Positron lifetime calculation of vacancy clusters in tantalum containing hydrogen and helium

Tantalum (Ta)-cladded tungsten (W) is predominantly used in a spallation target to obtain a high yield of neutrons. Investigation of changes in the mechanical properties and evolution of microstructures in Ta under irradiation of high-energy neutrons is essential to maintain the safety of the target of the spallation neutron source. In this study, the effects of helium (He) and hydrogen (H) on the positron lifetimes of vacancy clusters were examined. The calculation results indicate that the positron lifetime of perfect Ta is 118.3 ps and the positron lifetime is 216.5 ps when single vacancies exist. The positron lifetime increases with increase in the number of vacancies in the vacancy cluster. The positron lifetime of single vacancies decreases to 191.7 ps when the vacancy contains an H atom, whereas it decreases to 158.6 ps when the vacancy contains a He atom. This shows that the effect of He on positron lifetime change is larger than that of H.