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Effect of helium irradiation fluence on HenVm cluster evolution in nickel studied by positron annihilation spectroscopy

Polycrystalline nickel was irradiated using 50 keV He ions at room temperature. The irradiated fluences were 5× 1013 He cm−2, 5× 1014 He cm−2, and 5× 1015 He cm−2, respectively. Positron annihilation Doppler broadening spectroscopy (DBS) was used to characterize the irradiation-induced defect evolution. The DBS results show that a large amount of vacancy defects were introduced in the specimens after helium irradiation. In addition, the DBS data could be also interpreted as the formation of helium-vacancy (HenVm) clusters due to combination between vacancies and helium atoms.