Modeling ²³²Th Backgrounds in the MAJORANA DEMONSTRATOR

The MAJORANA DEMONSTRATOR is a neutrinoless double beta decay (0νββ) experiment consisting of ~30 kg of germanium detectors enriched to 88% in ⁷⁶Ge and ~14 kg of natural germanium detectors. The detectors are divided between two cryostats and surrounded by a graded passive shield. The measured background of 11.9 ± 2.0 cts/(FWHM t y) at the double beta decay Q-value is among the lowest background rates in 0νββ experiments but is still more than a factor of four higher than the projected background rate based on component assays. This discrepancy arises from an excess of events from the ²³²Th decay chain. The location(s) of the excess can potentially be revealed by comparing data to simulations through studies of the relative intensities of high and low energy peaks, comparisons of peak rates between detectors, and background model fits. Indications that near-detector locations are not consistent with the observed background have informed design decisions for components being used in the next-generation LEGEND experiment. More specific information on the source of the excess could provide additional input to LEGEND and other low-background experiments. Recent findings have identified a few suspected locations for the excess activity, motivating additional simulations and assays.