Speaker
Description
The MAJORANA DEMONSTRATOR is a neutrinoless double-beta decay experiment located at the Sanford Underground Research Facility (SURF) in Lead, South Dakota. The DEMONSTRATOR contains an array of 44 kg of Ge detectors, of which 30 kg are enriched in the neutrinoless double-beta decay candidate isotope $^{76}$Ge. From the initial 26 kg yrs of exposure, the count rate in the background estimation window was found to be 11.9 $\pm$ 2.0 counts/(FWHM t y), among the smallest background rates for 0υββ experiments, although a factor of 4 above what was projected based on radioassays. This has motivated multiple studies into the background model for the DEMONSTRATOR to understand possible sources for the excess background. In addition to simulations of the standard configuration, simulations of the MAJORANA DEMONSTRATOR configuration post hardware upgrade were performed. Some potential sources of background excess, not previously considered as significant in the original model, were identified; $^{232}$Th and $^{238}$U decay chain contamination in those sources was simulated and assessed. Assay campaigns following the decommissioning of Module 1 have been used to investigate hypotheses for sources of background excess by comparing to simulation results. Additionally, maximum likelihood fits using spectra from simulations of different decay chains in different component groups, including the new sources, have been performed. These fits will inform design considerations for the next generation neutrinoless double-beta decay experiment LEGEND.
Acknowledgements: This material is supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, the Particle Astrophysics and Nuclear Physics Programs of the National Science Foundation, and the Sanford Underground Research Facility.
| Collaboration | MAJORANA Collaboration |
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