A new radiochemical method for determining the concentration of F centres in X-or y-ray coloured crystals of alkali halides is described. The method is based on the fact that
A spin site exchange mechanism for metal ammonia solutionsThe time spectrum of positrons annihilated in lithium-ammonia solutions of various concentrations was analyzed. It was found that the decay rate of the longest lived component increases at the same time as the Li concentration increases; likewise its intensity increases until it attains 100% at high concentration. The results are interpreted in terms of electron exchange collisions which convert triplet positronium to singlet and vice versa. The triplet to singlet conversion rate y was found proportional to the number of Li atoms dissolved per unit volume.
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