Positron lifetimes of defects in Ni containing H or He were calculated, using Monte Carlo, density-functional theory and local-density approximations. It was found that the values of the positron lifetime in voids were lowered by the presence of H or He. The calculations showed a correlation between the micro-void size in Ni and the value of the

positron lifetime. With the growth of the void size the difference in the positron lifetime for clusters containing H or He became significant. Calculations showed a saturation of the positron lifetime for micro-voids containing more than 45 hydrogen atoms and around 30 He atoms. A dependence between the positron lifetime and the ratio of H or He atoms to the number of vacancies in Ni voids was established.

Positron Lifetime Computations of Defects in Nickel Containing Hydrogen or Helium. B.L.Shivachev, T.Troev, T.Yoshiie: Journal of Nuclear Materials, 2002, 306[2-3], 105-11