The nature of the point defects in samples which had been irradiated with fast neutrons at low temperatures was studied by means of the small-angle scattering of synchrotron radiation. The scattering curves were analyzed in terms of a distribution of depleted zones. After irradiation and annealing at 90K, single vacancies and small agglomerates were found in Fe. Some 70% of these were in depleted zones with an average radius of 1.26nm and a mean vacancy concentration of 1.1%. In the case of Cu, larger agglomerates (11 to 15 vacancies) were observed in depleted zones with an average radius of 0.82nm and a mean vacancy concentration of 5.2%. A strong scattering signal at very low scattering angles was attributed to correlations between individual cascade zones. During recovery, agglomeration of the vacancies was observed in both metals. Whereas, in Cu, the correlation between individual depleted zones survived annealing at 430K, the correlation vanished in Fe samples during annealing in stage III. A computer model of the correlation between individual cascades in Fe showed that the observed negative curvature of the resistivity damage rate could be explained by this correlation effect.

H.Franz, G.Wallner, J.Peisl: Radiation Effects and Defects in Solids, 1994, 128[3], 189-204