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The reach of sub-GeV dark-matter detectors is at present severely affected bylow-energy events from various origins. We present the theoretical methods tocompute the single- and few-electron events that arise from secondary radiationemitted by high-energy particles passing through detector materials and performsimulations to quantify them at (Skipper) CCD-based experiments, focusing onthe SENSEI data collected in the MINOS cavern at Fermilab. The simulationsaccount for the generation of secondaries from Cherenkov and luminescentrecombination; photo-absorption, reflection, refraction and thin-filminterference in detector materials; roughness of the interfaces and thedynamics of charges and partial charge collection (PCC) in the dopedCCD-backside. We consider several systematic uncertainties, notably thosestemming from the backside charge-diffusion modeling, which we estimate with a"fiducial'' and an "extreme'' model, with the former model presenting betteragreement with PCC data. We find that Cherenkov photons constitute about 40% ofthe observed single-electron events for both models; radiative recombinationrates are negligible for the fiducial model, but can dominate over theCherenkov rates for the extreme model. We also estimate the fraction of2-electron events from 1-electron event same-pixel coincidences, finding thatthe entire 2-electron rate can be explained by coincidences of radiative eventsand spurious charge. Accounting for backgrounds, we project the sensitivity offuture Skipper-CCD-based experiments to different dark-matter models. Forlight-mediator models with dark-matter masses of 1, 5, and 10 MeV, we find thatfuture experiments with 10-kg-year exposures and successful backgroundmitigation could have a sensitivity that is larger by 9, 3, and 2 orders ofmagnitude, respectively, when compared to an experiment without backgroundimprovements. (abridged)

Low-Energy Radiative Backgrounds in CCD-Based Dark-Matter Detectors