INTERACTION BETWEEN SLOW POSITRONS AND ATOMS

Relativistic many-body perturbation theory and a qualitative estimation of non-perturbative effects are used to calculate low energy scattering and annihilation of a positron with noble gas atoms. The existence of a virtual level (resonance) and an annihilation enhancement factor that is produced by the strong e+e- correlation, is demonstrated for heavy atoms (e.g. Xe and Rn). For molecules, (e+Xe(n), e+Rn(n), n greater-than-or-equal-to 2) the virtual level can transform into a real bound level for the positron. These results can explain the very high annihilation rate and non-linear dependence on atomic density observed in measurements of positron decay rates in dense xenon gas. The absence of such effects in a e+-Ne system completes the picture.