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Defect studies of Nb irradiated with 10 MeV electrons were performed in the present work by means of positron annihilation spectroscopy. The lattice defects were characterized by positron lifetime spectroscopy. Moreover, defect depth profiles were studied by slow positron implantation spectroscopy. The experimental investigations were accompanied by first principles theoretical calculations of positron parameters. It was found that irradiationinduced vacancies in Nb specimens are surrounded by H, which causes a shortening of the lifetime of trapped positrons. The influence of a Pd and Cr over-layer on the H concentration in the Nb specimens was examined.

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The aim of the present work was to investigate the microstructure of bulk niobium irradiated by 10 MeV electrons. Positron-annihilation spectroscopy was employed as a principal technique for the characterization of irradiation-induced defects. Experimental results were compared to first-principles theoretical calculations of positron characteristics. In addition to extended positron-annihilation studies, the specimens were characterized also by x-ray diffraction. It was found that irradiation-induced vacancies are surrounded by hydrogen. Complexes consisting of a Nb vacancy surrounded by one and two H atoms were identified in the irradiated specimens. The concentration of these vacancy-hydrogen complexes was estimated to be ͑18-24͒ ϫ 10 −5 at. %. Vacancy-2H complexes are found to represent the dominating type of defects. Hydrogen atoms surrounding a Nb vacancy cause a shortening of the lifetime of trapped positrons. Moreover, it was demonstrated that hydrogen attached to Nb vacancy can be identified by coincidence Doppler broadening technique. The effect of a thin Pd ͑or Cr͒ overlayer on the irradiation-induced defects was investigated also. It was found that the relative fraction of vacancy-2H complexes is higher in the specimens irradiated with such an overlayer.