A channelling study of helium-ion irradiation damage in niobium I. Effect of dissolved oxygen

The method of ion channelling/backscattering has been used to compare the defect structures produced in degassed and oxygenated single-crystal niobium by irradiation at 86 K with 2·0 MeV 4He+ ions to doses up to 3 × 1016 ions/cm2 non-channelled, corresponding to approximately 1·3 × 10−2 displacements per atom, near the surface, for an assumed effective displacement energy of 62 eV. Channelling measurements were made, without warm-up, with 2·0 MeV 4He+ ions incident along 〈110〉 and 〈111〉 directions, and dechannelling rates and minimum backscattered yields at the surface were derived from the spectra of backscattered ions. Both the dechannelling rate and the minimum yield increased as a result of the irradiation, with saturation occurring at doses of about 1016 ions/cm2, and it was found that the increases were larger in samples containing 0·18 or 0·35 at.% of dissolved oxygen than in degassed samples. Heat treatments at 500 K produced only minor changes in the backscattered spectra. Most of the de-channelling in the irradiated samples is attributed to the effect of lattice distortions around clusters of vacancies produced directly in energetic cascades; the average dechannelling cross-section of these clusters is larger in samples containing oxygen, and this is interpreted as being due to interactions between oxygen atoms and vacancies in the cascades. The results suggest that focused collision sequences in niobium are not affected by the presence of dissolved oxygen. It is thought that the increase of the minimum scattering yield is primarily caused by direct backscattering of channelled ions at clusters of interstitials, and that the effect of oxygen is to enhance the retention of interstitials in clusters.