Abstract
In-situ production of radioisotopes by cosmic muon interactions may generate a non-negligible background for deep underground rare event searches. Previous Monte Carlo studies for the Gerda experiment at Lngs identified the delayed decays of ^{77}Ge and its metastable state ^{77m}Ge as dominant cosmogenic background in the search for neutrinoless double beta decay of ^{76}Ge. This might limit the sensitivity of next generation experiments aiming for increased ^{76}Ge mass at background-free conditions and thereby define a minimum depth requirement. A re-evaluation of the ^{77(m)}Ge background for the Gerda experiment has been carried out by a set of Monte Carlo simulations. The obtained ^{77(m)}Ge production rate is (0.21pm 0.01) nuclei/(kgcdot year). After application of state-of-the-art active background suppression techniques and simple delayed coincidence cuts this corresponds to a background contribution of (2.7pm 0.3)times 10^{-6} cts/(keVcdot kgcdot year). The suppression achieved by this strategy equals an effective muon flux reduction of more than one order of magnitude. This virtual depth increase opens the way for next generation rare event searches.
Highlights
In-situ production of radioactive isotopes by cosmic muon interactions constitutes a non-negligible background for rare event searches and may define a minimum depth for experiments aiming for ultra-low backgrounds [1]
Distinct features in the production and the decay of radioactive isotopes from cosmic muon interactions allow for
State-of-the-art active background rejection techniques in combination with a simple delayed coincidence cut based on tagging muon events with accompanying isotope production by prompt and delayed coincidences between muon veto and germanium array will allow a reduction by more than one order of magnitude to (2.7 ± 0.3)×10−6 cts/(keV·kg·year) at an acceptable life-time loss of < 4%
Summary
In-situ production of radioactive isotopes by cosmic muon interactions constitutes a non-negligible background for rare event searches and may define a minimum depth for experiments aiming for ultra-low backgrounds [1]. A background contribution of (1.1 ± 0.2) × 10−4 cts/(keV·kg·year) at 0νββ relevant energies before active background rejection cuts was reported This is well below the background of Gerda Phase II, but might constitute a significant fraction of the background budget for the generation experiment Legend (Large Enriched Germanium Experiment for Neutrinoless ββ Decay) [4]. 37 of them are made from isotopically enriched germanium material with an enrichment fraction of 87% 76Ge. In Phase II 40 HPGe detectors are operated in a 7 string array configuration. The LAr volume around the array is instrumented with wavelength shifting fibers coupled to silicon photomultipliers (SiPMs) [6,7] and low-activity PMTs. The ability to detect scintillation light allows to reject backgrounds with coincident energy release in HPGe detectors and LAr [8]. A detailed description of the Gerda Phase II setup can be found in [11]
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