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&cause of a strong prevacancy effect in cadmium the assumptions of the trapping model seem t o be fulfilled rather badly. Therefore the formation enthalpy of monovacancies is also determined by methods using the trapping model only to some extent or not a t all. Hf,is obtained as IntroductionSince the positron lifetime is a measure of the overlapping of the positron-electron wave function in crystal lattices, its value depends sensitively on the concentration of electrons. The vacancies which appear in the crystal, act as an attractive potential to the positrons and will, therefore, trap them forming a bound state [l]. As a result of the smaller electron concentration in the trap the expected second lifetime will be considerably longer than the lifetime in the crystal bulk; in addition, the intensity of this component increases with increasing vacancy concentration. This phenomenon may be described by a simple kinetic model (trapping model) [l, 21. According to the two-state trapping model one obtains for the temperature dependence of the mean lifetime Z a characteristic S-shape, with the limiting values -ctl (lifetime in the

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