Further improvements in the efficiency of low-energy positron moderators

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.