Full background decomposition of the CONUS experiment

Abstract The CONUS experiment is searching for coherent elastic neutrino nucleus scattering of reactor anti-neutrinos with four low-energy threshold point-contact high-purity germanium spectrometers. Excellent background suppression within the region of interest below 1 keV (ionization energy) is absolutely necessary to enable signal detection. The collected data also make it possible to set limits on various models regarding beyond the standard model physics. These analyses benefit as well from the low background level of $$\sim $$ ∼ 10 d $$^{-1}$$ - 1 kg $$^{-1}$$ - 1 below 1 keV and at higher energies. The low background level is achieved by employing a compact shell-like shield that was adapted to the most relevant background sources at the shallow depth location of the experiment: environmental gamma radiation and muon-induced secondaries. Overall, the compact CONUS shield including the active anticoincidence muon-veto reduces the background by more than four orders of magnitude. The remaining background is described with validated Monte Carlo simulations which include the detector response. It is the first time that a full background decomposition in germanium operated at a reactor site has been achieved. Next to the remaining muon-induced background, $$^{210}$$ 210 Pb within the shield and cryostat end caps, cosmogenic activation and airborne radon are the most relevant background sources. The reactor-correlated background is negligible within the shield. The validated background model, together with the parameterization of the noise, is used as input to the likelihood analyses of the various physics cases.