Enabling efficient federated learning and fine-tuning for heterogeneous and resource-constrained devices

The goal of this PhD thesis is to develop methods that enhance resource efficiency in federated learning (FL), with particular attention to the constraints and heterogeneity of client resources. The work will first focus on the classical client-server FL architecture, before extending the investigation to decentralised FL settings. The proposed methods will be studied in the context of both federated model training and distributed fine-tuning of large models, such as large language models (LLMs).

Development of an online measurement method for radioactive gases based on porous scintillators

As the national metrology laboratory for ionizing radiation, the Henri Becquerel National Laboratory (LNE-LNHB) of the French Alternative Energies and Atomic Energy Commission (CEA) operates unique facilities dedicated to radionuclide metrology. These include various setups for producing liquid-phase standards, as well as systems for mixing radioactive gases. In previous research projects, a specific installation was developed for the generation of radioactive gas atmospheres [1], with the aim of creating new testing and calibration methods that meet the needs of both research and industry.

One of the major current challenges is to reproduce environmental conditions as realistically as possible in order to better address actual regulatory requirements—particularly regarding volumetric activity and measurement conditions. This general issue applies to all radioactive substances, but is especially critical for volatile radioactive substances. Over the past several years, through numerous projects and collaborations, CEA/LNHB has been exploring new detection methods that outperform traditional liquid scintillation techniques. Among these innovations are new porous inorganic scintillators [1], which enable not only online detection but also online separation (“unmixing”) of pure beta-emitting radionuclides—this technique has been patented [2].

The objective of this PhD project is to develop, implement, and optimize these measurement methods through applications to:

- Pure radioactive gases,
- Multicomponent mixtures of pure beta-emitting radioactive gases—using porous scintillators for unmixing and identification,
- Liquid scintillation counting, more generally, where this unmixing capability has recently been demonstrated at LNHB and is currently being prepared for publication.

The unmixing technique is of particular interest, as it significantly simplifies environmental monitoring by scintillation, especially in the case of ³H and ¹4C mixtures. Currently, such analyses require multiple bubbler samplings, mixing with scintillation cocktail, and triple-label methods—procedures that involve several months of calibration preparation and weeks of experimentation and processing.

This PhD will be closely aligned with a second doctoral project on Compton-TDCR [1] (2025–2028), aimed at determining the response curve of the scintillators.

The scientific challenges of the project are tied to radionuclide metrology and combine experimentation, instrumentation, and data analysis to develop innovative measurement techniques. Key objectives include:

- Developing a method for beta-emitter unmixing in scintillation, based on initial published and patented concepts.
- Assessing the precision of the unmixing method, including associated uncertainties and decision thresholds.
- Validating the unmixing technique using the laboratory’s radioactive gas test bench [1], with various radionuclides such as 3H, 14C, 133Xe, 85Kr, 222Rn,... or via conventional liquid scintillation counting.
- Enhancing the unmixing model, potentially through the use of machine learning or artificial intelligence tools, particularly for complex multicomponent mixtures.

New experimental constraints on the weak interaction coupling constants by coincidence measurements of complex decay schemes

Accurate experimental knowledge of forbidden non-unique beta transitions, which constitute about one third of all known beta transitions, is an important and very difficult subject. Only a few reliable studies exist in the literature. Indeed, the continuous energy spectrum of these transitions is difficult to measure precisely for various reasons that cumulate: high diffusivity of electrons in matter and non-linearity of the detection system, unavailability of some radionuclides and presence of impurities, long half-lives and complex decay schemes, etc. Accurate theoretical predictions are equally difficult because of the necessity of coupling different models for the atomic, the nuclear and the weak interaction parts in the same, full-relativistic formalism. However, improving our knowledge of forbidden non-unique beta transitions is essential in radioactivity metrology to define the becquerel SI unit in the case of pure beta emitters. This can have a strong impact in nuclear medicine, for the nuclear industry, and for some studies in fundamental physics such as dark matter detection and neutrino physics.
Our recent study, both theoretical and experimental, of the second forbidden non-unique transition in 99Tc decay has highlighted that forbidden non-unique transitions can be particularly sensitive to the effective values of the weak interaction coupling constants. The latter act as multiplicative factors of the nuclear matrix elements. The use of effective values compensates for the approximations used in the nuclear structure models, such as simplified correlations between the nucleons in the valence space, or the absence of core excitation. However, they can only be adjusted by comparing with a high-precision experimental spectrum. The predictability of the theoretical calculations, even the most precise currently available, is thus strongly questioned. While it has already been demonstrated that universal values cannot be fixed, effective values for each type of transition, or for a specific nuclear model, are possible. The aim of this thesis is therefore to establish new experimental constraints on the weak interaction coupling constants by precisely measuring the energy spectra of beta transitions. Ultimately, establishing robust average effective values of these coupling constants will be possible, and a real predictive power for theoretical calculations of beta decay will be obtained.
Most of the transitions of interest for constraining the coupling constants have energies greater than 1 MeV, occur in complex decay schemes and are associated to the emission of multiple gamma photons. In this situation, the best strategy consists in beta-gamma detection in coincidence. The usual detection techniques in nuclear physics are appropriate but they must be extremely well implemented and controlled. The doctoral student will rely on the results obtained in two previous theses. To minimize self-absorption of the electrons in the source, they will have to adapt a preparation technique of ultra-thin radioactive sources developed at LNHB to the important activities that will be required. He will have to implement a new apparatus, in a dedicated vacuum chamber, including a coincidence detection of two silicon detectors and two gamma detectors. Several studies will be necessary, mechanical and by Monte Carlo simulation, to optimize the geometric configuration with regard to the different constraints. The optimization of the electronics, acquisition, signal processing, data analysis, spectral deconvolution and the development of a complete and robust uncertainty budget will all be topics covered. These instrumental developments will make possible the measurement with great precision of the spectra from 36Cl, 59Fe, 87Rb, 141Ce, or 170Tm decays. This very comprehensive subject will allow the doctoral student to acquire instrumental and analytical skills that will open up many career opportunities. The candidate should have good knowledge of nuclear instrumentation, programming and Monte Carlo simulations, as well as a reasonable knowledge of nuclear disintegrations.

Development of ultra-high-resolution magnetic microcalorimeters for isotopic analysis of actinides by X-ray and gamma-ray spectrometry

The PhD project focuses on the development of ultra-high-resolution magnetic microcalorimeters (MMCs) to improve the isotopic analysis of actinides (uranium, plutonium) by X- and gamma-ray spectrometry around 100 keV. This type of analysis, which is essential for the nuclear fuel cycle and non-proliferation efforts, traditionally relies on HPGe detectors, whose limited energy resolution constrains measurement accuracy. To overcome these limitations, the project aims to employ cryogenic MMC detectors operating at temperatures below 100 mK, capable of achieving energy resolutions ten times better than that of HPGe detectors. The MMCs will be microfabricated at CNRS/C2N using superconducting and paramagnetic microstructures, and subsequently tested at LNHB. Once calibrated, they will be used to precisely measure the photon spectra of actinides in order to determine the fundamental atomic and nuclear parameters of the isotopes under study with high accuracy. The resulting data will enhance the nuclear and atomic databases used in deconvolution codes, thereby enabling more reliable and precise isotopic analysis of actinides.

Simulation of interaction phenomena between ultrasonic waves and metallic microstructures for imaging and characterization

The interaction of waves with matter strongly depends on the frequency of these waves and on the scale of their wavelengths relative to the properties of the medium under consideration. In the context of ultrasonic imaging applications that are of interest to us, the relevant length scales for metals are generally on the order of millimeters (from tenths to several tens of millimeters). Depending on the manufacturing processes used, metallic media—often anisotropic—may also exhibit microstructures with heterogeneities of similar characteristic dimensions. As a result, ultrasonic waves propagating through metals can, under certain circumstances, be significantly affected by these microstructures. This may hinder some ultrasonic techniques (due to attenuation or structural noise), or conversely, offer an opportunity to estimate local properties of the inspected metal.

The general objective of the proposed PhD thesis is to deepen the understanding of the relationship between microstructure and ultrasonic wave behavior for broad classes of materials, leveraging the combined expertise of LEM3 in virtual microstructure generation and CEA in ultrasonic wave propagation simulation.

The proposed work will combine the acquisition and analysis of experimental data (both material and ultrasonic), the use of simulation tools, and statistical data processing. This will enable an analysis of wave behavior across material classes, and possibly the development of inversion procedures to characterize a microstructure based on ultrasonic datasets. The combination of these methods will support a holistic approach, contributing to significant advancements in the field.

Development of new strategies for robotic computed tomography with variable magnification

The Department of Numerical Instrumentation joins expertise on different research fields through experimental and software platforms. In the Monitoring, Control and Diagnostic Unit, one of the important research areas is the industrial inspection with X-ray methods. Within this framework, a robotic inspection facility is being employed for innovative research and validation of new algorithms and on instrumentation aspects.
One of the most important features of robotic inspection is the possibility to scan large samples. In most application cases, region-of-interest areas are defined, for which a higher spatial resolution is targeted. In this context, a research program covering several topics is proposed, with the main objective of facilitating the setup and the use of the robotic inspection configuration for industrial application cases. A focus can be set on one or two research topics, depending on the background and on personal R&D interests and initiatives of the candidate.

A first topic will consist of developing CT reconstruction algorithms for a scan configuration using a variable magnification ratio, in a first phase with analytical algorithms such as the one proposed by Dennerlein [1] and then to adapt iterative reconstruction algorithms of SART type.

A second topic will consist on a work related to adapt the algorithms towards a multi-resolution representation of the reconstructed volumes, through octree or wavelet decomposition. An approach involving the correlation of experimental data to the CAD model of the sample will allow a better implementation in order to improve the VOI (volume of interest) tomography.

A third topic focused on instrumentation will deal with the experimental validation and additionally it will aim to develop a system capable to verify the accurate positioning of the scene elements with the help of precision distance sensors. A simultaneous measure of the distance source - to part surface together with the radiographic image will allow implementing corrections for positioning errors for every scan point and in a second phase to use this additional information directly in the reconstruction process.

architecture for embedded system of Automated and Reliable Mapping of indoor installations

The research focuses on the 3D localization of data from measurements inside buildings, where satellite location systems, such as GPS, are not operational. Different solutions exist in the literature, they rely in particular on the use of SLAM (Simultaneous Localization And Mapping) algorithms, but the 3D reconstruction is generally carried out a posteriori. In order to be able to propose this type of approach for embedded systems, a first thesis was carried out and led to a choice of algorithms to embed and a draft of the electronic architecture. A first proof of concept was also realized. Continuing this work, the thesis will have to propose a method allowing the localization device to be easily embedded on a wide range of nuclear measuring equipment (diameter, contamination meter, portable spectrometry, etc.). The work is not limited to a simple integration phase; it requires an architectural exploration, which will be based on adequacy between algorithm and architecture. These approaches will make it possible to respect different criteria, such as weight and small size so as not to compromise ergonomics for the operators carrying out the maps and quality of the reconstruction to ensure the reliability of the input data for the Digital Twin models.

Development of a transportable and high sensibility gamma/neutron spectro-imager to reconstruct and identify radioactive hotspots during decomissioning and dismantling operations

Radioactive hotspots reconstruction is a significant challenge when characterising radioelements in environments that have been impacted by radiological or nuclear activity. A thesis proposal aims to address this issue by developing a compact, highly sensitive multimodal instrument for assessing and characterising gamma-ray and neutron contributions. This system will help to meet the encountered challenges, during decommissioning and dismantling (D&D) operations, in nuclear industry sites. To do this, it will incorporate spectro-imaging specifications to ensure the identification and location of present radioelements. The state of art has already demonstrated the advantages and benefits of combining ionising radiations spectrometry and imaging. However, the suggested solutions show difficulties in deploying measurement systems (size, weight), as well as a sensitivity incompatible with the ground constraints. Results obtained in the frame of thesis works, carried out at SIMRI (Service Instrumentation et Métrologie des Rayonnements Ionisants), have led to the development of a gamma and neutron spectro-imager prototype.

Development of the Compton-TDCR Method for Scintillator Metrology

The objectives of this PhD thesis lie upstream of the applied domain, specifically in the field of radionuclide metrology. The research aims to obtain essential information for a deeper understanding of scintillation mechanisms. This topic represents a new discipline within the national metrology laboratory, currently nonexistent in other laboratories, and focuses specifically on scintillator metrology. The work will be centered on instrumentation and data analysis, enabling a refined understanding of the underlying physical phenomena. The PhD will be co-supervised by Benoit Sabot (expert in radioactivity metrology) and Christophe Dujardin (expert in scintillation).
One of the primary experimental objectives of this PhD is the development and implementation of the new Compton-TDCR setup [7], designed for the absolute measurement of scintillation yield as a function of electron energy. This system will be designed using 3D printing technology and will integrate high-purity germanium (GeHP) detectors to enhance measurement precision. After characterizing these detectors in terms of energy resolution and efficiency, they will be integrated into the final experimental setup. The PhD candidate will be responsible for signal processing using a digital module generating List-Mode files. The data will then be analyzed using an existing Rust-based software with a Python interface, which is currently limited to four channels. Given that the new setup will incorporate up to three GeHP detectors in addition to three photomultiplier channels, the software must be adapted to ensure optimal processing of the acquired data. Following fine-tuning of the electronics and a series of experimental tests, the required software modifications will be implemented to enable full data exploitation from the platform.
Once this initial phase is completed and the platform is fully operational, the candidate will focus on investigating scintillation phenomena. The first studies will examine standard scintillating materials, such as organic (liquid or plastic) and inorganic scintillators. Subsequently, the research will extend to less explored materials, such as porous scintillators. This phase will involve close collaboration with the University of Lyon, particularly with the Institut Lumière Matière, where complementary measurements will be performed to refine the analysis of scintillation phenomena, complete the laboratory findings, and develop simulations that integrate various experimental approaches.
The ultimate goal of this setup is to establish a metrology methodology for scintillators, enabling access to the response curve of these materials as a function of the energy of electrons interacting within them, as well as their temporal properties. This work will pave the way for new ionizing radiation measurement techniques and will make a significant contribution to the scientific community in this field.

Diamond Beam Monitor for FLASH Therapy

Optimizing tumor dose delivery requires advanced treatment techniques. One promising approach focuses on refining beam delivery through ultra-high dose rate irradiation (UHDR), with temporal optimization being a key strategy. Recent studies highlight the effectiveness of FLASH irradiation using electrons, demonstrating similar tumor inhibition capabilities as gamma rays but with reduced damage to healthy tissue. To fully harness this potential, we are exploring innovative beams, such as high energy electron beams, which offer instantaneous dose rates and per-pulse doses many times higher than those produced by conventional radiation sources. However, accurately monitoring and measuring these beams remains a significant challenge, primarily due to the high dose rate.
The Sensors and Instrumentation Laboratory (CEA-List) will collaborate with the Institut Curie as part of the FRATHEA project. We propose the development of a novel diamond-based monitor, connected to associated electronics, to achieve precise measurements of dose and beam shape for high-rate electron and proton beams. Interdisciplinary experimental techniques, including diamond growth, device microfabrication, device characterization under radioactive sources, and final evaluation with electron beam, will be used for prototyping and testing the diamond beam monitor.
As part of the FRATHEA project, the PhD student will work on the following tasks:
• Growth of optimized single-crystal chemical vapor-deposited (scCVD) diamond structures
• Characterization of the electronic properties of the synthesized diamond materials
• Estimation of the dose response characteristics of a simplified prototypes
• Fabrication of a pixelated beam monitor
• Participation in beam times at the Institut Curie (an other institutes) for devices testing in clinical beams
Required Skills:
• Strong background in semiconductor physics and instrumentation
• Knowledge of radiation detectors and radiation-matter interactions
• Ability to work effectively in a team and demonstrate technical rigor in measurements
Additional Skills:
• Knowledge of electronics, including signal processing, amplifiers, oscilloscopes, etc.
• Familiarity with device fabrication and microelectronics
• Previous experience working with diamond materials
Profile:
• Master's level (M2) or engineering school, with a specialization in physical measurements
• Adherence to radiation protection regulations (category B classification required)
PhD Duration: 3 years
Start Date: Last semester of 2025
Contact:

Michal Pomorski : michal.pomorski@cea.fr
Guillaume Boissonnat: guillaume.boissonnat@cea.fr

Top