The Cosmic Muon-Induced Background for the LEGEND-1000 Alternative Site at LNGS

The in-situ production of long-lived radio-isotopes by cosmic muon interactions may generate a non-negligible background for rare event searches in the deep subsurface. The delayed decay of 77(m)Ge has been identified as the dominant in-situ cosmogenic contributor for a neutrinoless double-beta decay search with 76Ge. The future ton-scale LEGEND-1000 experiment requires a total background of ≤ 10−5 cts/(keV-kg-yr). Dedicated Monte Carlo studies of the 77(m)Ge background at the alternative LNGS site were performed. The addition of passive neutron moderators, in combination with a delayed coincidence strategy, results in a background contribution of 8.6 × 10−7 cts/(keV.kg.yr) with an additional dead time of < 9%.