Positron lifetime computations of defects in nickel containing hydrogen or helium

Positron lifetime of defects in nickel containing hydrogen or helium has been calculated, using Monte-Carlo, density-functional theory and local-density approximation. It is found that the values of positron lifetime in voids are lowered by the presence of hydrogen or helium. The calculations show a correlation between the microvoid size in nickel and the value of the positron lifetime. With the growth of the void size the difference in the positron lifetime for clusters containing hydrogen or helium becomes significant. Calculations show a saturation of the positron lifetime for microvoids containing more than 45 hydrogen atoms and around 30 helium atoms. A dependence between the positron lifetime and the ratio of hydrogen or helium atoms to the number of vacancies in nickel voids is established.