Modeling Backgrounds for the MAJORANA DEMONSTRATOR

. The M AJORANA D EMONSTRATOR is a neutrinoless double-beta decay (0 νββ ) experiment containing ∼ 30 kg of p-type point-contact germanium detectors enriched to 88% in 76 Ge and ∼ 14 kg of natural germanium detectors. The detectors are housed in two electroformed copper cryostats and surrounded by a graded passive shield with an active muon veto. An extensive radioassay campaign was performed prior to installation to insure the use of ultra-clean materials. The D EMONSTRATOR achieved one of the lowest background rates in the region of the 0 νββ Q-value, 15.7 ± 1.4 cts/(FWHM t y) from the low-background configuration spanning most of the 64.5 kg-yr active exposure. Nevertheless this background rate is a factor of five higher than the projected background rate. This discrepancy arises from an excess of events from the 232 Th decay chain. Background-model fits aim to explain the deviation from assay-based projections, potentially determine the source(s) of observed backgrounds, and allow a precise measurement of the two-neutrino double-beta decay half-life. The fits agree with earlier simulation studies, which indicate the origin of the 232 Th excess is not from a near-detector component and have informed design decisions for the next-generation LEGEND experiment. Recent findings have narrowed the suspected locations for the excess activity, motivating a final simulation and assay campaign to complete the background model.