Monitoring criticality risk through neutron noise in degraded nuclear environments

Our team at CEA/Irfu is working with ASNR to study the possibility of using neutron noise measurements, i.e., stochastic variations in neutron flux, to estimate the reactivity of subcritical nuclear systems. The aim is to propose this technique for online measurement of the reactivity of the corium at Fukushima Daiichi during future decommissioning operations. The thesis work will focus on evaluating a solution based on Micromegas-type neutron detectors (nBLM detectors) developed by IRFU, which are adapted to the extreme gamma radiation expected in the vicinity of the Fukushima Daiichi corium. The student will participate in experiments at nuclear research facilities in Europe and the United States to test this technical solution and measure neutron noise for a wide range of reactivities. He/she will be responsible for analyzing the data and evaluating the various inversion methods used to estimate reactivity from neutron noise measurements.

Measurement of low lying dipole excitations using neutron inelastic scattering

The pygmy dipole resonance is a vibration mode observed in neutron-rich nuclei and which has initially been described as the oscillation of a neutron skin against a symmetric core in term of proton and neutron numbers. But experimental studies have revealed a more complex structure. Few years ago, we have proposed to take benefit of the high intensity neutron flux from SPIRAL2-NFS to study the pygmy resonance with an original approach: the neutron inelastic scattering. Following the success of the first experiment carried out in 2022, we propose to continue our program in a new region of the nuclear chart. The objective of the thesis is to study the pygmy dipole resonance in 88Sr by inelastic neutron scattering. The thesis will consist of: i) participation in the experiment, ii) data analysis, and iii) interpretation of the results in collaboration with theorists.

Development of the Micromegas CyMBaL Detector and study of gluon saturation for the future electron-ion collider

The future Electron-Ion Collider (EIC), to be constructed at Brookhaven National Laboratory (NY, USA) is a next-generation facility designed to explore the inner structure of protons and nuclei with unprecedented precision. It will explore how quarks and gluons generate the mass, spin, and structure of visible matter, and study the increase of gluon density at small Bjorken-x. To meet its ambitious physics goals, innovative detectors are being developed — including the Micromegas CyMBaL system, a gaseous tracker for the central region of the first EIC experimental apparatus ePIC.
This PhD project combines experimental detector R&D and physics simulations:
* Prototype characterization: build and test full-scale Micromegas detectors; measure efficiency, gain uniformity, and spatial resolution in laboratory and beam environments. Test and validate the prototypes with the new ASIC SALSA developed at CEA for gasesous detectors at ePIC.
* Detector simulations: integrate the CyMBaL geometry into the EIC framework and assess global tracking and performance requirements.
* Physics studies: simulate key processes sensitive to gluon saturation (e.g. final-state di-hadron correlations) to understand QCD at small-x and evaluate how detector performance influences physics sensitivity.
The PhD student will have opportunities to participate in the development of state-of-the-art gaseous detectors and to work within an international community of hadronic physicists on topics at the forefront of the field, with trips to Brookhaven National Laboratory (NY, USA) and opportunities for test-beam campaigns at accelerator facilities.

INVESTIGATION OF THE NUCLEAR TWO-PHOTON DECAY

The nuclear two-photon, or double-gamma decay is a rare decay mode in atomic nuclei whereby a nucleus in an excited state emits two gamma rays simultaneously. This second-order electromagnetic process, well known in atomic physics, has been little studied for the atomic nucleus due to the largely predominant first-order processes. Even-even nuclei with a first excited 0+ state are favorable cases to search for a double-gamma decay branch, since the emission of a single gamma ray is strictly forbidden for 0+ to 0+ transitions by angular momentum conservation. The double-gamma decay still remains a very small decay branch (<1E-4) competing with the dominant (first-order) decay modes of atomic internal-conversion electrons (ICE) or internal positron-electron (e+-e-) pair creation (IPC).

The thesis project has two distinct experimental parts: First, we store bare (fully-stripped) ions in their excited 0+ state in the heavy-ion storage ring (ESR) at the GSI facility to search for the double-gamma decay in several nuclides. For neutral atoms the excited 0+ state is a rather short-lived isomeric state with a lifetime of the order of a few tens to hundreds of nanoseconds. At relativistic energies available at GSI, however, all ions are fully stripped of their atomic electrons and decay by ICE emission is hence not possible. If the state of interest is located below the pair creation threshold the IPC process is not possible either. Consequently, bare nuclei are trapped in a long-lived isomeric state, which can only decay by double-gamma emission to the ground state. The decay of the isomers is identified by so-called time-resolved Schottky Mass Spectroscopy. This method allows to distinguish the isomer and the ground state by their (very slightly) different revolution time in the ESR, and to observe the disappearance of the isomer peak in the mass spectrum with a characteristic decay time. Successful experiment establishing the double-gamma decay in several nuclides (72Ge, 98Mo, 98Zr) were already performed and a new experiment to study the nuclide 194Pb has been accepted by the GSI Programme Committee and its realization is planned for 2027.

The second part concerns the direct observation of the emitted photons using gamma-ray spectroscopy. While the storage ring experiments allow to measure the partial lifetime for the double gamma decay, further information on the nuclear properties can be only be achieved by measuring the photon themselves. A test experiment has been performed to study its feasibility and the plans a more detailed study should be developed with the PhD project.

Contribution of artificial intelligence to the study of fission

Nuclear fission is an extreme process during which a heavy nucleus deforms until it reaches a point of no return leading to its separation into two fragments. The process goes with a significant release of energy, mainly as kinetic energy of the newly formed fragments, but also as excitation energy (about 15 MeV/fragment). In addition, the fragments are also produced with a high angular momentum. It is through the emission of neutrons and photons that fission fragments evacuate their energy and angular momentum. The ultimate experiment in fission would consist of identifying each fragment in mass and charge; measuring their kinetic energy; and characterize in energy and multiplicity the neutrons and photons they emit. This data set would make it possible to access the global energy of the fission process and to completely characterize the deexcitation of the fragments. Due to the significant complexity of such an exclusive measurement, this data set is always missing.

Our team is moving towards such measurement and this thesis work aims to explore the benefits that machine learning techniques can bring in this perspective.
The thesis will consist of taking advantage of all the experimentally accessible multi-correlated data in order to feed machine learning algorithms whose purpose will be to identify fission fragments and determine their properties.
The developed techniques will be applied to a first data set using a twin ionization chamber for the detection of fission fragments coupled to a set of neutron detectors. The data will be acquired at the beginning of the thesis.
In a second step, a more exploratory study will consist of applying the same techniques to data obtained during the thesis using a temporal projection chamber as a fission fragment detector. It will be a matter of demonstrating that the energy resolution is compatible with the study of fission.

Study of heavy nuclei: from the mass measurement to the spectroscopy of Americium nuclei and commissioning of the double Penning trap PIPERADE

The atomic nucleus is a complex system that continues to be actively studied more than a century after its discovery. Among the open questions, the question of the limits of existence of the nucleus remains central: what are the numbers of protons and neutrons that allow a bound nucleus to form? This question can be addressed using mass measurements that provide access to the binding energy of the nucleus, one of its most fundamental properties. The objective of this thesis is, on the one hand, to perform high-precision mass measurements of the isotopes 234-238Am (Z = 95) isotopes at the University of Jyväskylä, Finland (experiment planned in 2026), and, on the other hand, to participate in the installation and commissioning of the PIPERADE double Penning trap (PIèges de PEnning pour les RAdionucléides à DESIR) at GANIL in Caen.
The americium nuclei that will be studied in this thesis are at the boundary between two regions of particular interest: the octupole deformation region (pear-shaped nuclei) and the fission isomer region (meta-stable states of nuclei decaying by fission), and measuring their mass will provide a better understanding of the properties of these exotic nuclei.
PIPERADE is a device that can be used to perform high-precision mass measurements. Currently in the characterisation phase in Bordeaux, its installation at GANIL will enable the study of a wide range of exotic nuclei by measuring their mass. Currently undergoing characterisation in Bordeaux, its installation at GANIL (planned for 2027) will enable the study of a wide range of exotic nuclei by measuring their mass, but also by using separation techniques to purify the radioactive beams before sending them to other experimental devices.

STUDY OF THE NUCLEAR COLLECTIVE PROPERTIES OF 232TH WITH THE AGATA SPECTROMETER

The study of so-called ‘deformed’ atomic nuclei with a non-spherical charge distribution is essential for testing nuclear interactions and structural models. These deformed nuclei exhibit a very particular pattern of excited states, known as ‘rotational bands’. These bands can be constructed on states with different deformations or different intrinsic structures (shape coexistence). The subject of the thesis is the experimental study of the macroscopic and microscopic properties of the nucleus 232Th. This nuclide exhibits a wide variety of rotational bands that are thought to be due to vibrations of the nuclear surface known as quadrupole and octupole vibrations. In particular the latter have attracted a great deal of interest recently, as octupolar deformed nuclei can be used to determine nuclear electric dipole moments, a fundamental question in physics in general. In our particular case, the aim is to characterise for the first time the quadruplet of octupole bands expected in a strongly deformed nucleus. Furthermore, this nucleus is the only example with a rotational band built on a double quadrupole vibration.

We will study these various shapes using the powerful technique of Coulomb excitation, which is the most direct method for determining the shape of nuclei in their excited states. The experiment will be carried out using AGATA, a new-generation gamma spectrometer consisting of a large number of finely segmented germanium crystals, which can identify each point of interaction of a gamma ray inside the detector and then, using the innovative concept of ‘gamma-ray tracking’, reconstruct the energies of all the gamma rays emitted and their emission angles with unprecedented precision. A complementary experiment will be carried out at HIL Warsaw, which will enable better interpretation of the highly complex data provided by AGATA.

Dimensionality reduction method applied to the deformed coupled cluster ab initio many-body method

The theoretical description from first principles, i.e. in a so-called ab initio manner, of atomic nuclei containing more than 12 nucleons has only recently become possible thanks to the crucial developments in many-body theory and the availability of increasingly powerful high-performance computers. These ab initio techniques are successfully applied to study the structure of nuclei, starting from the lightest isotopes and now reaching all medium-mass nuclei containing up to about 80 nucleons. The extension to even heavier systems requires decisive advances in terms of storage cost and computation time induced by available many-body methods. In this context, the objective of the thesis is to develop the dimensionality reduction method based on the factorization of tensors involved in the non-perturbative many-body theory known as deformed coupled cluster (dCC). The proposed work will exploit the latest advances in nuclear theory, including the use of nuclear potentials from chiral effective field theory and renormalization group techniques, as well as high-performance computing resources and codes.

Characterization and calibration of cryogenic detectors at the 100 eV scale for the detection of coherent neutrino scattering (CEvNS)

DESCRIPTIONS:

The NUCLEUS experiment [1] aims to detect reactor neutrinos via coherent elastic neutrino–nucleus scattering (CEvNS). Predicted in 1974 and first observed in 2017, this process provides a unique opportunity to test the Standard Model at low energies. Because the scattering is coherent over the entire nucleus, the cross section is enhanced by several orders of magnitude, making CEvNS also promising for reactor monitoring using neutrinos.

The NUCLEUS experimental setup is currently being installed near the EDF nuclear reactors in Chooz (Ardennes, France), which constitute an intense neutrino source. The only physical signal of a CEvNS event is the tiny recoil of the target nucleus, with an energy below 1 keV. To detect this, NUCLEUS uses CaWO4 crystals of about 1 g, placed in a cryostat cooled to 15 mK. The nuclear recoil produces vibrations in the crystal lattice, equivalent to a temperature rise of about 100 µK, measured with a Transition Edge Sensor (TES) deposited on the crystal. These detectors achieve excellent energy resolutions of only a few eV and detection thresholds on the order of ~10 eV [2]. The NUCLEUS setup was successfully tested and validated in 2024 at TU Munich [3], and data taking at Chooz is scheduled to start in summer 2026, simultaneously with the beginning of the PhD. An initial contribution will involve data acquisition and analysis at the reactor site. More specifically, the PhD student will be responsible for the characterization of the deployed cryogenic CaWO4 detectors — stability, energy resolution, calibration, and intrinsic background of the crystal.

Calibration at the sub-keV scale is a crucial challenge for CEvNS (and dark matter) experiments. Until recently, it was extremely difficult to generate nuclear recoils of known energy to characterize detector responses. The CRAB method [4, 5] addresses this issue by using thermal neutron capture (25 meV) on nuclei that constitute the cryogenic detector. The resulting compound nucleus has a well-known excitation energy — the neutron separation energy — between 5 and 8 MeV, depending on the isotope. When it de-excites by emitting a single gamma photon, the nucleus recoils with a precisely determined energy given by two-body kinematics. A calibration peak in the desired energy range of a few hundred eV then appears in the detector’s energy spectrum. A first measurement in 2022, using a NUCLEUS CaWO4 detector and a commercial ²5²Cf neutron source, validated this method [6].

The second part of the PhD will take place within the “high-precision” phase of the project, which consists in performing measurements with a pure thermal neutron beam from the TRIGA-Mark-II reactor in Vienna (TU Wien, Austria). The calibration setup was successfully installed and characterized in 2025 [7]. It consists of a cryostat housing the cryogenic detectors to be characterized, surrounded by large BaF2 crystals for coincidence detection of the nuclear recoil and the gamma ray that induced it. The whole setup is placed directly on the neutron beam axis, which provides a flux of about 450 n/cm²/s. This coincidence technique will significantly reduce background and extend the CRAB method to a wider energy range and to materials used in most cryogenic detectors. These measurements are expected to provide a unique characterization of the response of cryogenic detectors in the energy region of interest for light dark matter searches and coherent neutrino scattering. In parallel with the measurement of nuclear recoils, the installation of a low-energy X-ray source in the cryostat will generate electronic recoils, enabling a direct comparison between the detector responses to sub-keV energy deposits produced by nuclear and electronic recoils.

The arrival of the PhD student will coincide with the completion of the measurement program on CaWO4 and Al2O3 detectors of NUCLEUS and with the start of the measurement programs on Ge (TESSERACT project) and Si (BULLKID project) detectors.
The high-precision measurements will also open a new sensitivity window to subtle effects coupling nuclear physics(nuclear de-excitation times) and solid-state physics (nuclear recoil times in matter, and the creation of crystal defects induced by nuclear recoils) [8].

The PhD student will be deeply involved in all aspects of the experiment: simulation, data analysis, and interpretation of the obtained results.

WORK PLAN:

The PhD student will actively participate in data taking and in the analysis of the first results from the NUCLEUS cryogenic CaWO4 detectors at Chooz. This work will be carried out in collaboration with the Nuclear Physics Department (DPhN), the Particle Physics Department (DPhP) of CEA-Saclay, and the TU Munich team. It will begin with familiarization with the CAIT analysis framework used for cryogenic detectors. The student will focus in particular on detector calibration, studying the detector response to electronic recoils induced by optical photon pulses injected through fibers and by X-ray fluorescence generated by cosmic rays. Once this calibration is established, two types of backgrounds will be investigated: Nuclear recoils in the keV range induced by cosmogenic fast neutrons, and a low-energy background, known as the Low Energy Excess (LEE), intrinsic to the detector.
The comparison between the experimental and simulated fast neutron background spectra will be analyzed in light of the differences between nuclear and electronic recoil responses measured in the CRAB project. The long data-taking periods at the Chooz site will also be used to study the time evolution of the LEE background. This work will be conducted in collaboration with solid-state physics experts from the Institute for Applied Sciences and Simulation (CEA/ISAS) to better understand the origin of the LEE, which remains a major open question in the cryogenic detector community.
The analysis skills acquired on NUCLEUS will then be applied to the high-precision CRAB measurement campaigns planned for 2027 at the TRIGA reactor (TU Wien) with Ge and Si detectors. The student will be deeply involved in the setup, data acquisition, and analysis of results. The planned measurements on germanium, using both phonon and ionization channels, have the potential to resolve the current ambiguity in the ionization yield of low-energy nuclear recoils, a key factor for the sensitivity of future experiments.
The high calibration precision will also be exploited to study fine effects in nuclear and solid-state physics, such as timing effects and crystal defect formation induced by nuclear recoils in the detector. This study will be conducted in synergy with teams from CEA/IRESNE and CEA/ISAS, who provide detailed simulations of nuclear de-excitation gamma cascades and molecular dynamics simulations of nuclear recoil propagation in matter.

Through this work, the student will receive comprehensive training as an experimental physicist, including strong components in simulation and data analysis, as well as hands-on experience with cryogenic techniques during the commissioning of the NUCLEUS and CRAB detectors. The proposed contributions are expected to lead to several publications during the PhD, with high visibility in the CEvNS and dark matter communities. Within the CEA, the student will also benefit from the exceptionally cross-disciplinary nature of this project, which already
fosters regular interaction among the communities of nuclear physics, particle physics and condensed matter physics.

COLLABORATIONS:

NUCLEUS: Germany (TU-Munich, MPP), Austria (HEPHY, TU-Wien), Italy (INFN), France (CEA-Saclay).
CRAB: Germany (TU-Munich, MPP), Austria (HEPHY, TU-Wien), Italy (INFN), France (CEA-Saclay, CNRS-IJCLab, CNRS-IP2I, CNRS-LPSC).

BIBLIOGRAPHY:

[1] NUCLEUS Collaboration, Exploring CE?NS with NUCLEUS at the Chooz nuclear power plant, The European Physical Journal C 79 (2019) 1018.
15, 48, 160, 174
[2] R. Strauss et al., Gram-scale cryogenic calorimeters for rare-event searches, Phys. Rev. D 96 (2017) 022009. 16, 18, 78, 174
[3] H. Abele et al., Particle background characterization and prediction for the NUCLEUS reactor CE?NS 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 TUWien TRIGA reactor: status and related physics program, (https://arxiv.org/abs/2505.15227)
[8] G. Soum-Sidikov et al., Study of collision and ?-cascade times following neutron-capture processes in cryogenic detectors Phys. Rev. D
108, 072009 (2023) (https://arxiv.org/abs/2305.10139)