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 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 at the 2039 keV Q-value of 6.23×10^-3 cnts/(keV kg y). However, this background rate is significantly higher than the rate of 1.17×10^-3 cnts/(keV kg y) projected by material assays and simulations. Spectral fits and supplementary background studies indicate that this discrepancy primarily arose from an excess of events from the ²³²Th decay chain. This excess was non-uniformly distributed between the DEMONSTRATOR’s two modules and was dominated by sources outside the detector array. Results of background model fits and a measurement of the 2νββ half-life, incorporating studies of sources of systematic uncertainties, is presented.