The formation of dislocation loops in copper during neutron irradiation

The thin film transmission electron microscope technique has been used to make a quantitative study of neutron damage in copper. The density of observable defect clusters has been measured as a function of diameter after various neutron doses between 0.61 × 1018 and 3.79 × 1018 fast n. cm−2. The total number of clusters and point defects in the visible damage is approximately linear with dose and indicates that ∼50 point defect pairs are formed per primary knock-on atom. The data have been compared with the calculated results of various models of nucleation and growth and the results show that the homogeneous theory of nucleation by the chance collision of isolated point defects does not fit the experimental evidence. A simple heterogeneous theory in which the nuclei for the loops are formed directly by the irradiation can be fitted to the results, however, and it is suggested that nuclei are formed in the displacement spikes by the primary knock-on atoms. The concept of a ‘critical’ nucleus size is introduced to account for the experimental observations on the formation of loops in various metals bombarded with different particles.