Direct observation of the primary state of damage of ion-irradiated tungsten I. Three-dimensional spatial distribution of vacancies

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Abstract
The results of an extensive field-ion microscope (FIM) investigation of the primary state of damage of ion-irradiated tungsten are presented. Two-pass zone-refined single crystals of tungsten were irradiated in situ, at ≤ 15 K, with a magnetically analysed beam of various ions at a background pressure of (5–10) × 10−10 Torr in the absence of the imaging electric field. The value of the standard fluence was small enough (5 × 1012 cm−2) to guarantee that each depleted zone (DZ) detected was associated with a single projectile ion. After an irradiation each specimen was examined on an atom-by-atom basis employing the pulse field-evaporation technique. The two main experimental programmes were: (1) the determination of the effect of the mass of the projectile ion (M 1) on the three-dimensional spatial distribution of vacancies in DZs, in specimens which had been irradiated with 30 keV W+, Mo+, Kr+ Cu +, Cr+, or Ar+ ions; and (2) the characterization of the effect of the initial energy of the projectile ion (E 1) on the vacancy structure of DZs created by 15, 30, 45, 60 or 70 keV Kr ions. Three-dimensional visualizations are presented of a number of the DZs detected, based on the use of the OR TEP program. The average number of vacancies (〈v〉) per DZ is 174 ± 48 for E 1 = 30 keV, independent of M 1, for the range of ion masses employed. The value of the average vacancy concentration per DZ decreases from ≈ 16 to 2 at.% as M 1, is decreased from ∼184 a.m.u. (W) to ∼40 a.m.u. (Ar) for E 1 = 30 keV. For the Kr ion irradiations the value of 〈v〉 increases linearly as E 1 is increased from 15 to 70 keV. Many other detailed physical properties of the DZs are presented.