Single atom sputtering events

The three-dimensional spatial arrangement of vacancies contained in depleted zones (DZs) of ion-irradiated tungsten specimens, has been determined with atomic resolution by the field-ion microscope (FIM) technique. These DZs were detected in the near-surface region of specimens which had been irradiated in situ at <=15 K with 20 keV W+, 30 keV W+, Kr+, Cu+ or Ar+ ions. The values of the ion dose employed were small (<=1013 ions cm-2); therefore, each DZ analysed was the result of the impact of a single projectile ion. At the irradiation temperature both the self-interstitial atoms and vacancies were immobile, so that the primary state of radiation damage was preserved. Most of the properties of the near-surface DZs had similar values to those of the DZs detected in the bulk of the FIM specimens. The total number of vacancies detected in the near-surface region was approximately consistent with theoretical estimates of the average sputtering yield. The sputtering yield of individual DZs exhibited significant fluctuations from the measured average sputtering yield. (author)