A Review of Methods for Determining the Nature of Small Defect Clusters

Abstract

When a crystalline material is damaged by fast neutron irradiation, the most common defects encountered are small aggregates of vacancies or interstitial atoms. The images observed range from about 150Å down to the resolution limits of the contrast producing mechanism. An understanding of these defects is quite important to theories of irradiation damage, and a significant effort has been directed towards unraveling their nature. The development of the appropriate diffraction contrast techniques has been marked by a number of controversies. Since many are now resolved, this seems a salutary time to put forward a review of the present state of affairs.An image of a defect cluster is a result of the strain field, either expansion or contraction, in the surrounding lattice. Calculations of the contrast from such a strain field have been performed by Ashby and Brown, and more recently by Rühle et al., and McIntyre and Brown. These authors used the two-beam wave-optical dynamical theory of diffraction contrast including anomalous absorption, and the images predicted have two-lobes, one darker and the other lighter than the background.