Irradiation-induced lattice–site change of oxygen in niobium

Abstract

Abstract The effect of 2·0 MeV 4He+ irradiation at 86 K and 293 K on the lattice site of oxygen (18O) dissolved in single-crystal niobium has been studied by use of the proton-channelling mothod. With an incident beam of 700 keV protons, angular scans of the yield from the nuclear reaction 18O(p, α)uN have been performed about the 〈100〉, 〈111〉 and {100} channels in Nb/0·18 at.% 180. The results indicate that oxygen atoms occupy octahedral interstitial sites in an unirradiated niobium crystal, but that the lattice site is altered when defects are introduced into the sample by irradiation at 86 K or 293 K with 2 MeV helium ions to a dose of 2–3 × 1018 ions cm−2. Using calculated channelled-flux distributions it is shown that the angular scans in irradiated samples are consistent with oxygen atoms being displaced by 0·5 ±0·1 A from the octahedral sites, the displacements occurring with equal probability in all 〈100〉 lattice directions. It was found that the angular scans in irradiated samples were not altere...