  Understanding and predicting a material's performance in response to
high-energy radiation damage, as well as designing future materials to be used
in intense radiation environments, requires the knowledge of the structure,
morphology and amount of radiation-induced structural change. We report the
results of molecular dynamics simulations of high-energy radiation damage in
iron in the range 0.2-0.5 MeV. We analyze and quantify the nature of collision
cascades both at the global and local scale. We find that the structure of
high-energy collision cascades becomes increasingly continuous as opposed to
showing sub-cascade branching reported previously. At the local length scale,
we find large defect clusters and novel small vacancy and interstitial
clusters. These features form the basis for physical models aimed at
understanding the effects of high energy radiation damage in structural
materials.
