Installation of a Kr moderator in the high-brightness beam at Brandeis

A solid Kr moderator has been installed in the high-brightness beam at Brandeis. Its raw moderation efficiency for the an encapsulated 22Na source has been found to be as high as 4.0 × 10−4. The longitudinal energy spread of the re-emitted slow positrons has been measured by a high-resolution (0.1 eV) parallel plane geometry retarding field analyser comprised of the source plane and a parallel grid and is found to be 2.5 ± 0.1 eV. This value can be regarded as intrinsic to the positron moderation by solid Kr and not an artifact of the energy analyser. Implications of these measurements with regard to electrostatic focusing of positron beams are discussed.