The direct recombination of interstitial atoms and vacancies in an irradiation damage cascade

Abstract

Computer calculations have been made of the fraction of interstitial atoms and vacancies that directly recombine by diffusion in a damage cascade, neglecting the effect of defect clustering. The shape of the cascade is idealized to be either cylindrical or spherical, which should provide a useful guide to the more complicated shapes occurring in practice, and the point defects are given a gaussian distribution initially. A special feature of the calculations is that the initial spread of the interstitials is allowed to exceed that of the vacancies, due to the propagation of dynamic crowdions during the formation of the cascade. The present calculations have the merit that (i) a rapid estimate of the recombination can be made, (ii) a complete anneal is performed whereas often only partial anneals can be achieved with more sophisticated atomic models, and (iii) allowance is made for the trapping of interstitial atoms at other sinks. The results are presented in a convenient non-dimensional form as a function of the size and density of the cascade. They seem to be most relevant to higher temperatures, where less defect clustering occurs, and show a marked dependence on the initial spread of the interstitials relative to the vacancies. The diffusion of the vacancies has little effect on the recombination provided D_v < D_i /100, which is usually the case. The application of the results to several wellknown computed cascades in α-Fe suggests that about 85% of the defects in these cascades are likely to recombine at high temperatures. Such a high level of recombination would markedly affect the estimated damage rate (dpa/unit time) produced by neutrons or heavy particles, which is relevant to the interpretation of quantitative void growth experiments. However there is a possibility that the defects may be too dense in these cascades and further work on more realistic cascades is required to decide if the damage rate need be modified.