Cascade and subcascade structure in fission neutron irradiated fcc metals and their correlation to fusion neutron irradiation

Thin foil specimens of Cu and Au were irradiated with fission neutrons at 285–295 K to neutron fluences of 7 × 1021 n m−2 (>0.1 MeV), using the Kyoto University Reactor. The structure of cascades and subcascades was examined by fitting the experimentally observed distribution of vacancy-type defect clusters to the calculated primary recoil energy spectrum, and was compared directly with that for fusion neutron irradiation. In Cu, 70% of cascades consist of only one defect cluster and 2% of more than four clusters. This is in contrast to larger cascades produced by 14 MeV neutrons that contain more than 10 clusters and extend 60 nm in diameter. The subcascade energy was 18 and 15 keV for Cu and Au, respectively. These values are about 30–60% higher than those obtained for fusion neutron irradiation.