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The DEAP-3600 experiment is searching for WIMP dark matter with a 3.3 tonnesingle phase liquid argon (LAr) target, located 2.1 km underground at SNOLAB.The experimental signature of dark matter interactions is keV-scale $^{40}$Arnuclear recoils (NR) producing 128 nm LAr scintillation photons observed byPMTs. The largest backgrounds in DEAP-3600 are electronic recoils (ER) inducedby $\beta$ and $\gamma$-rays originating from internal and externalradioactivity in the detector material. A background model of the ERinteractions in DEAP-3600 was developed and is described in this work. Themodel is based on several components which are expected from radioisotopes inthe LAr, from ex-situ material assay measurements, and from dedicatedindependent in-situ analyses. This prior information is used in a Bayesian fitof the ER components to a 247.2 d dataset to model the radioactivity in thesurrounding detector materials. While excellent discrimination between ERs andNRs is reached with pulse shape discrimination, utilizing the large differencebetween fast and slow components of LAr scintillation light, detailed knowledgeof the ER background and activity of detector components, sets valuableconstraints on other key types of backgrounds in the detector: neutrons andalphas. In addition, the activity of $^{42}$Ar in LAr in DEAP-3600 isdetermined by measuring the daughter decay of $^{42}$K. This cosmogenicallyactivated trace isotope is a relevant background at higher energies for otherrare event searches using atmospheric argon e.g. DarkSide-20k, GERDA or LEGEND.The specific activity of $^{42}$Ar in the atmosphere is found to be $40.4 \pm5.9$ $\mu$Bq/kg of argon.

Electromagnetic Backgrounds and Potassium-42 Activity in the DEAP-3600 Dark Matter Detector