Depth Requirements for a Tonne-scale 76 Ge Neutrinoless Double-beta Decay Experiment

AbstractNeutrinoless double-beta decay experiments can potentially determine theMajorana or Dirac nature of the neutrino, and aid in understanding the neu-trino absolute mass scale and hierarchy. Future 76 Ge-based searches target ahalf-life sensitivity of >10 27 y to explore the inverted neutrino mass hierarchy.Reaching this sensitivity will require a background rate of <1 count tonne 1 y 1 in a 4-keV-wide spectral region of interest surrounding the Qvalue of thedecay. We investigate the overburden required to reach this background goalin a tonne-scale experiment with a compact (copper and lead) shield basedon Monte Carlo calculations of cosmic-ray background rates. We nd that, inlight of the presently large uncertainties in these types of calculations, a sitewith an underground depth &5200 meters water equivalent is required for atonne-scale experiment with a compact shield similar to the planned 40-kgMajorana Demonstrator. The required overburden is highly dependenton the chosen shielding con guration and could be relaxed signi cantly if, forexample, a liquid cryogen and water shield, or an active neutron shield wereemployed. Operation of the Majorana Demonstrator and GERDA de-tectors will serve to reduce the uncertainties on cosmic-ray background ratesand will impact the choice of shielding style and location for a future tonne-scale experiment.Keywords: neutrinoless double-beta decay, Germanium, cosmic rays2