Loop formation in Cu and Al after low-temperature fast-neutron irradiation

Single crystals of Cu and Al were irradiated with 2.4*1018 fast neutrons/cm2 (E>0.1 MeV) at 4.6 K. Asymptotic diffuse scattering of X-rays was measured after irradiation and after several annealing steps up to 300 K. The formation of dislocation loops of interstitial and vacancy type was observed; their size distribution was determined. In Cu after recovery at 60 K most of the interstitials are contained in loops with a mean loop radius of 11 Å. On further annealing, the mean radius increases to 18 Å at 300 K, where 25% of the initially induced defects are retained. At 60 K, between 20% and 40% of the vacancies are contained in loops of a mean radius of about 10 Å. During further recovery the total number of vacancies in loops as well as the mean vacancy loop size remain rather constant. In Al, no evidence for loop formation was found.