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

Abstract

Experiments aiming to directly detect dark matter through particle recoils can achieve energy thresholds of O(10 eV). In this regime, ionization signals from small-angle Compton scatters of environmental γ-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 γ-ray source over several months. Features associated with the silicon K, L1, and L2,3-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.

Confirmation of the excess of ionization events in DAMIC at SNOLAB with skipper CCDs

“…We thus assumed a flat background model below 0.5 keV 𝑒𝑒 . This is a conservative assumption, given the non-increasing nature of the tritium and Compton [5] spectra Cosmic muons cross CCDs from top to bottom and thus provide complete depth information on charge diffusion in the CCD. below 0.5 keV 𝑒𝑒 .…”

Shake-up and shake-off effects in neutrinoless double-beta decay

“…For experiments with the ββ isotope embedded in the detector's active region, these atomic de-excitation ejecta will generate signals and contribute to the collected event energy. Extremely low-energy ejecta may have their energy deposition quenched relative to higher-energy depositions by β's due to atomic physics that modifies scattering cross sections at the lowest incident energies [15]. However this only occurs at energies far below the best-available energy resolutions and can be neglected.…”