Background characterization of the CONUS+ experimental location

E. Sánchez García; N. Ackermann; S. Armbruster; H. Bonet; C. Buck; K. Fülber; J. Hakenmüller; J. Hempfling; G. Heusser; E. Hohmann; M. Lindner; W. Maneschg; K. Ni; M. Rank; T. Rink; I. Stalder; H. Strecker; R. Wink; J. Woenckhaus

doi:10.1140/epjc/s10052-025-14160-7

2023 Impact factor 4.2

Particles and Fields

Open Access

Eur. Phys. J. C (2025) 85: 465
https://doi.org/10.1140/epjc/s10052-025-14160-7

Regular Article - Experimental Physics

Background characterization of the CONUS`+` experimental location

E. Sánchez García¹^a, N. Ackermann¹, S. Armbruster¹, H. Bonet¹, C. Buck¹, K. Fülber², J. Hakenmüller¹^,5^,5, J. Hempfling¹, G. Heusser¹, E. Hohmann⁴, M. Lindner¹, W. Maneschg¹, K. Ni¹, M. Rank³, T. Rink¹, I. Stalder³, H. Strecker¹, R. Wink² and J. Woenckhaus⁴

¹ Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117, Heidelberg, Germany
² PreussenElektra GmbH, Kernkraftwerk Brokdorf, 25576, Osterende, Brokdorf, Germany
³ Kernkraftwerk Leibstadt AG, 5325, Leibstadt, Switzerland
⁴ Paul Scherrer Institut, Forschungsstrasse 111, 5232, Villigen, Switzerland
⁵ Duke University, 27708, Durham, NC, USA

^a esanchez@mpi-hd.mpg.de

Received: 18 December 2024
Accepted: 7 April 2025
Published online: 26 April 2025

Abstract

CONUS+ is an experiment aiming at detecting coherent elastic neutrino-nucleus scattering (CE $ν$ $\nu$ 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 $1.5 \cdot 10^{13}$ $1.5\cdot 10^{13}$ s $^{- 1}$ $^{-1}$ cm $^{- 2}$ $^{-2}$ . The CE $ν$ $\nu$ 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 $ν$ $\nu$ 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 $(2.3 \pm 0.1) \cdot 10^{4}$ $(2.3\pm 0.1)\cdot 10^{4}$ neutrons d $^{- 1}$ $^{-1}$ cm $^{- 2}$ $^{-2}$ during reactor operation. The $γ$ $\gamma$ -ray background was studied with a HPGe detector without shield, paying special attention to the thermal power correlated $^{16}$ $^{16}$ N decay and other neutron capture $γ$ $\gamma$ -lines. The muon flux was examined using a liquid scintillator detector measuring (107 ± 3) muons s $^{- 1}$ $^{-1}$ m $^{- 2}$ $^{-2}$ , 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 $γ$ $\gamma$ -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.

Present address: Duke University, Durham, NC, 27708, USA

© The Author(s) 2025

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