# Probing positron cooling in noble gases via annihilation $\gamma$ spectra

## TL;DR

This paper calculates gamma spectra from positron annihilation in noble gases to study positron cooling dynamics, revealing that spectral shape parameters effectively track the evolution of positron momentum during cooling.

## Contribution

It introduces a method combining many-body theory and time-dependent spectra to probe positron cooling, offering a new approach complementary to lifetime spectroscopy.

## Key findings

- Gamma spectra are sensitive to positron momentum evolution.
- Shape parameters effectively monitor positron cooling.
- Method provides new insights into positron interactions in noble gases.

## Abstract

$\gamma$ 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 $\gamma$ spectra produced during positron cooling in noble gases. The $\gamma$-spectra and their $\bar{S}$ and $\bar{W}$ 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.