FIELD ION MICROSCOPE STUDY OF THE POINT DEFECT STRUCTURE OF A DEPLETED ZONE IN ION (W ⁺ ) IRRADIATED TUNGSTEN.

The depleted zone was found to consist of 41 vacant lattice sites clustered in a cubic region of approximately 13A on a side. No contrast effects which could be attributed to self-interstitial atoms were found in or near the depleted zone or in the lattice surrounding it. If only first nearest neighbor vacant lattice sites are considered then the distribution of cluster sizes is as follows; (1) three monovacancies; (2) one divacancy; (3) one trivacancy; (4) one six vacancy cluster; and (5) one huge cluster of 27 vacancies. A diffusion model is also presented which explains the temperature dependence of the size of a depleted zone in terms of the mean range of a focused collision replacement sequence at a given irradiation temperature.