Displacement cascades with wide ranges of primary knock-on atom energy and mass in iron were simulated using molecular dynamics. New visualisation techniques were used to show how the shock-front dynamics and internal structure of a cascade developed over time. This revealed that the nature of the final damage was determined early in the cascade process. A zone (termed 'spaghetti') was defined in which atoms were moved to new lattice sites, and it was shown how it was created by a supersonic shock-front expanding from the primary recoil event. A large cluster of self-interstitial atoms could form on the periphery of the spaghetti if hypersonic recoil created damage with a supersonic shock ahead of the main supersonic front. When the two fronts meet, the main one injected atoms into the low-density core of the other: becoming interstitial atoms during rapid recovery of the surrounding crystal. The hypersonic recoil occurred in less than 0.1ps after the primary recoil and the interstitial cluster was formed before the onset of the thermal spike phase of the process. The corresponding number of vacancies was then formed in the spaghetti core as the crystal cools, i.e. at times one to two orders of magnitude longer. By using the spaghetti zone to define cascade volume, the energy density of a cascade was shown to be almost independent of the PKA mass. This threw doubt on the conventional energy-density interpretation of an increased defect yield with increasing PKA mass in ion irradiation.

On the Origin of Large Interstitial Clusters in Displacement Cascades. A.F.Calder, D.J.Bacon, A.V.Barashev, Y.N.Osetsky: Philosophical Magazine, 2010, 90[7-8], 863-84