An in situ study was made of atomic-vacancy production, and its evolution during irradiation, by using a slow-positron beam. Thermal stability of the vacancy produced during irradiation was investigated under elevated temperature conditions. An annealed Ni specimen was irradiated with 400keV He ions at three different temperatures of 296, 368 and 713K. Doppler broadenings of positron-annihilation γ-rays were measured and variation of the line-shape parameter S was observed under beam-on (during irradiation) and beam-off (non-irradiation) conditions. Results indicate that variation of the S depends on the specimen temperature, showing that vacancy cluster consisting of about 15 vacancies was formed predominantly via thermal evolution of atomic-sized vacancies under irradiation at high temperatures. It was found that formation of the activated vacancies occurred during irradiation, which leads to vacancy clustering.

Vacancy Evolution in Ni during Irradiation at High Temperatures Studied by in situ Positron Annihilation Spectroscopy. H.Tsuchida, T.Iwai, S.Kasai, H.Tanaka, N.Oshima, R.Suzuki, T.Yoshiie, A.Itoh: Journal of Physics - Conference Series, 2011, 262[1], 012060