The nature of radiation-induced point defect clusters

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Abstract
The electron microscope has been, used to examine point defect clusters produced in copper and aluminium foils bombarded with l·4 × 1017 alpha-particles cm−2. The foils were bombarded as a stack and, in copper, both those through which the alpha-particles had passed and that in which they came to rest appeared similar, containing dislocation loops (r ∼ 200 Å) and a background of more numerous and smaller dots (r∼20 Å). The behaviour at grain boundaries suggested that these two types of cluster were due to different point defects. Whereas the dislocation loops normally annealed out at about 350°c, in the foil in which the alpha-particles came to rest (containing ∼ 1015 atoms of helium cm−2) they grew to form a dislocation tangle, and eventually small helium bubbles (r ∼ 40 Å) appeared in the same number as the original small dots. These results indicate that the dislocation loops are due to the clustering of interstitial atoms and the small dots are clusters formed from vacancies. It is also inferred that the helium atoms are in interstitial positions during the bombardment, and form bubbles by nucleating upon the dusters.