γ spectra for positron annihilation in noble-gas atoms are calculated
using many-body theory for positron momenta up to the positronium-formation
threshold. This data is used, together with time-evolving positron-momentum
distributions determined in [arXiv:1706.01434 (2017)], to calculate the
time-varying γ spectra produced during positron cooling in noble gases.
The γ-spectra and their Sˉ and Wˉ shape parameters are
shown to be sensitive probes of the time evolution of the positron momentum
distribution, and thus provide a means of studying positron cooling that is
complementary to positron lifetime spectroscopy.Comment: This version contains minor update to references, otherwise unchange