Positron annihilation in thermal equilibrium on pure and carbon doped α-iron

Positron annihilation measurements in thermal equilibrium on extermely pure α-iron (total interstitial impurity content -- 5 at.ppm.) and on 50 at.ppm carbon doped α-iron are presented. In pure iron, no positron trapping is observed in the ferromagnetic state. Consequently, the vacancy formation enthalpy in this state cannot be determined from the analysis of equilibrium data. However, in carbon-doped iron trapping is already observed at 7000C. This is interpreted in terms of localization of the positron at other sites than single vacancies, presumably carbon-vacancy pairs. (author)