Searching for neutrinoless double-beta decay of germanium-76 in the presence of backgrounds

Abstract

The neutrino, an elementary particle, has been the subject of experimental investigation for over 50 years. Recent experiments have shown that neutrinos have mass and oscillate, but questions about fundamental properties of the neutrino remain. The observation of neutrinoless double-beta decay could determine whether lepton number is violated, discover whether the neutrino is its own antiparticle, and provide information about the absolute scale of neutrino mass. The MAJORANA Collaboration will search for neutrinoless double-beta decay of germanium-76 in an array of germanium detectors. Previous experiments indicate that the half life of this decay mode is greater than 10^25 years. To be sensitive to this rate, MAJORANA must construct an ultra-low-background detector. MAJORANA is building the DEMONSTRATOR, a 40-kg detector array, at the Sanford Underground Research Facility in Lead, South Dakota. The neutrinoless double-beta decay of 76-Ge would produce 76-Se and two electrons with 2039 keV of energy. The physics reach of the DEMONSTRATOR will be determined by the background count rate in a 4-keV energy region surrounding the 2039-keV Q-value. MAJORANA has a background goal of less than three counts in the energy region of interest per tonne-year of DEMONSTRATOR exposure. Projections of the DEMONSTRATOR's sensitivity are determined from a background energy-spectrum model based on material assay data and Monte Carlo simulation results. Understanding and minimization of backgrounds is critical to the success of the DEMONSTRATOR. MAJORANA Collaborators operate a low-background detector in a shielded environment at the Kimballton Underground Research Facility near Ripplemeade, Virginia. The contents of the detector cryostat are well known, making it a good candidate for testing the MAJORANA background model. This dissertation describes the creation of a background energy- spectrum model for the Kimballton detector. Energy spectra measured with the detector at Kimballton are compared to results of the background model, and implications for the DEMONSTRATOR are explored.