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.
Joint simulation-based inference of tSZ maps and Euclid's weak lensing
Context:
The Euclid mission will provide weak lensing measurements with unprecedented precision, which have the potential to revolutionise our understanding of the Universe. However, as the statistical uncertainties decrease, controlling systematic effects becomes even more crucial. Among these, baryonic feedback, which redistributes gas within galaxies and clusters, remains one of the key astrophysical systematic effects limiting Euclid’s ability to constrain the equation of state of dark energy. Understanding baryonic feedback is one of the urgent challenges of cosmology today.
The thermal Sunyaev-Zel’dovich (tSZ) effect provides a unique window into the baryonic component of the Universe. This effect arises from the scattering of cosmic microwave background (CMB) photons by hot electrons in galaxy groups and clusters. This is the same hot gas that has been redistributed by baryonic feedback and is particularly relevant for weak lensing cosmology. The cross-correlation between tSZ and weak lensing (WL) probes how baryons trace and modify the cosmic structures, allowing joint constraints on cosmology and baryonic physics.
Most current tSZ-WL analyses rely on fitting angular power spectra under the assumption of a Gaussian likelihood. However, the tSZ signal is highly non-Gaussian, as it traces the massive structures of the Universe, and the power spectra fail to fully capture the information in the data. To unlock the scientific potential of the tSZ-WL analyses, it is essential to move beyond these simplifying assumptions.
PhD thesis:
The goal of this PhD project is to develop a novel simulation-based framework to jointly analyse tSZ and Euclid’s WL data. This framework will combine physically motivated forward models with advanced statistical and machine-learning techniques to provide accurate measurements of baryonic feedback and cosmological parameters. By jointly analysing tSZ and WL measurements, this project will increase the accuracy of Euclid’s cosmological analyses and improve our understanding of the dark matter-baryon connection.
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.
Studying the structural dynamics of vitamin B12 -dependent photoreceptors in view of biotechnological applications
This integrated structural biology project aims at gaining a mechanistic understanding of the recently discovered family of vitamin B12 -dependent photoreceptors. In particular, we aim at visualising protein conformational changes upon photoactivation from the photochemical timescales (femtoseconds) to the photobiological timescales (milliseconds -seconds). To do so, we will use time-resolved X-ray crystallography and X-ray solution scattering at X-ray free electron lasers (XFEL) and at synchrotrons. By establishing the modus operandi of these newly discovered B12 photoreceptors we will open a window to their rational modification for biotechnological exploitation as optogenetic components.
Synthesis and optical properties of quantum dots
Graphene as a constituent of graphite was close to us for almost 500 years. However, it is only in 2005 that A. Geim and K. Novoselov (Nobel Prize in 2010) reported for the first time the obtaining of a nanostructure composed by a single layer of carbon atom. The exceptional electronic properties of graphene make it a very promising material for applications in electronic and renewable energies.
For many applications, one should be able to modify and control precisely the electronic properties of graphene. In this context, we propose to synthesize chemically graphene nanoparticles and study their absorption and photoluminescence properties. We will focus on families of elongated nanoparticles, with the aim of studying how size can enable us to observe and control multiexcitonic processes in these materials. This project will be developed in collaboration with Physicists so the candidate will work in a multidisciplinary environment.
Real-time measurement of edge plasma parameters for the optimization of the WEST tokamak performance
The control of heat fluxes at the plasma edge, and particularly in the divertor — a dedicated volume where these fluxes are focused — is a major challenge in research on magnetic confinement fusion. In future devices, heat fluxes will need to be dissipated by radiation to reduce the heat conducted to the divertor. However, the operational window for these high-radiation regimes is quite narrow and requires precise control of the edge plasma. The PhD first objective is to develop a real-time measurement of the density and temperature profiles at the plasma edge from the Thomson scattering diagnostics. By leveraging a large experimental database and simulations performed with edge plasma modeling tools and plasma/wall interaction models, the student will then develop meta-models to create a real-time control algorithm for WEST scenarios, particularly for high radiation discharges. This development will rely on continuous iteration between simulations, experimental observations, and real-time control performance requirements. This thesis is part of a collaborative framework involving French universities and international collaborations, with a high level of expected scientific visibility.
Regulation of gene expression by acetylation and lactylation of histone proteins
In eukaryotic cells, DNA wraps around histone proteins to form chromatin. Dynamic modification of histones by various chemical structures allows for fine regulation of gene expression. Alterations in these complex regulatory mechanisms are responsible for many diseases. Acetylation of histone lysines is known to induce gene expression. Other structures can be added to histones, whose effects on transcription remain largely unclear. Most of them, such as lactylation discovered in 2019, depend on cellular metabolism. We are studying this new modification in murine spermatogenesis: this process of cell differentiation is an ideal model for studying transcription regulation, due to dramatic changes in chromatin composition and gene expression patterns. We have established the distribution of acetylated and lactylated marks on three lysines of histone H3 across the genome. The aim of this thesis is to contribute to deciphering the “histone language,” first by studying the role of lactylations on the transcriptional program. Next, the prediction of chromatin states will be refined by integrating our new data with numerous available epigenomic data within neural network models.
Theoretical studies of orbital current and their conversion mechnism for leveraging spin-orbit torques based devices performances
The proposed PhD thesis aims at understanding and identifying the key parameters governing the conversion of orbital moments into spin currents, with the goal of enhancing the write efficiency of spin-orbit torque magnetic random-access memory (SOT-MRAM) devices. The work will employ a multiscale modeling approach comprising ab initio, tight-binding and atomistic calculations of the Orbital Hall Effect (OHE) and Orbital Rashba-Edelstein Effect (OREE). These phenomena exhibit larger magnitudes and diffusion lengths compared to their spin counterparts, Spin Hall Effect (SHE) and Rashba-Edelstein Effect (REE). Furthermore, they are present in a broader range of materials, including low-resistivity light metals. This opens very interesting opportunities for more efficient and conductive materials, potentially lifting the barriers limiting the technological deployment of SOT-MRAM.
This thesis will play a key role in a close collaboration between SPINTEC and LETI laboratories at CEA. The PhD student will conduct ab initio calculations at SPINTEC to unveil fundamental material characteristics to exploit the described orbitronic phenomena, and will construct multi-orbital Hamiltonians at LETI to calculate orbital and spin transport, in strong interaction/synergy with experimentalists working on SOT-MRAM development. The PhD will be co-supervised by M. Chshiev, K. Garello at Spintec and J. Li at LETI. This PhD project will be at the heart of collaborations with leading theoretical and experimental groups at national and international level.
Highly motivated candidates with a strong background in solid-state physics, condensed matter theory, and numerical simulations are encouraged to apply. The selected candidate will perform calculations using Spintec’s computational cluster, leveraging first-principles DFT-based packages and other simulation tools. Results will be rigorously analyzed, with opportunities for publication in international peer-reviewed journals.
Effect of gamma-ray irradiation on ferroelectric, hafnia-based, non-volatile memory for use in extreme environments
The emergence of hafnia-based ferroelectric (FE) memories has opened a new paradigm for ultra-low-power edge computing. Hafnia is fully compatible with CMOS technology and is ultra low-power—three orders of magnitude less than other emerging memory technologies.
These advantages align with strategic applications in space, defense, medical, nuclear safety, and heavy-duty transport, where electronics face harsh radiation environments.
Imprint induces a shift of the Polarization-Voltage (P-V) curve along the voltage axis and is attributed to charge trapping/detrapping, domain pinning and charged defects. All may be accentuated under irradiation.
The project will use advanced photoelectron spectroscopy techniques including synchrotron radiation induced Hard X-ray photoelectron spectroscopy and complementary structural analysis including high-resolution electron microscopy, X-ray diffraction and near field microscopy. The experimental characterization will be accompanied by theoretical calculations to simulate the material response to irradiation
The work will be carried out in the framework of close collaboration between the CEA/Leti in Grenoble providing the samples, integrated devices and wafer scale characterization and the CEA/Iramis in Saclay for the fundamental analysis of the material properties, irradiation experiments and device scale characterizations.
Machine-learning methods for the cosmological analysis of weak- gravitational lensing images from the Euclid satellite
Weak gravitational lensing, the distortion of the images of high-redshift galaxies due to foreground matter structures on large scales, is one of the most promising tools of cosmology to probe the dark sector of the Universe. The statistical analysis of lensing distortions can reveal the dark-matter distribution on large scales, The European space satellite Euclid will measure cosmological parameters to unprecedented accuracy. To achieve this ambitious goal, a number of sources of systematic errors have to be quanti?ed and understood. One of the main origins of bias is related to the detection of galaxies. There is a strong dependence on local number density and whether the galaxy's light emission overlaps with nearby objects. If not handled correctly, such ``blended`` galaxies will strongly bias any subsequent measurement of weak-lensing image distortions.
The goal of this PhD is to quantify and correct weak-lensing detection biases, in particular due to blending. To that end, modern machine- and deep-learning algorithms, including auto-differentiation techniques, will be used. Those techniques allow for a very efficient estimation of the sensitivity of biases to galaxy and survey properties without the need to create a vast number of simulations. The student will carry out cosmological parameter inference of Euclid weak-lensing data. Bias corrections developed during this thesis will be included a prior in galaxy shape measurements, or a posterior as nuisance parameters. This will lead to measurements of cosmological parameters with a reliability and robustness required for precision cosmology.