Field assisted positron moderation by surface charging of rare gas solids

The field assisted moderation of positrons (e + ) has been achieved by charging the surfaces of argon and krypton rare gas solid (RGS) films. Following deposition the RGS surface was coated with a thin layer of molecular oxygen which serves to trap some of the charge which was then applied by low energy electron bombardment. Energy spectra of both the low energy e + s and secondary electrons emitted from a charged argon surface were found to be consistent with the production of a surface potential of approximately -20 V. This resulted in a threefold enhancement of the slow e + yield and a reduction of around a factor of five in the emission of secondary electrons. Electron bombardment of RGS films not exposed to oxygen showed no such surface charge effects.