he NUCLEUS experiment aims to demonstrate the bolometric detection of reactor antineutrinos based on a process called coherent neutrino-nucleus scattering. After a year dedicated to installing shielding and cryogenic infrastructure at the Technical University of Munich, the project reached a major milestone by concluding a 50-day data-taking run last fall. The primary goals were to demonstrate a routine operation of the experiment, validate the performance and stability of a first version of the detection system, and study backgrounds. These objectives were essential to achieve before moving to the Chooz nuclear power plant in the French Ardennes!
Detecting the coherent scattering of reactor antineutrinos on nuclei
Coherent scattering is a process in which a low-energy neutrino (< 50 MeV) interacts with the entire nucleus of an atom, without distinguishing its internal nucleonic structure [1]. In this regime, the probability of neutrino interaction is enhanced compared to the interaction channels usually exploited to study them, such as inverse beta decay (also called antineutrino capture on proton). Mastering the detection of low-energy neutrinos via this process gives hope to reduce the size of current neutrino detection devices, which could open very interesting prospects for neutrino astronomy or nuclear energy applications in the long term. Coherent neutrino-nucleus scattering (CEvNS) is also sensitive to a wide range of new physics and allows testing the Standard Model in an energy regime still little explored. In particular, a precision measurement could help better understand certain properties of the neutrino (see [2] for some examples).
Observing this process is not without difficulties, the main one being that the signal is reduced to a tiny nuclear recoil induced by the interaction of a neutrino, similarly to a small table tennis ball colliding a huge bowling ball. For accelerator neutrinos with energies in the tens of MeV range, the process has been detected and studied since 2017 [3]. The induced nuclear recoils, in the order of tens of keV, are then more easily measurable. The challenge is quite different for reactor antineutrinos, as their energies are at least ten times lower, giving nuclear recoils below or well below the keV regime. After about ten years of effort, the CONUS collaboration recently announced the first observation of the process with a semi-conductor germanium detector reaching a threshold below the keV range and deployed at the Leibstadt nuclear power plant in Switzerland [4].
A promising way to measure CEvNS with precision is that of cryogenic calorimeters. These devices, originally developed for direct dark matter searches and neutrinoless double beta decay, are capable of reaching thresholds below 100 electronvolts while keeping a sufficiently large detection volume. The NUCLEUS collaboration has embarked on this path and is developing a system of cryogenic calorimeters equipped with superconducting transition edge sensors (TES) with a payload of about 10 grams of CaWO4 [5]. These detectors, whose design is directly derived from those of the CRESST experiment, reach detection thresholds around 20 eV [6].
A first successful and insightful commissioning run
The NUCLEUS collaboration aims for a first measurement at the Chooz nuclear power plant by 2025. To this end, the teams from the LRI at DIS and DETECS at DEDIP have joined forces with those from EDF to complete the installation of a new experimental room located less than 100 meters from the two reactor units in the basement of a tertiary building (see Figure 2). Reducing and controlling particle backgrounds are two other major challenges to overcome, mainly because the experimental site offers little protection against cosmic radiation, but also because a nuclear recoil is a much less obvious and irrefutable signature of the interaction of a reactor antineutrino than the products of inverse beta decay, the channel usually used to detect them. Thanks to its past expertise with the Double Chooz, Nucifer, and STEREO experiments, the LCAP at DIS has therefore designed and implemented the entire system of shielding in collaboration with the teams from DPhP and DPhN, from the external muon veto [7] to the “cryogenic outer veto” (COV), a cryogenic shield composed of high-purity germanium crystals surrounding the cryogenic calorimeters and read in ionization mode (see Figure 2 and [8]). The entire system was installed in 2023 at the Technical University of Munich under the aegis of the DETECS laboratory at DEDIP, with however a simplified version of the COV. A first commissioning run, coordinated from start to finish by the team of physicists from DPhP and DPhN last summer, successfully operated a simplified version of the cryogenic calorimeter system for 9 weeks without interruptions or failures.
This commissioning run first demonstrated the stability and performance of all subsystems (cryostat, muon veto, shielding, and cryogenic calorimeters), an essential condition to ensure before moving the experiment to Chooz. It was also about setting up all the tools necessary for acquisition, diagnosis, reduction, and analysis of a large volume of data (~0.2 TB/day), an objective successfully achieved by developing a complete analysis chain by the DPhP team. Finally, this commissioning run was also an opportunity to study the energy response of cryogenic calorimeters in detail thanks to an onboard calibration system and to study backgrounds, two essential aspects for the future measurement of CEvNS for which the physicists from DPhP and DPhN also played a key role in data analysis and simulation work.
The adventure at Chooz is about to begin
The analysis of the data from the first commissioning run is nearing completion with the objective of a publication before the summer of 2025. In particular, an important simulation work is in progress to separate and better understand two background components in the region of interest of the CEvNS signal:
i. events from external radiation (natural radioactivity, cosmic radiation, etc.) and whose majority is attenuated by the shielding system,
ii. and what the bolometric detector community calls the EXCESS [9], an unknown source of background which particularly rises at energies close to the threshold.
This latter is a currently major obstacle to measuring a CEvNS signal and is the subject of many speculations. The most consensus interpretation is that of a relaxation of constraints in the crystals at the level of the mechanical supports holding them and/or at the interface between the crystal and the thermometer, releasing bursts of phonons and creating signals reconstructed at low energies. This scenario was partially studied by a cryogenic calorimeter with double TES readout during the commissioning run, without however firmly concluding.
In addition to the complete validation of the shielding system in real background conditions, studying the origin of the excess with a new module containing an instrumented mechanical support is therefore the major objective of the next technical run scheduled for 2025 at Chooz. Stay tuned, more news will soon arrive, especially since the experiment’s shielding has already completed its journey to Chooz to be reassembled and put back into service by the spring of 2025!
Contacts
Matthieu Vivier (Irfu/DPhP)
Fabien Jeanneau (Irfu/DEDIP)
References
[1] D. Freedman, Phys. Rev. D 9 (1974)
[2] J. Billard et al., JCAP 11 016 (2018)
[3] D. Akimov et al., Science 03 (2017)
[4] N. Ackermann et al., arXiv : 2501.05206 (2025)
[5] G. Angloher et al., EPJC 79 1018 (2019)
[6] R. Strauss et al., Phys. Rev. D 96 022009 (2017)
[7] V. Wagner et al., JINST 17 T05020 (2022)
[8] C. Goupy et al., NIM A 1064, 169383 (2024)
[9] P. Adari et al., Sci. Post. Phys. Proc. 9, 001 (2022)