Solid neon moderator for producing slow positrons

Slow positrons can be obtained by moderating the energetic β+ particles from a radioactive source. We find that solid Ne makes a more efficient moderator than any other material known to date. The efficiency ε, defined as the number of slow positrons per β+ emitted by the source, is (0.30±0.02)% for a flat layer of Ne covering a 22Na deposit. In a cylindrical geometry, ε is (0.70±0.02)%, more than twice the previous best efficiency obtained with single-crystal tungsten. The energy spectrum for Ne has a full width at half-maximum of 0.58 eV, somewhat broader than the spectrum of positrons from a single-crystal metal. Moderators made from the other solid rare gases have a much lower efficiency and a larger energy spread.