Vacancy recovery in irradiated niobium

Positron lifetime and annihilation lineshape measurements are used to study defect recovery in pure and H-doped Nb after low-temperature electron and neutron irradiation. In neutron-irradiated specimens correlated vacancy migration and the concomitant vacancy clustering within the collision cascades are observed around 160 K. However, in electron-irradiated Nb, no sign of vacancy annealing is seen until above 380 K where the vacancies agglomerate strongly. According to earlier ideas this is interpreted as being due to hydrogen impurities bound at vacancies. Vacancy migration and clustering are possible only after the dissociation of the vacancy-hydrogen pairs at 380 K. The results also indicate that hydrogen impurities play an important role in the stability of small three-dimensional vacancy clusters in Nb. (author)