Positron lifetime measurements were carried out at temperatures which ranged from 300 to 5K. The positron lifetime of 189ps, at 300K, was found to decrease to 179ps at 5K. No change in lifetime was detected across the superconducting transition at about 16K. The observed magnitude of the lifetime, and its temperature variation, were considered with regard to theoretical estimates of the positron density distribution and the annihilation characteristics of a perfect crystal and of various plausible defect configurations. On the basis of the calculations, it was deduced that the lifetime and its temperature dependence were governed by positron annihilation from C vacancies.

C.S.Sundar, A.Bharathi, Y.Hariharan, T.S.Radhakrishnan, Z.Hossain, R.Nagarajan, L.C.Gupta, R.Vijayaraghavan: Physical Review B, 1996, 53[6], R2971-4