Description

The NUCLEUS experiment aims to detect reactor neutrinos via coherent elastic neutrino-nucleus scattering (CEvNS). Predicted in 1974 and first observed in 2017, this process provides unique access to low-energy tests of the Standard Model.
The NUCLEUS experimental setup is currently being installed near the EDF nuclear reactors in Chooz (Ardennes, France), which are intense sources of neutrinos. The only physical signature of a CEvNS event is the minute recoil of the target nucleus, with a very low energy, below 1 keV. To detect this, NUCLEUS uses ~1 g CaWO₄ crystals, cooled to 15 mK in a cryostat. The nuclear recoil induces vibrations in the crystal lattice, which are detected by a Transition Edge Sensor (TES) deposited on the crystal. This technology achieves detection thresholds on the order of ~10 eV. The NUCLEUS setup was successfully tested and validated in 2024 at TU-Munich, and data taking at Chooz is expected to begin in summer 2026, in parallel with the PhD. An initial contribution will focus on data taking and commissioning at the reactor site and analysis. More specifically, the student will be in charge of characterizing the deployed CaWO₄ cryogenic detectors: stability, energy resolution, calibration, and the crystal's intrinsic background.
Sub-keV calibration is a crucial challenge for CEvNS and dark matter experiments. Until recently, generating nuclear recoils of known energy for detector characterization was extremely difficult. The CRAB method addresses this by exploiting thermal neutron capture on the nuclei within the cryogenic detector. The resulting compound nucleus has a well-known excitation energy. If it de-excites by emitting a single gamma photon, the nucleus recoils with a precisely known energy, defined by two-body kinematics. The gamma escapes the cm-scale detector easily, leaving a clean, calibrated nuclear recoil signal in the desired range of a few hundred eV. This method was validated by a first measurement using a CaWO₄ NUCLEUS detector and a commercial neutron source.
The second part of the PhD project takes place within the 'high-precision' phase of the CRAB program, using a pure thermal neutron beam from the TRIGA-Mark-II reactor in Vienna (TU-Wien, Austria). The experimental setup was successfully installed and characterized this year. It consists of a cryostat housing the cryogenic detectors to be characterized, surrounded by large BaF₂ crystals for coincidence detection of both the nuclear recoil and the gamma ray that caused it. The setup is placed directly on the neutron beam axis. This coincidence method will significantly reduce background and allow extension of the CRAB technique to a wider energy range and to materials used in most cryogenic detectors. These measurements are expected to provide a unique characterization of cryogenic detector response in an energy domain of interest for light dark matter searches and CEvNS studies.
The PhD work will begin as the measurement program on NUCLEUS CaWO₄ and Al₂O₃ detectors is finalized, and as new campaigns begin on Ge (TESSERACT project detector) and Si (BULLKID project detector).
High-precision studies will also open a new sensitivity window to subtle effects at the intersection of nuclear physics (nuclear de-excitation times) and solid-state physics (nuclear recoil time within the material, creation of crystal defects during recoil).
The student will be deeply involved in all aspects of the experiment: simulation, data analysis, and interpretation of results.

Skills required

Training in experimental physics with a specialization in particle physics or nuclear physics or nuclear reactor physics. Programming skills : Python, C++, ROOT (is an asset), Geant4 (is an asset)

Bibliography

[1] NUCLEUS Collaboration, Exploring CEνNS with NUCLEUS at the Chooz nuclear power plant, The European Physical Journal C 79 (2019) 1018.
[2] R. Strauss et al., Gram-scale cryogenic calorimeters for rare-event searches, Phys. Rev. D 96 (2017) 022009.
[3] H. Abele et al., Particle background characterization and prediction for the NUCLEUS reactor CEνNS experiment, https://arxiv.org/abs/2509.03559
[4] L. Thulliez, D. Lhuillier et al. Calibration of nuclear recoils at the 100 eV scale using neutron capture, JINST 16 (2021) 07, P07032 (https://arxiv.org/abs/2011.13803)
[5]https://irfu.cea.fr/dphp/Phocea/Vie_des_labos/Ast/ast.php?id_ast=4970
[6] H. Abele et al., Observation of a nuclear recoil peak at the 100 eV scale induced by neutron capture, Phys. Rev. Lett. 130, 211802 (2023) (https://arxiv.org/abs/2211.03631)
[7] H.Abele et al., The CRAB facility at the TU Wien TRIGA reactor: status and related physics program, (https://arxiv.org/abs/2505.15227)
[8] G. Soum-Sidikov et al., Study of collision and γ-cascade times following neutron-capture processes in cryogenic detectors Phys. Rev. D 108, 072009 (2023)

Keywords

neutrino, cryogenic detectors, nuclear physics, calibration, solid state physics