Positron cooling and annihilation in noble gases

Abstract

Understanding the dynamics of positron cooling in gases, including the fraction of positrons surviving to thermalisation, is critical for accurate interpretation of positron lifetime spectra, for the development of efficient positron cooling in traps and accumulators, and for a cryogenically cooled, ultra-high-energy-resolution, trap-based positron beam. Here, positron cooling and annihilation in noble gases is simulated using accurate scattering and annihilation cross sections calculated \emph{ab initio} with many-body theory. It is shown that a strikingly small fraction of positrons survive to thermalisation: $\sim$0.1 in He, $\sim$0 in Ne (due to cooling effectively stalling in the relatively deep momentum-transfer cross-section minimum), $\sim$0.15 in Ar, $\sim$0.05 in Kr and $\sim$0.01 in Xe. For Xe, the time-varying annihilation rate $\bar{Z}_{\rm eff}(\tau)$ is shown to be highly sensitive to the depletion of the distribution due to annihilation, conclusively explaining the long-standing discrepancy between gas-cell and trap-based measurements in Xe. The \emph{ab initio} calculations enable the first simultaneous probing of the energy dependence of the the scattering cross section and annihilation rate. Overall, the use of the accurate atomic data gives $\bar{Z}_{\rm eff}(\tau)$ in close agreement with experiment for all noble gases except Ne, the experiment for which is proffered to have suffered from incomplete knowledge of the fraction of positrons surviving to thermalisation and/or the presence of impurities.