A Radiogenic Background Model for the MAJORANA DEMONSTRATOR

Abstract

Neutrinoless double-beta decay (0vbb) is a lepton-number-violating process whose existence would indicate that neutrinos are Majorana fermions. The MAJORANA collaboration is searching for 0vbb in germanium-76 using a modular array of high-purity germanium (HPGe) detectors with support and shielding constructed from low-background materials and housed at the 4850' level of the Sanford Underground Research Facility in Lead, South Dakota. The two modules of the experiment contain 44.8 kg of p-type point-contact high-purity germanium detectors, 29.7 kg of which are enriched in germanium-76. Both modules have been in operation since August 2016, and with 26 kg-yr of exposure have achieved a limit of 2.7*10^25 yr on the decay half life. The DEMONSTRATOR has achieved an excellent energy resolution of 0.1% FWHM at the 2039 keV region-of-interest (ROI), and has among the lowest ROI backgrounds of current generation 0vbb searches. Although the MAJORANA DEMONSTRATOR has achieved these low backgrounds, trace amounts of radioactivity remain detectable in the array. This work will present the results of an in-depth study of the backgrounds visible to the array, and the details behind a model for the remaining radiogenic backgrounds. The background model is a simultaneous fit of simulated energy spectra to groups of detectors and data sets, and has been validated with comparisons to Monte Carlo generated and detector calibration data.