Characterization of the background spectrum in DAMIC at SNOLAB

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 eV(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 H from silicon cosmogenic activation and surface Pb-210 from radon plate-out. We observe the bulk background rate of the DAMIC at SNOLAB CCDs to be as low as 3.1 +/- 0.6 counts kg(-1) day(-1) keV(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 eV(ee).