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.