Positron-annihilation $S$-parameter measurements in thermal equilibrium on pure and carbon-doped (50 and 750 at. ppm) $\alpha$-iron are presented. It is shown that trapping of positrons in both monovacancies and carbon-vacancy pairs occurs, even far above the dissociation temperature of the vacancy pairs. Therefore, a three-state trapping model is used in the analysis of the measured $S$ curves. The vacancy-formation enthalpy in both the paramagnetic and ferromagnetic state is deduced: It is found to be 1.79 ± 0.10 eV in the paramagnetic state and 2.0 ± 0.2 eV in the ferromagnetic state. These values are larger than those published so far. The activation enthalpy for vacancy migration obtained by combining the values cited above with recently published self-diffusion enthalpy values confirms the applicability of the one-interstitial model in $\alpha$-iron.

• Received 8 November 1982

DOI:https://doi.org/10.1103/PhysRevB.27.5257