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Abstract

Experiments searching for neutrinoless double beta (0νββ) decay which could occur if neutrinos are Majorana particles require very low background due to the very long expected half-life. The GERDA experiment uses germanium detectors operated in liquid argon to search for 0νββ decay of 76Ge. Since the signature is a peak at the Q-value of 2039 keV, any energy deposition around the Q-value is potential background. Neutrons can produce such background due to the production of long-lived radioisotopes which are hard to veto. Long-lived isotopes can be produced either during processing of the detector materials above ground or by muon-induced neutrons below ground. Monte Carlo simulations of the propagation of cosmic-ray neutrons and the production of muon-induced neutrons for different commonly used shielding materials were performed. They were used to estimate the total neutron flux and spectra for different shielding configurations. Muon-induced neutron production rates, especially for high-Z materials, are not well understood. The MINIDEX experiment located at the Tübingen Shallow Underground Laboratory aims to investigate the muon-induced neutron production rates for high-Z materials. The muon flux inside the laboratory was simulated which was used as input for Monte Carlo simulations of the expected muon-induced neutron production rates in MINIDEX. While muon-induced radionuclides are not the dominant background for current experiments searching for 0νββ decay, this kind of background becomes more relevant for future experiments like LEGEND. A relevant background contribution in the region of interest is expected from beta decay of 77(m)Ge which can be produced by capture of muon-induced neutrons on 76Ge. The predicted production rates of 77(m)Ge in GERDA from different Monte Carlo simulations differ by a factor of two. GERDA data were analyzed to search for signatures of the 77(m)Ge and 75(m)Ge production. No signal was found. Upper limits on the production rates of 4.1 nuclei/(kg yr) (90% CI) for 77(m)Ge and of 0.63 nuclei/(kg yr) (90% CI) for 75(m)Ge were set. Using this result an upper limit on the expected background due to the decay of 77(m)Ge can be extracted. After applying analysis cuts, the upper limit is BI < 6.4·10^−5 cts/(keV kg yr) and BI < 4.7·10^−5 cts/(keV kg yr) for coaxial detectors and BEGe detectors, respectively. Limits on the integrated neutron flux around the GERDA detector array were derived to be Phi_n < 36 n/(m^2 h). The set limits are 1 − 2 orders of magnitude larger than the ones expected from simulations. LEGEND-200 will have the capability to determine the 77(m)Ge and 75(m)Ge production rates as well as the BI from 77(m)Ge beta decay. This will allow to verify that this background component will not deteriorate the sensitivity of future tonne-scale experiments