# Measurement of low energy ionization signals from Compton scattering in a CCD dark matter detector

## TL;DR

This paper presents detailed measurements of low-energy Compton scattering spectra in silicon CCDs, improving background understanding for low-mass dark matter detection and providing a new parametrization for such spectra.

## Contribution

It provides the first accurate measurement of atomic-structure features in low-energy Compton spectra in silicon and develops a predictive model for background estimation in dark matter experiments.

## Key findings

- Spectra agree with theoretical predictions
- Atomic structure features are accurately measured
- A new parametrization for low-energy Compton spectra is derived

## Abstract

An important source of background in direct searches for low-mass dark matter particles are the energy deposits by small-angle scattering of environmental $\gamma$ rays. We report detailed measurements of low-energy spectra from Compton scattering of $\gamma$ rays in the bulk silicon of a charge-coupled device (CCD). Electron recoils produced by $\gamma$ rays from $^{57}$Co and $^{241}$Am radioactive sources are measured between 60 eV and 4 keV. The observed spectra agree qualitatively with theoretical predictions, and characteristic spectral features associated with the atomic structure of the silicon target are accurately measured for the first time. A theoretically-motivated parametrization of the data that describes the Compton spectrum at low energies for any incident $\gamma$-ray flux is derived. The result is directly applicable to background estimations for low-mass dark matter direct-detection experiments based on silicon detectors, in particular for the DAMIC experiment down to its current energy threshold.