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 obtaining better conversion efficiencies, namely by use of the field assisted positron technique. The basic principles of field assisted moderation are first reviewed. Theoretical estimates of possible expected positron yields from various semiconductor based field assisted moderators are then presented, the general aim being to give some guidance to experimentalists seeking to test such potential moderator systems. The calculations are based on positron affinities calculated using density functional ab initio pseudopotential theory and are found to be in good accord with experimental data where 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 positron band states that favor positron emission. However, these materials also suffer from short positron lifetimes and low densities. In contrast, materials such as GaP, GaAs and ZnSe are also favored as positron emitters albeit with lower emission energy, but these materials have the advantage of longer lifetimes and higher densities that favor more efficient moderation.link_to_subscribed_fulltex