The recent resurgence of interest in positron transport in gases has been driven both by new fundamental positron-atom/molecule cross scattering sections [1, 2], and by the richness and novelty of the associated transport phenomena [3, 4]. In particular the phenomenon of negative differential conductivity (NDC) induced by positronium formation continues to be a focus of attention of kinetic theorists and modellers [5, 6], and there are other interesting effects yet to be explained. Although this work in gaseous systems is largely motivated by intrinsic physical interest, there are a number of important technological and medical applications (e.g. PET scans) which provide an additional imperative for such studies and in particular their extension to dense systems (liquid\ud
and soft-condensed), the subject of this presentation. Investigations of positron transport in dense systems so far have been very limited [7]. One can draw to some extent on the extensive transport theory literature for electrons in dense gases and liquids [8], and both electrons and positrons in gases, but there is no straightforward way of directly adapting this existing transport theory: A new theory is needed, in which the effects of both non-reactive coherent scattering by many atoms in the dense gaseous and liquid phases, and reactive collisions are accounted for, through a dynamic structure factor S(K,W) and the positronium formation at cross section respectively
