Abstract
Low background experiments need a suppression of cosmogenically induced events. The GERDA experiment located at LNGS is searching for the neutrinless double beta decay of $^{76}$Ge. It is equipped with an active muon veto the main part of which is a water Cherenkov veto with 66 PMTs in the watertank surrounding the GERDA cryostat. With this system 806 live days have been recorded, 491 days were combined muon-germanium data. A muon detection efficiency of $\varepsilon_{\mu d}=(99.935\pm0.015)$ \% was found in a Monte Carlo simulation for the muons depositing energy in the germanium detectors. By examining coincident muon-germanium events a rejection efficiency of $\varepsilon_{\mu r}=(99.2_{-0.4}^{+0.3})$ \% was found. Without veto condition the muons by themselves would cause a background index of $\textrm{BI}_{\mu}=(3.16 \pm 0.85)\times10^{-3}$ cts/(keV$\cdot$kg$\cdot$yr) at $Q_{\beta\beta}$.
Highlights
Muons may cause a substantial background to rare event searches like Gerda (Germanium Detector Array) by generating counts in the region of interest (ROI) either through direct energy deposition in the detectors or through e.g. decay radiation of spallation products
Gerda was constructed in the underground laboratory of Laboratori Nazionali del Gran Sasso (Lngs) of Infn in Italy, which offers an overburden of 3500 m water equivalent (m.w.e.) of rock and a reduction of the muon flux by a factor of ∼106 to a rate of ∼3.4×10−4/(s·m2)
This remaining muon flux, is sufficient to cause a non-negligible background in the region of interest around Qββ = 2039 keV when increasing the sensitivity beyond T10/ν2> 1025 year or when requiring a background index BI
Summary
Muons may cause a substantial background to rare event searches like Gerda (Germanium Detector Array) by generating counts in the region of interest (ROI) either through direct energy deposition in the detectors or through e.g. decay radiation of spallation products. Gerda was constructed in the underground laboratory of Laboratori Nazionali del Gran Sasso (Lngs) of Infn in Italy, which offers an overburden of 3500 m water equivalent (m.w.e.) of rock and a reduction of the muon flux by a factor of ∼106 to a rate of ∼3.4×10−4/(s·m2). This remaining muon flux, is sufficient to cause a non-negligible background in the region of interest around Qββ = 2039 keV when increasing the sensitivity beyond T10/ν2> 1025 year or when requiring a background index BI
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