Characteristics of condensed gas moderators for the generation of very slow polarized muons

Motivated by the possibility of using condensed gas moderators to produce very slow (epithermal) polarized muons we have studied the moderator properties of rare gas solids and solid N2 thin films as a function of the growing parameters (deposition temperature and rate, heat treatment) and of the film thickness. For Ar and Kr moderators the moderation efficiency is found to depend on the growth temperature and an annealing effect is observed. The dependence can be interpreted in terms of the changes in granularity and porosity of the condensed gas layers when the growth parameters are varied. From the thickness dependence of the moderation efficiency the escape depth of the very slow muons was determined. Its large value indicates a suppression of electronic energy loss mechanisms for the epithermal muons. The time dependent decay properties of the moderators were also investigated and found to depend solely on the rest gas pressure. At a pressure of 10−10 mbar the moderation efficiency remains stable over a period of days.