Positron lifetime studies of pure Ni from 4.2 to 1700K

Positron annihilation lifetime measurements are made for Ni over a temperature range from 4.2 to 1700K. It is found that there is almost no change in the lifetime from 4.2 to 900K indicating a very small thermal-expansion effect. A small precursor effect is observed before the onset of significant vacancy trapping. A monovacancy formation enthalpy of 1.54 +0.2 -0.1 eV is extracted from an analysis in which divacancies are not taken into consideration. No detrapping from monovacancies is observed even at the higher temperatures. An analysis wherein E F is deduced from a 'threshold temperature' for the lifetime data produces a value lower than that obtained in similar analyses involving Doppler broadening or angular-correlation measurements. The lower value of the threshold temperature found in this work is largely attributable to the value for the precursor slope obtained from lifetime measurements, being smaller than the values obtained by use of the other two methods.