Rare gas moderated electrostatic positron beam

Abstract

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 ε_m∼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 ε_m 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.