The vacancy formation enthalpy in Ni determined by positron annihilation

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Abstract
The monovacancy formation enthalpy, H1vF, was determined for Ni using the Doppler broadening technique. The 58Co positron source was ion implanted in Ni of 99.995 wt.% purity. A 103Ru gamma -ray line (497 keV) was recorded simultaneously with the positron annihilation 511 keV line, and was used in stabilising the data. Measurements were performed on three similar Ni samples, which were annealed in an ultra-high vacuum chamber (10-8-10-9 Torr) at approximately 30 degrees C below their melting point for at least two hours prior to the in situ measurements. For one of the samples, a comparison was made of H1vF values deduced prior to and after in situ melting. A value of H1vF=1.8+or-0.1 eV was determined from the present data. In addition, a value for the monovacancy migration enthalpy, H1vM=1.1+or-0.1 eV, has been deduced from a comparison of the present H1vF measurement with previous self-diffusion results.