Precision measurement of Compton scattering in silicon with a skipper CCD for dark matter detection

TL;DR

This study precisely measures low-energy Compton scattering in silicon using a skipper CCD, revealing discrepancies with standard models and providing data crucial for dark matter detection experiments.

Contribution

It provides the first experimental validation of Compton scattering spectra at energies below 100 eV in silicon, challenging existing simulation models.

Findings

Relativistic impulse approximation does not match measured spectra below 0.5 keV.

Detected scattering on valence electrons below 100 eV for the first time.

Data aligns better with ab initio calculations than with standard Monte Carlo models.

Abstract

Experiments aiming to directly detect dark matter through particle recoils can achieve energy thresholds of $\mathcal{O}(1\,\mathrm{eV})$. In this regime, ionization signals from small-angle Compton scatters of environmental $\gamma$-rays constitute a significant background. Monte Carlo simulations used to build background models have not been experimentally validated at these low energies. We report a precision measurement of Compton scattering on silicon atomic shell electrons down to 23$\,$eV. A skipper charge-coupled device (CCD) with single-electron resolution, developed for the DAMIC-M experiment, was exposed to a $^{241}$Am $\gamma$-ray source over several months. Features associated with the silicon K, L$_{1}$, and L$_{2,3}$-shells are clearly identified, and scattering on valence electrons is detected for the first time below 100$\,$eV. We find that the relativistic impulse approximation for Compton scattering, which is implemented in Monte Carlo simulations commonly used by direct detection experiments, does not reproduce the measured spectrum below 0.5$\,$keV. The data are in better agreement with $ab$ $initio$ calculations originally developed for X-ray absorption spectroscopy.