changes of interstitial clusters.