Atomic displacement cascade distributions in iron

Full molecular dynamics (MD) and its binary collision approximation (BCA) are used in a complementary way in order to study displacement cascade distributions in iron. Frenkel pair distributions are particularly narrow and symmetrical. They are thus well described by their first moment. Therefore, quantitative estimates by MD are possible. The comparison between the dependence of Frenkel pair production on primary energy predicted by both computational techniques suggests a post-cascade recombination model. Its physical grounds are discussed. The variability between spatial distributions of individual cascades is particularly large as a consequence of instability, which takes place in the early stage of the cascade development. The subsequent loss of correlation with initial conditions is statistically demonstrated on the basis of BCA simulations of 5000–15 000 cascades. Sufficient statistics can be reached by MD in order to characterise spatial distributions within cascades. It comes out of systematic comparison between MD and its BCA that, after the ballistic phase, the spatial extent of both vacancies and interstitials tends to increase. This phenomenon correlates with atomic mixing in the cascade core. This mixing is not predicted in the BCA. It is suggested to be responsible for the fragmentation of vacancy clusters formed during the ballistic phase of the cascades.