SOLID KR MODERATOR FOR PRODUCING SLOW POSITRONS

We find that proper annealing of a solid Kr positron moderator results in a fast positron to slow positron conversion efficiency 90% as good as that of solid Ne. Our results are in agreement with Grund, Maier, and Seeger [Mater. Sci. Forum 105-110, 1879 (1992)] except that our absolute efficiencies (almost-equal-to 0.2%) are higher presumably due to the better vacuum conditions of our experiment. Since the Kr does not have to be cooled below 10 K, the moderator can be made using an inexpensive refrigerator and is suitable for a large surface area intense positron source.