  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.
