The search for interstitial dislocation loops produced in displacement cascades at 20K in copper

A low-temperature in situ ion-irradiation and annealing experiment has been performed in copper. Most clusters which persisted through an anneal to 120 K showed no size changes within the resolution (0.5 nm) of a new weak-beam sizing technique. Of the 55 defects measured under a range of weakly diffracting conditions, seven showed measurable size decreases while three showed size increases. We argue that these clusters are likely to be of vacancy and interstitial nature, respectively. Also on annealing to 120 K a fraction of about 25% of the clusters formed by irradiation with 600 kV Cu+ ions at 20 K disappeared, while a similar number of clusters appeared at different locations. The remaining defects persisted through the anneal, sometimes however with modified morphologies. Video microscopy suggested that the disappearance and appearance of clusters occurred gradually and was unlikely to be due to loop movement. Some arguments on the possible nature of these clusters are presented. On warming specimens to room temperature, a high density of small stacking-fault tetrahedra appeared close to the electron-exit surface of the foil in regions which had been exposed to the electron beam at low temperatures. These are most likely due to the clustering of vacancies produced by sputtering at the back surface