Studies of positron moderation in surface charged rare gas solids

With the use of surface charging the efficiency of a rare gas solid (RGS) positron (e+) moderator has been determined as a function of applied electric field. This technique relies on electron (e-) capture by adsorbed molecular species such as oxygen. These studies have shown that the emitted e+ yield from an RGS moderator can be enhanced by at least a factor of three by negative surface charging or almost totally reduced by positive surface charging. Even under ultra high vacuum conditions significant surface charging of RGS films can occur and such effects should be considered if moderating materials of this type are to be used. (orig.)