A channelling study of helium-ion irradiation damage in niobium II. Effect of irradiation temperature

The method of ion channelling/backscattering has been used to compare the defect structures produced in niobium by irradiation with 2·0 MeV 4He+ ions at temperatures of 86 and 293 K. Samples of degassed niobium and of niobium containing 0·18 at.% of oxygen were irradiated to doses up to 3 × 1016 ions/cm2 (non-channelled). Channelling measurements were made without warm-up at the irradiation temperatures with 2·0 MeV 4He+ ions incident along 〈110〉 and 〈111〉 directions. The defects produced by irradiation at 293 K were found to cause more dechannelling than those produced by irradiation with the same dose and dose-rate at a temperature of 86 K. The difference is attributed to dechannelling caused by interstitial-type dislocation loops, which grow to larger size at the higher temperature and have a larger dechannelling cross-section per defect in the loop. In contrast, although there was a clear increase, δX 0≃1·0%, in the surface minimum scattering yield as a result of irradiation at 86 K, no significant change occurred during irradiation at 293 K. The increase in X 0 at 86 K is interpreted as being due to direct backscattering of channelled ions at small interstitial clusters, and the lack of increase at 293 K as due to the formation of larger interstitial loops which are believed to have a significantly smaller direct backscattering cross-section per interstitial.