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. 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 \((2.3\pm0.1)\cdot 10^{4}\)~neutrons~d\(^{-1}\)cm\(^{-2}\) during reactor operation. The \(\gamma\)-ray background was studied with a HPGe detector without shield. The muon flux was examined using a liquid scintillator detector measuring (107\(\pm\)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.