An assay-based background projection for the MAJORANA DEMONSTRATOR using Monte Carlo Uncertainty Propagation

The background index is an important quantity which is used in projecting and calculating the half-life sensitivity of neutrinoless double-beta decay (0νββ) experiments. A novel analysis framework is presented to calculate the background index using the specific activities, masses and simulated efficiencies of an experiment's components as distributions. This Bayesian framework includes a unified approach to combine specific activities from assay. Monte Carlo uncertainty propagation is used to build a background index distribution from the specific activity, mass and efficiency distributions. This analysis method is applied to the MAJORANA DEMONSTRATOR, which deployed arrays of high-purity Ge detectors enriched in ⁷⁶Ge to search for 0νββ. The framework projects a mean background index of [8.95 ± 0.36] × 10⁻⁴cts/(keV kg yr) from ²³²Th and ²³⁸U in the DEMONSTRATOR's components.