Characterization and use of a fluorescence X-ray source for low-energy calibration of cryogenic detectors.

Full description

The CRAB method aims to calibrate the cryogenic detectors used in dark matter research and coherent neutrino scattering experiments. These experiments share the fact that the signal sought is a very low-energy nuclear recoil (around 100 eV) requiring detectors with a resolution of a few eV and a threshold of O(10 eV), namely bolometers. However, until now, it has been very difficult to generate nuclear recoils of known energy to characterize the response of these detectors. The main idea of the CRAB method, detailed here [1,2], is to induce a capture reaction with thermal neutrons (energy of 25 meV) on the nuclei constituting the cryogenic detector. The resulting compound nucleus has a well-known excitation energy, the neutron separation energy ranging from 5 to 8 MeV depending on the isotopes. In the case where it de-excites by emitting a single gamma photon, the nucleus will recoil with an energy that is also perfectly known as given by two-body kinematics. A calibration peak, in the sought range of a few hundred eV, then appears in the energy spectrum of the cryogenic detector. A first measurement carried out in 2022 with a CaWO4 cryogenic detector from the NUCLEUS experiment (coherent neutrino scattering experiment) validated the method [3].
The second phase of this project, currently underway, involves conducting high-precision measurements at the TRIGA-Mark-II reactor in Vienna (TU-Wien, Austria) [5]. Beyond the absolute calibration of cryogenic detectors with unmatched precision, this measurement campaign is part of a transdisciplinary program offering original tests of nuclear physics (level schemes and de-excitation times of the nucleus) and solid-state physics (the temporal progression of displacement cascades, study of crystal defects induced by nuclear recoil).
In 2025-2026, CaWO4 and Al2O3 crystals cooled to 15 mK in a dilution cryostat will be irradiated by a pure beam of thermal neutrons, provided by the TRIGA-Mark-II reactor.
The context of the internship is the comparison of the energy response of these detectors to nuclear recoils and electronic recoils in the range of 100-1000 eV. In current experiments, calibrations rely on electronic recoils because they are easy to implement with X-ray sources or laser pulses. However, the physical signal of the interaction of neutrinos or dark matter is a nuclear recoil, whose energy losses in matter are very different from those of an electron. The proposed comparison thus achieves several objectives: it provides an absolute calibration for the signal of interest from nuclear recoils, establishes an intercalibration between the two types of recoils allowing the propagation of the absolute anchoring established in Vienna to measurements in other cryostats, and finally represents an original probe of the different energy loss processes at work (notably the creation of crystal defects by nuclear recoils).

A low-energy X-ray source is currently under development at the CEA [6,7]. The intern will participate in its characterization at Saclay, in the installation of the cryogenic detectors and this source in the cryostat in Vienna, as well as in data collection and analysis. The work will be conducted within the DPhN team, consisting of two permanent physicists, a postdoctoral researcher, and a PhD student. The interpretation of the results will be done in collaboration with our colleagues from the materials research and physical chemistry department, who are conducting dedicated molecular dynamics simulations for low-energy nuclear recoils.
Through this internship, the student will gain a comprehensive insight into a precision physical measurement. They will be able to make proposals and test their own ideas in the development of the analysis. The results obtained will be presented to the international CRAB collaboration.

TRAINING AND REQUIRED SKILLS
* Internship for a Master’s student (M2) or engineering school student lasting 3 to 6 months.
* Basic knowledge in nuclear physics, particle physics, and instrumentation.
* Programming skills in C++ and Python.
* Familiarity with Geant4 and ROOT software is a plus.

CONTINUATION INTO A PHD PROGRAM
A PhD position is proposed as a continuation of this internship.

WORKING ENVIRONMENT
The intern will work at the CEA-Saclay site, within the nuclear physics department of Irfu, and in close collaboration with the particle physics department and the materials research and physical chemistry department.

CONTACTS
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BIBLIOGRAPHY
[1] 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)
[2]https://irfu.cea.fr/dphp/Phocea/Vie_des_labos/Ast/ast.php?id_ast=4970
[3] 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) (https://arxiv.org/abs/2305.10139)
[4] 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)
[5] H. Abele et al., The CRAB facility at the TU Wien TRIGA reactor: status and related physics program, Eur. Phys. J. C (2025) accepted (https://arxiv.org/abs/2505.15227)
[6] E. Leblanc et al. Bolometer characterisation with a specially developed cryogenic source having more than five peaks in the 1–6 kev range. Nucl. Instrum. Methods Phys. Res. Sect. A 370(1), 208–210 (1996). https://doi. org/ 10. 1016/ 0168- 9002(95) 01119-6
[7] H. Abele, Sub-keV Electron Recoil Calibration for Macroscopic Cryogenic Calorimeters using a Novel X-ray Fluorescence Source, J Low Temp Phys (2025). https://doi.org/10.1007/s10909-025-03330-2 (https://arxiv.org/abs/2505.17686)

Location

Saclay

Internship conditions

• Internship duration: 6 months
• Level of study: Bac+5
• Training: Master 2
• Continuation in PhD thesis: Yes
• Application deadline: 1 janvier 2026

Computer languages and software:
* Programmation: C++, Python
* Software: GEANT4 et ROOT is a plus

Supervisor

THULLIEZ Loïc et LHUILLIER David
Phone: +33 1 69 08 74 53
Email :

Head of the laboratory DPhN / LEARN

Phone: