Atomic displacement cascade distributions in iron

Full molecular dynamics (MD) and its binary collision approximation (BCA) a re used in a complementary way in order to study displacement cascade distr ibutions in iron. Frenkel pair distributions are particularly narrow and sy mmetrical. They are thus well described by their first moment. Therefore, q uantitative estimates by MD are possible. The comparison between the depend ence of Frenkel pair production on primary energy predicted by both computa tional techniques suggests a post-cascade recombination model. Its physical grounds are discussed. The variability between spatial distributions of in dividual cascades is particularly large as a consequence of instability, wh ich takes place in the early stage of the cascade development. The subseque nt loss of correlation with initial conditions is statistically demonstrate d on the basis of BCA simulations of 5000-15000 cascades. Sufficient statis tics can be reached by MD in order to characterise spatial distributions wi thin 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 mix ing in the cascade core. This mixing is not predicted in the BCA. It is sug gested to be responsible for the fragmentation of vacancy clusters formed d uring the ballistic phase of the cascades. (C) 2001 Elsevier Science B.V, A ll rights reserved.