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In the bombardment of materials with heavy particles a large amount of energy can be deposited in a very small region by a primary knock-on atom and the local atomic arrangement can be thereby drastically disrupted. Various measurements of physical properties of such irradiated materials indicate the presence of distributions of defects which are removed in a step-like manner by annealing. One of the more interesting physical property changes accompanying fast particle irradiation is the attendant change in mechanical properties of irradiated crystals. The defect which is responsible for the mechanical property changes of irradiated crystals is only removed at high temperatures, temperatures coresponding to self diffusion. This observation, as well as others, has led to the model of a depleted zone as being responsible for the changes of mechanical properties of irradiated crystals. A depleted zone is envisioned as a region of crystal where a high local concentration of point defects exists - a belt of interstitials surrounding a multiply connected complex of vacancy clusters. We would like to present here some evidence which lends support to the existence of such defects.