Developments in the MAJORANA DEMONSTRATOR Background Model

The MAJORANA DEMONSTRATOR neutrinoless double-beta decay experiment operated 44 kg of Ge detectors, of which 30 kg were enriched in the candidate isotope ⁷⁶Ge. The DEMONSTRATOR achieved one of the lowest background rates in the region of the 2039 keV Q-value, 15.7 cnts/(FWHM t y). This background rate, however, 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 unevenly distributed between the two modules of the DEMONSTRATOR 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 indicate 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 radioassays have further narrowed the suspected locations for the ²³²Th excess. A study of systematic uncertainties is also underway with the overall goal of completing a background model.