Modeling Backgrounds in the MAJORANA DEMONSTRATOR

The MAJORANA DEMONSTRATOR is a neutrinoless double-beta decay experiment containing ~30kg of p-type point contact germanium detectors enriched to 88% in ⁷⁶Ge and ~14kg of natural germanium detectors. The detectors are divided between two cryostats and surrounded by a graded passive shield. The DEMONSTRATOR achieved one of the lowest background rates in the region of the 0?????? Q-value, ~16 cnts/(FWHM t y). Nevertheless this background rate exceeds the projected background rate of 2.9 cnts/(FWHM t y). This discrepancy arises from an excess of events from the ²³²Th decay chain and is non-uniformly distributed between the two modules. Background model fits aim to understand this deviation from assay-based projections, potentially determine the source location(s) of observed backgrounds, and allow a precision measurement of the two-neutrino double-beta decay half-life. Fits indicate the origin of the ²³²Th excess is not from a near-detector component, which 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 in-situ assay campaign to complete the background model.