Background characterization of the CONUS+ experimental location

CONUS+ is an experiment aiming at detecting coherent elastic neutrino-nucleus scattering (CE$ν$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$ν$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 $γ$-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 $γ$-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.