We construct the first comprehensive radioactive background model for a dark matter search with charge-coupled devices (CCDs). We leverage the well-characterized depth and energy resolution of the DAMIC at SNOLAB detector and a detailed geant4-based particle-transport simulation to model both bulk and surface backgrounds from natural radioactivity down to $50{\mathrm{eV}}_{\mathrm{ee}}$. We fit to the energy and depth distributions of the observed ionization events to differentiate and constrain possible background sources, for example, bulk ${}^3\mathrm{H}$ from silicon cosmogenic activation and surface ${}^{210}\mathrm{Pb}$ from radon plate-out. We observe the bulk background rate of the DAMIC at SNOLAB CCDs to be as low as $3.1\pm 0.6\mathrm{counts}{\mathrm{kg}}^{-1}{\mathrm{day}}^{-1}{\mathrm{keV}}_{\mathrm{ee}}^{-1}$, making it the most sensitive silicon dark matter detector. Finally, we discuss the properties of a statistically significant excess of events over the background model with energies below $200{\mathrm{eV}}_{\mathrm{ee}}$.

• Received 29 October 2021
• Accepted 22 February 2022

DOI:https://doi.org/10.1103/PhysRevD.105.062003

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

Published by the American Physical Society