Positron trapping and annihilation at vacancies in BCC refractory metals

Theoretical calculations of the positron-vacancy binding energies in the BCC refractory metals Nb, Ta and W have been carried out using the authors' previously-developed supercell method (1977). The results indicate that the positron is sufficiently tightly bound to the vacancy in these metals that significant thermal detrapping is highly unlikely. Positron annihilation characteristics in the Bloch and vacancy-trapped states of the positron were also calculated for W. The general shapes of the core-electron contributions to the angular correlation curves in the two states were found to be different.