Recent Progress in the MAJORANA DEMONSTRATOR Background Model

The MAJORANA DEMONSTRATOR was a neutrinoless double-beta decay experiment that operated 44 kg of Ge detectors, of which 30 kg were enriched in the candidate isotope ⁷⁶Ge. The DEMONSTRATOR concluded in March 2021 and set a 0νββ half-life limit of T_1/2 > 8.3×10²⁵ yrs based on its full exposure. The DEMONSTRATOR also achieved one of the lowest background rates in the region of the 2039 keV Q-value, 15.7 cnts/(FWHM t y). However, this background rate is significantly higher than the rate of 2.9 cnts/(FWHM t y) projected by material assays and simulations. An excess of ²³²Th decay chain events has been observed. 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 confirm the origin of the ²³²Th excess is not from near-detector components, which informed design decisions for the next-generation LEGEND experiment. Recent findings from background model fits and material assays have suggested some potential sources of the ²³²Th excess. Studies of systematic uncertainties are ongoing, and will help complete a background model.