Further improvements in the efficiency of low-energy positron moderators

Abstract

Low‐energy positrons are emitted by negative‐positron‐affinity moderator surfaces irradiated by the β⁺ spectrum of a radioactive source. The slow‐positron conversion efficiency ε (flux of slow positrons/total β⁺ activity) of a Cu(111) single‐crystal moderator increases 30% when the positron affinity is made more negative by exposure of the Cu to H₂S in situ. Upon cooling the moderator crystal to 100 K, ε increases an additional 50% to ε=(1.5±0.3)×10⁻³ using a low‐self‐absorption β⁺ source in a backscattering geometry.