A theoretical search for possible high efficiency semiconductor based field assisted positron moderators

The need to obtain more intense positron beams is an ever-pressing concern in experimental positron physics. The present paper explores one route of o btaining better conversion efficiencies, namely by use of the field assiste d positron technique. The basic principles of field assisted moderation are first reviewed. Theoretical estimates of possible expected posit on yields from various semiconductor based field assisted moderators are then presen ted, the general aim being to give some guidance to experimentalists seekin g to test such potential moderator systems. The calculations are based on p ositron affinities calculated using density functional ab initio pseudopote ntial theory and are found to be in good accord with experimental data wher e such exist. The work suggests that wide-gap materials such as GaN, C and SiC tend to compress the positron wavefunction leading to higher energy pos itron band states that favor positron emission. However, these materials al so suffer from short positron lifetimes and low densities. In contrast, mat erials such as GaP, GaAs and ZnSe are also favored as positron emitters alb eit with lower emission energy, but these materials have the advantage of l onger lifetimes and higher densities that favor more efficient moderation. (C) 1999 Elsevier Science B.V. All rights reserved.