Trapping of hydrogen impurities in helium-implanted niobium and tantalum

Trapping of hydrogen impurities at helium precipitates in helium-implanted niobium and tantalum has been studied. The samples were implanted at room temperature with 100 keV 4He+ ions to doses of (0.6–36)×1016 cm−2. Some of the samples were postirradiated with 60 keV and 4 MeV protons, and annealed at elevated temperatures up to 1070 K. The helium and hydrogen concentration distributions were characterized simultaneously using the elastic-recoil-detection-analysis technique. The hydrogen distributions were also measured by the nuclear resonance reaction 1H(15N,αγ) 12C. The observed hydrogen distributions show that defect-hydrogen complexes at He bubbles are built from the ion-irradiation-induced and pre-existing vacancies and pre-existing hydrogen impurities migrated to the associated internal surfaces and that the hydrogen impurities saturate the surfaces. Recovery energies of about 2.1 eV in Nb and about 2.7 eV in Ta were observed for the hydrogen trapping defects.