Study of the Mg-Cd system by positron annihilation methods

The problem of preferential positron annihilation in binary alloys has not been satisfactorily solved in the past. We examine this effect experimentally in Mg-Cd alloys using the new technique of Doppler broadening measurements with background reduction which allows us to observe positron annihilation with core electrons. Conventional positron lifetime spectroscopy is applied as well. We measure the Doppler spectra and positron lifetimes for selected alloys in the whole concentration range of the Mg-Cd system. The analysis of experimental results is supported by theoretical calculations. The conclusion is given that a small positron preferential annihilation occurs at Mg sites. Besides, the temperature dependencies of the peak counting rate of the angular correlation curve are measured for several samples and the vacancy formation energies are established. In the case of Mg-20 at.% Cd alloy, the increase of the peak counting rate characteristic for generation of thermal vacancies shows a discontinuity close to the order-disorder transition temperature. (author)