ELECTRIC-FIELD-ASSISTED MODERATOR FOR GENERATION OF INTENSE LOW-ENERGY POSITRON BEAMS

In this paper we reexamine an old idea of using an electric-field-assi sted moderator (FAM) based on a rare-gas solid (RGS) to form intense l ow-energy positron beams. Contrary to common belief, the main body of a successful FAM should consist of a dirty (a high level of molecular impurities) RGS which will allow for a high e(+) drift velocity. On to p of this layer is condensed a thin layer of a highy-purity RGS. When the e(+) enters this latter region its energy will heat up, and e(+) e mission into vacuum will become possible despite a positive e(+) affin ity of the RGS. Simple calculations show that the e(+) intensity can b e increased by a factor of 100 over what can be achieved using a RGS m oderator without an applied electric field and, equally important, the e(+) transverse energy is reduced to less than 0.3 eV. This leads to a gain in brightness by a factor of 1000.