Positron Annihilation in Nearly Perfect Crystals of Aluminum during Slow Cooling

Abstract

Doppler-broadening lineshapes of positron annihilation in nearly perfect crystals of aluminum were measured as a function of temperature during cooling down from 650 K to room temperature with various cooling rates, 1.9–27.4 K/h. The lineshape parameter S reflected the existence of excess vacancies depending on the cooling rate, and exhibited a linear temperature dependence below about 500 K. Its associated temperature coefficient was larger than the corresponding quantity for the equilibrium measurements of S in the prevacancy region, and increased with the excess vacancy concentration. The results, in conjunction with the data for the quenched-state below 200 K, suggested that the temperature dependence of S for the vacancy-trapped state of the positron is approximately linear, over the range 100–500 K, with a temperature coefficient about twice that found for the free state.