Defect retention in copper during electron irradiation at 80°K

Abstract

Electron irradiation damage in copper at 80°K is analyzed by a reciprocal damage rate approach and by a method relating normalized damage and normalized fluence. The latter is more sensitive to changes in effective trapping concentrations: the unsaturable trap model has not been found to strictly hold for any sample irradiated here or described in the literature. More importantly, the increase in relative trapping radius with respect to the nucleation of interstitials varies at least by a factor of 30 according to sample pre-irradiation treatment. These results and those of stage II irradiations provide information regarding the relative size and nature of trapping centers. Trapping probabilities based upon interstitial diffusion in both the correlated and uncorrelated “stages” I_D and I_E, rather than I_E alone, predict some nonlinear reciprocal damage rate curves-even for constant trapping radii-due to the increasing encroachment of stage I_E into I_D during irradiation. Values of initial damage rates and their concentration dependence are also predicted by this calculation. Use of these trapping probabilities in an integral equation for normalized damage allows a determination of effective trapping concentration from a single damage-fluence data point, and has been found to accurately describe irradiations over 3 orders of normalized fluence magnitude and of samples differing by 4 orders of magnitude in impurity concentration. The concentration dependence of the effective uncorrelated interstitial production factor is found by comparison of this approach with one involving I_E diffusion alone. Irradiation data at early normalized fluence and initial damage rate data suggest the absence of residual interstitial sink concentrations and suggest that interstitial specific resistivities decrease slightly upon trapping.