The temperature dependence of the diffusion constant of positrons in Ge single crystals is discussed. Experimental data on the fraction (fp, ) of 0 -5-keV incident positrons reemitted from the surface as positronium are presented for Ge over the temperature range 300& T & 1020 K. The positron diffusion constant is deduced, at various temperatures, from an analysis using a onedimensional diffusion model for fp, which is determined as a function of incident positron energy E. The magnitude and temperature dependence of the positron diffusion constant are compared with the predictions of the conventional weak carrier-phonon scattering theory that is used to describe the mobilities of electrons and holes in the elemental semiconductors.Conventional carrier-phonon scattering does not adequately represent the observed temperature dependence of the positron diffusion constant, and we investigate some possible origins of these discrepancies. Quantitative calculations identify several of these possibilities as implausible. We tentatively conclude that some form of positron-lattice coupling is the dominant interaction governing the positron motion.

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An electrostatically guided positron beam which is variable in energy from 0 to 60 keV is described. A frozen inert gas (Ar, Kr, and Xe) is used as the positron moderator. The efficiency, ε, defined as the ratio of the number of slow positrons at the target to the number of positrons emitted by the source has been measured; the values obtained for Ar, Kr, and Xe were ε∼ 2.5×10−4, 1.6×10−4, and 1.1×10−4, respectively. We estimate a source/moderator efficiency of εm∼7.6×10−4, 4.7×10−4, and 3.0×10−4 for Ar, Kr, and Xe, respectively, which includes source effects, grid transmission, geometrical losses, and beam transport. These values are comparable to the best reported values for thin metal foil moderators. The measured εm is specific to the electrostatic system; significantly higher values for the same moderator should be attainable in magnetically guided beams, where higher transmission for moderators with large energy spreads is usually obtained.

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