The role of cascade energy and temperature in primary defect formation in iron

Molecular dynamics (MD) simulations have been used to investigate the formation of atomic displacement cascades in iron with energies up to 50 keV (corresponding to a primary knock-on atom (PKA) energy of 79 keV) at 100 K, up to 20 keV at 600 K, and up to 10 keV at 900 K. The cascade damage production has been characterized in terms of several parameters: the number of surviving point defects, the fraction and type of surviving point defects found in clusters, and the size distributions of the in-cascade point defect clusters. A sufficient number of simulations have been completed at each condition to evaluate the statistical variation in these primary damage parameters as a function of irradiation temperature and cascade energy. The energy dependence of stable defect formation can be conveniently separated into three regimes with the number of defects in each regime correlated by a simple power law with a characteristic exponent. The primary effects of cascade energy on defect formation at high energies are explained in terms of subcascade formation. Only a modest effect of temperature is observed on defect survival, while irradiation temperature increases lead to a slight increase in the in-cascade interstitial clustering fraction and a decrease in the vacancy clustering fraction. Cascade energy has little effect on the in-cascade clustering fractions above about 5 keV. However, there is a systematic change in the cluster size distribution, with higher energy cascades producing larger clusters. The loosely coupled nature of the in-cascade vacancy clusters persists at higher energies.