The effect of cluster formation on the ‘temperature shift’ for accelerator simulation of neutron irradiation

The effect of the nature of the incident particles on the temperature dependence of irradiation-induced void formation in steel is considered. During neutron irradiation small clusters of vacancies are formed in ‘spikes’. At temperatures below the maximum in the swelling-temperature curve these clusters tend to dissolve slowly and hence provide recombination sites for interstitials as well as vacancies which have escaped from other clusters. Consideration of the dissolution kinetics of these clusters leads to the theoretical prediction of a minimum temperature for void formation (under neutron irradiation) of around 350°C. This result is more consistent with experimental observation than the much lower temperatures predicted by previous theory and hence lends support to the cluster dissolution theory. Due to the nature of the interaction of the high-energy electrons with the lattice, no clusters are formed during HVEM simulation of neutron damage, and cluster dissolution kinetics are therefore not controlling. Clusters are, however, produced in heavy-ion bombardment. Consequently the rate-dependent temperature shifts which are used to compare accelerator void formation data with neutron irradiation are expected to be different for electrons and for heavy ions.