Abstract :
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. q1999 Elsevier Science B.V. All rights reserved
