Vacancies in thermal equilibrium were investigated by measuring the Doppler broadening of the $2\gamma$ positron-annihilation line of Ta from 35 K to the melting temperature. Experimental evidence for detrapping of positrons from vacancies at high temperatures was found. The consequences of the values obtained for monovacancy-formation and -migration enthalpies, ${H_{\mathrm{IV}}}^F\approx 2.2$ eV, ${H_{\mathrm{IV}}}^M\approx 1.9$ eV, for the interpretation of radiation damage and cold-work experiments are discussed.

• Received 29 December 1976

DOI:https://doi.org/10.1103/PhysRevLett.39.484