Measurement of the ionization produced by sub-keV silicon nuclear recoils in a CCD dark matter detector

TL;DR

This study measures how silicon CCD detectors respond to very low-energy nuclear recoils, crucial for detecting low-mass dark matter particles, revealing deviations from existing models and demonstrating CCD sensitivity.

Contribution

First measurement of ionization efficiency for sub-keV silicon nuclear recoils in CCDs, showing deviations from Lindhard model extrapolation and confirming CCDs' potential in dark matter detection.

Findings

Ionization efficiency measured down to 60 eV electron equivalent.

Deviation from Lindhard model at low energies.

Demonstrated CCD sensitivity to low-energy nuclear recoils.

Abstract

We report a measurement of the ionization efficiency of silicon nuclei recoiling with sub-keV kinetic energy in the bulk silicon of a charge-coupled device (CCD). Nuclear recoils are produced by low-energy neutrons ($<$24 keV) from a $^{124}$Sb-$^{9}$Be photoneutron source, and their ionization signal is measured down to 60 eV electron equivalent. This energy range, previously unexplored, is relevant for the detection of low-mass dark matter particles. The measured efficiency is found to deviate from the extrapolation to low energies of the Lindhard model. This measurement also demonstrates the sensitivity to nuclear recoils of CCDs employed by DAMIC, a dark matter direct detection experiment located in the SNOLAB underground laboratory.