Numerical Study of Interstellar Turbulence in the Exascale Era
This PhD project aims to better understand interstellar medium turbulence, a key phenomenon governing the formation of stars and galactic structures. This turbulence—magnetized, supersonic, and multiphase—influences how energy is transferred and dissipated, thereby regulating the efficiency of star formation throughout the history of the Universe. Its study is complex, as it involves a wide range of spatial and temporal scales that are difficult to reproduce numerically. Advances in high-performance computing, particularly the advent of GPU-based exascale supercomputers, now make it possible to perform much more refined simulations.
The Dyablo code, developed at IRFU, will be used to carry out large-scale three-dimensional simulations with adaptive mesh refinement to resolve the regions where energy dissipation occurs. The study will progress in stages: first, simulations of simple isothermal flows will be conducted, followed by models that include heating, cooling, magnetic fields, and gravity. The turbulent properties will be analyzed using power spectra, structure functions, and density distributions, in order to better understand the formation of dense regions that give birth to stars. Finally, the work will be extended to the galactic scale, in collaboration with other French institutes, to investigate the large-scale energy cascade of turbulence across entire galaxies.
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.
Mining LEP data for fragmentation: A TMD-oriented analysis of pi+pi- pairs in e+e- collisions
This project aims to advance our understanding of quark and gluon fragmentation by performing the first-ever extraction of Transverse-Momentum-Dependent Fragmentation Functions (TMDFFs) for charged pions using archived data from LEP experiments like DELPHI or ALEPH.
Fragmentation Functions, which describe how partons form detectable hadrons, are non-perturbative and must be determined from experimental data. TMDFFs provide more detailed information about the transverse momentum of these hadrons. An ideal process to study them is the production of back-to-back pi+pi- pairs in electron-positron annihilations, a measurement surprisingly absent from both past and current experiments.
The project will leverage CERN OpenData initiative to access this historical data. The work is structured in three key steps: first, overcoming the technical challenge of accessing the data using potentially obsolete software; second, extracting relevant physical distributions, such as the transverse momentum of the pion pairs; and third, using Monte Carlo simulations (e.g., Pythia8) to interpret the results.
A crucial part of the analysis will be to identify the observables most sensitive to TMDFFs through simulations. The final data analysis will employ modern techniques to ensure a robust estimate of all uncertainties. Once completed, this pioneering measurement will be incorporated into a global analysis of TMD data, significantly improving the accuracy of TMDFFs and pushing the boundaries of our knowledge of non-perturbative QCD.
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.
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.