A quantitative field ion microscope study was made of the defect structure of two depleted zones detected in high purity (≤ 1.5 · 10-6 at.fr. impurity level) tungsten specimens irradiated in situ under ultra-high vacuum conditions at a specimen temperature of 18°K with 20 keV W+ ions to a dose of ∼ 1.1012 W+ ions cm-2. The irradiated specimens were examined by the pulse field evaporation technique at 18°K, hence their structure is characteristic of this temperature. The depleted zone detected in the (111) plane had a local vacant site concentration of ∼ 8.7 at.% and a self-interstitial atom concentration (SIA) of ∼ 0.92 at.%, while the depleted zone detected in the (141) plane of a second specimen had values of ∼ 10 and ∼ 0.5 at.% for these same quantities. The SIAs found locally were on the peripheral surfaces of the depleted zones. In addition to this large local imbalance in point defect concentrations the depleted zones were elongated along 〈011〉 directions. It was shown that each of these depleted zones was created by a single incident ion, and a comparison of the number of displaced atoms in each zone with the value predicted by the Kinchin-Pease model showed that this model strongly overestimates the number of displacements created per incident ion. For the depleted zone detected in the (111) plane it was demonstrated that 23 of 25 SIAs found in the lattice around this zone could have been propagated away from it along 〈011〉 and 〈111〉 directions as focused replacement sequences. The measured distances of these SIAs away from the depleted zone along 〈011〉 and 〈111〉 directions were between 45-150 and 45-85Å, respectively. The upper and lower values of these distances were determined by a geometric sampling problem and not by the intrinsic range of focused replacement sequences.
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