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.
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.
Exoplanets: phase curves observed by JWST
The James Webb Space Telescope (JWST), launched by NASA on December 25, 2021, is revolutionizing our understanding of the cosmos, particularly in the field of exoplanets. With more than 6,000 exoplanets detected, a great variety of worlds have been discovered, some with no equivalent in our Solar System, such as « hot Jupiters » or « super-Earths ». JWST now enables detailed characterization of exoplanetary atmospheres thanks to its spectroscopic instruments covering wavelengths from 0.6 to 27 µm and its large light-collecting area (25 m²). This capability allows determination of molecular composition, the presence of clouds or aerosols, the pressure–temperature profile, and the physical and chemical processes at work in these atmospheres.
The main method used is the so-called transit method, which observes variations in brightness when a planet passes in front of or behind its star (secondary eclipse). Nevertheless, observations over the entire orbital period (phase curve)—which also includes a transit and two eclipses—provide even more information. With phase curves, the energy budget, longitudinal structure, and atmospheric circulation can be directly observed. JWST has already obtained phase-curve data of exceptional quality. Many of these datasets are now publicly available and contain a wealth of information, though they are only partially exploited. The length of these observations, the extremely faint signals (a few tens of ppm), and the presence of subtler instrumental effects make the analysis of these data more complex.
The proposed PhD will first focus on studying and correcting these instrumental effects, then on extracting atmospheric properties using the TauREx software (https://taurex.space/), under the co-supervision of Quentin Changeat (University of Groningen) and Pierre-Olivier Langage (CEA Paris-Saclay). This PhD will contribute to preparing the scientific exploitation of the ESA Ariel mission (launch planned for 2031), entirely dedicated to the study of exoplanetary atmospheres and expected to observe nearly 50 phase-curves.
Magneto-convection of solar-type stars: flux emergence and origin of starspots
The Sun and solar-type stars possess rich and variable magnetism. In our recent work on turbulent convective dynamos in this type of star, we have been able to highlight a magneto-rotational history of their secular evolution. Stars are born active with short magnetic cycles, then slow down due to braking by their magnetized particle wind, their magnetic cycle lengthens to become commensurate with that of the Sun (lasting 11 years) and finally, for stars that live long enough, they end up with a loss of cycle and a so-called anti-solar rotation (slow equator/fast poles). The agreement with observations is excellent, but we are missing an essential element to conclude: What role do sunspots/starspots play in the organization of the magnetism of these stars, and are they necessary for the appearance of a stellar magnetic cycle, e.g. the so-called “paradox of spotty dynamos”? Indeed, our HPC simulations of solar dynamos do not have yet the angular resolution to resolve the spots, and yet we do observe cycles in our simulations of stellar dynamos for Rossby numbers < 1. So, are the spots simply a surface manifestation of an internal self-organization of the cyclic magnetism of these stars, or do they play a decisive role? Furthermore, how do the latitudinal flux emergence and the size and intensity of the spots forming on the surface evolve during the magneto-rotational evolution of these stars? To answer these key questions in stellar and solar magnetism in support of the ESA space missions Solar Orbiter and PLATO, in which we are involved, new HPC simulations of stellar dynamos must be developed, allowing us to get closer to the surface and thus better describe the process of magnetic flux emergence and the possible formation of sun/starspots. Recent tests showing that magnetic concentrations inhibiting local surface convection form in simulations with a higher magnetic Reynolds number and smaller-scale surface convection strongly encourage us to continue this project beyond the ERC Whole Sun project (ending in April 2026). Thanks to the Dyablo-Whole Sun code that we are co-developing with IRFU/Dedip, we wish to study in detail the convective dynamo, the emergence of magnetic flux, and the self-consistent formation of resolved spots, using its adaptive mesh refinement capability while varying global stellar parameters such as rotation rate, convective zone thickness, and surface convection intensity to assess how their number, morphology and latitude of emergence change and if they contribute or not to the closing of the cyclic dynamo loop.
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.
PtSeipin : linking lipid droplets biogenesis and degradation in the diatom Phaeodactylum tricornutum
Microalgae encompass a wide diversity of organisms and have attracted increasing interest due to their ability to produce biomolecules of biotechnological and industrial relevance. In particular, they can accumulate oil within lipid droplets (LDs) in response to abiotic stresses such as nitrogen deprivation. This oil accumulation holds great potential for biofuel production.We recently demonstrated that knockout of the gene encoding Seipin, a protein involved in LD biogenesis, leads to a strong oil accumulation in the diatom Phaeodactylum tricornutum. Moreover, this accumulation appears to result from an absence of LD degradation in the Seipin-deficient mutants. These findings suggest that, in this diatom, LDs are programmed to undergo degradation soon after their formation, thus inhibiting LD degradation could prove a promising strategy to increase their oil content.This project aims to elucidate the mechanisms underlying LD degradation and, more specifically, the connections between their biogenesis and degradation. Three main research axes will be pursued:
1. Identify PtSeipin interaction partners involved in LD degradation, using both candidate-based and unbiased approaches.
2. Characterize the LD degradation pathways disrupted in PtSeipin knockout mutants by combining electron microscopy with transcriptomic and proteomic analyses.
3. Investigate how microalgae utilize oil during the recovery phase after stress, through fluxomic approaches.
Development of a Modular Enzymatic Platform for the In Silico Design and Synthesis of Novel Therapeutic Peptides via Protein Splicing
The rise of antimicrobial resistance (AMR) has developed into a slow-moving epidemic, fueled by the overuse and misuse of antibiotics, coupled with a stagnation in the development of new antimicrobial agents over the past four decades. Addressing this crisis requires not only more judicious use of existing antibiotics but also the development of innovative drugs capable of overcoming resistant pathogens. In this context, the abundant genomic data generated in the omics era has facilitated the resurgence of natural products as a vital source of novel compounds. Among these, natural peptides—with their unique and diverse chemical properties—have garnered particular interest as potential antibiotics, anticancer agents, and inhibitors targeting specific pathological processes.
The aim of this PhD project is to develop a novel, modular enzymatic tool that enables the in silico design and synthesis of peptides with unprecedented chemical diversity. Central to this approach is the exploitation of a unique chemical reaction: protein splicing. This innovative reaction allows precise removal or editing of specific peptidic sequences, thereby providing a powerful platform to generate hybrid peptides with tailored functionalities, including potential therapeutic agents.
This project will integrate structural and functional studies, computational peptide design and enzyme engineering, aiming to expand the chemical and functional diversity of peptide-based molecules. The successful candidate will work in a state-of-the-art research setting, equipped with cutting-edge facilities and collaborative opportunities, fostering innovative approaches and impactful contributions to the field.
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.
Surface technologies for enhanced superconducting Qubits lifetimes
Materials imperfections in superconducting quantum circuits—in particular, two-level-system (TLS) defects—are a major source of decoherence, ultimately limiting the performance of qubits. Thus, identifying the microscopic origin of possible TLS defects in these devices and developing strategies to eliminate them is key to superconducting qubit performance improvement. This project proposes an original approach that combines the passivation of the superconductor’s surface with films deposited by Atomic Layer Deposition (ALD), which inherently have lower densities of TLS defects, and thermal treatments designed to dissolve the initially present native oxides. These passivating layers will be tested on 3D Nb resonators than implemented in 2D resonators and Qubits and tested to measure their coherence time. The project will also perform systematic material studies with complementary characterization techniques in order to correlate improvements in qubit performances with the chemical and crystalline alteration of the surface.