Optimized growth conditions for solid Ar and Kr moderators in slow positron beam

Solid rare-gas moderators can be used for producing slow positrons. The most natural choices for this kind of moderators are Ar and Kr because they are easily available, and their solidification temperature can be achieved by a normal two-stage helium cryopump. In this work an encapsulated 22Na source and a cone-shaped moderator were installed in a helium cryopump for testing the solid Ar and Kr moderators. As a result we found a moderation efficiency of (9.5 ± 1.0) × 10−4 for Ar and (1.2 ± 0.1) × 10−3 for Kr. the yield of these moderators decreased 12% and 7%, respectively, in 10 h.