The COSINE-100 dark matter search experiment is an array of NaI(Tl) crystal detectors located
in the Yangyang Underground Laboratory (Y2L). To understand measured backgrounds in the
NaI(Tl) crystals we have performed Monte Carlo simulations using the Geant4 toolkit and de-
veloped background models for each crystal that consider contributions from both internal and
external sources, including cosmogenic nuclides. The background models are based on compar-
isons of measured data with Monte Carlo simulations that are guided by a campaign of material
assays and are used to evaluate backgrounds and identify their sources. The average background
level for the six crystals (70 kg total mass) that are studied is 3.5 counts/day/keV/kg in the (2−6) keV energy interval. The dominant contributors in this energy region are found to be 210 Pb and
3 H.