Defect studies in alpha-irradiated and deformed niobium by positron annihilation

The positron annihilation technique has been used to study the recovery of defects in alpha-irradiated Nb. A comparative study on plastically deformed Nb has also been performed. An antirecovery behaviour of Doppler broadening lineshape parameter, s, is observed only in alpha-irradiated specimens between 350-6000C. This has been identified to arise from the presence of He and to the formation of He-vacancy complexes. A further sharp rise in S occurring in both irradiated and deformed specimens at about 6000C is presumed to be due to the release of vacancies from dissociating vacancy-impurity complexes. Above 6500C, S decreased towards the reference value. In irradiated specimens this recovery is seen till 9500C in two stages, i.e. rearrangement in the configuration and dissociation of He-vacancy complexes, respectively. (author)