Background characterization of the CONUS+ experimental location

CONUS+ is an experiment aiming at detecting coherent elastic neutrino-nucleus scattering (CE\(\nu \)NS) of reactor antineutrinos on germanium nuclei in the fully coherent regime, continuing the CONUS physics program conducted at the Brokdorf nuclear power plant (KBR), Germany. The CONUS+ experiment is installed in the Leibstadt nuclear power plant (KKL), Switzerland, at a distance of 20.7 m from the 3.6 GW reactor core, where the antineutrino flux is \(1.5\cdot 10^{13}\) s\(^{-1}\) cm\(^{-2}\). The CE\(\nu \)NS signature will be measured with four point-contact high-purity low energy threshold germanium (HPGe) detectors. A good understanding of the background is crucial, especially events correlated with the reactor thermal power are troublesome, as they can mimic the predicted CE\(\nu \)NS interactions. A large background characterization campaign was conducted during reactor on and off times to find the best location for the CONUS+ setup. On-site measurements revealed a correlated, highly thermalized neutron field with a maximum fluence rate of \((2.3\pm 0.1)\cdot 10^{4}\) neutrons d\(^{-1}\) cm\(^{-2}\) during reactor operation. The \(\gamma \)-ray background was studied with a HPGe detector without shield, paying special attention to the thermal power correlated \(^{16}\)N decay and other neutron capture \(\gamma \)-lines. The muon flux was examined using a liquid scintillator detector measuring (107 ± 3) muons s\(^{-1}\)m\(^{-2}\), which corresponds to an average overburden of 7.4 m of water equivalent. The new background conditions in CONUS+ are compared to the previous CONUS ones, showing a 30 times higher flux of neutrons, but a 26 times lower component of reactor thermal power correlated \(\gamma \)-rays over 2.7 MeV. The lower CONUS+ overburden increases the number of muon-induced neutrons by 2.3 times and the flux of cosmogenic neutrons. Finally, all the measured rates are discussed in the context of the CONUS+ background, together with the CONUS+ modifications performed to reduce the impact of the new background conditions at KKL.