Single atom sputtering events Direct observation of near-surface depleted zones in ion-irradiated tungsten

Abstract

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 ( 15 K) both the self-interstitial atoms and vacancies were immobile, so that the primary state of radiation damage was preserved. The following properties of each DZ were determined : (1) the total number of vacancies; (2) the number of vacancies in the near-surface region; (3) the spatial extent-that is, the dimensions required to determine a volume; (5) the average vacancy concentration associated with the near-surface region; (6) the first-nearest-neighbour cluster distribution for the vacancies in the near-surface region; (7) the radial distribution function of all the vacancies; (8) the distribution of vacancies as a function of depth normal to the irradiated surface; and (9) the sputtering yield, Most of the above 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 exibited significant fluctuations from the measured average sputtering yield.