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 ex perimentally in Mg-Cd alloys using the new technique of Doppler broade ning measurements with background reduction which allows us to observe positron annihilation with core electrons. Conventional positron life time spectroscopy is applied as well. We measure the Doppler spectra a nd positron lifetimes for selected alloys in the whole concentration r ange of the Mg-Cd system. The analysis of experimental results is supp orted by theoretical calculations. The conclusion is given that a smal l positron preferential annihilation occurs at Mg sites. Besides, the temperature dependencies of the peak counting rate of the angular corr elation curve are-measured for several samples and the vacancy formati on energies are established. In the case of Mg-20 at.% Cd alloy, the i ncrease of the peak counting rate characteristic for generation of the rmal vacancies shows a discontinuity close to the order-disorder trans ition temperature.