Building a new effective nuclear interaction model and propagating statistical errors
At the very heart of any « many-body » method used to describe the fundamental properties of an atomic nucleus, we find the effective nucleon-nucleon interaction. Such an interaction should be capable of taking into account the nuclear medium effects. In order to obtain it, one has to use a specific fitting protocol that takes into account a variety of nuclear observables such as radii, masses, the centroids of the giant resonances or the properties of the nuclear equation of state around the saturation density.
A well-known model of the strong interaction is the Gogny model. It is a linear combination of coupling constants and operators, plus a radial form factor of the Gaussian type [1]. The coupling constants are determined via a fitting protocol that typically uses the properties of spherical nuclei such as 40-48Ca, 56Ni, 120Sn and 208Pb.
The primary goal of this thesis is to develop a consistent fitting protocol for a generic Gogny interaction in order to access some basic statistical information, such as the covariance matrix and the uncertainties on the coupling constants, in order to be able to perform a full statistical error propagation on some selected nuclear observables calculated with such an interaction [2].
After having analysed the relations between the model parameters and identified their relative importance on how well observables are reproduced, the PhD candidate will explore the possibility of modifying some terms of the interaction itself such as the inclusion of a real three-body term or beyond mean-field effects.
The PhD candidate will work within a nuclear physics group at CEA/IRESNE Cadarache. The work will be done in close collaboration with CEA/DIF. Employment perspectives are in academic research and nuclear R&D labs.
[1] D. Davesne et al. "Infinite matter properties and zero-range limit of non-relativistic finite-range interactions." Annals of Physics 375 (2016): 288-312.
[2] T. Haverinen and M. Kortelainen. "Uncertainty propagation within the UNEDF models." Journal of Physics G: Nuclear and Particle Physics 44.4 (2017): 044008.
Microscopic nuclear structure models to study de-excitation process in nuclear fission
The FIFRELIN code is being developed at CEA/IRESNE Cadarache in order to provide a detailed description of the fission process and to calculate all relevant fission observables accurately. The code heavily resides on the detailed knowledge of the underlying structure of the nuclei involved in the post-fission de-excitation process. When possible, the code relies on nuclear structure databases such as RIPL-3 that provide valuable information on nuclear level schemes, branching ratios and other critical nuclear properties. Unfortunately, not all these quantities have been measured, nuclear models are therefore used instead.
The development of state-of-the-art nuclear models is the task of the newly-formed nuclear theory group at Cadarache, whose main expertise is the implementation of nuclear many-body solvers based on effective nucleon-nucleon interactions.
The goal of this thesis is to quantify the impact of the E1/M1 and E2/M2 strength functions on fission observables. Currently, this quantity is estimated using simple models such as the generalized Lorentzian. The doctoral student will be tasked with replacing these models by fully microscopic ones based on effective nucleon-nucleon interaction via QRPA-type techniques. A preliminary study shows that the use of macroscopic (generalized Lorentzian) or microscopic (QRPA) has a non-negligible impact on fission observables.
Professional perspectives for the student include academic research as well as theoretical and applied nuclear R&D.
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. 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 has been accepted by the GSI Programme Committee and its realization is planned for 2025.
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.
Probing Gluon Dynamics in the Proton via the Exclusive Phi Meson Photoproduction with CLAS12
Protons and neutrons are made of partons (quarks and gluons) that interact via the strong force, governed by Quantum Chromodynamics (QCD). While QCD can be computed at high energies, its complexity reveals itself at low energies, requiring experimental inputs to understand nucleon properties like their mass and spin. The experimental extraction of the Generalized Parton Distributions (GPDs), which describe the correlation of the partons longitudinal momenta and transverse positions within nucleons, provide critical insights into these fundamental properties.
This thesis focuses on analyzing data from the CLAS12 detector, an experiment part of Jefferson Lab's research infrastructure, one the 17 National Laboratory in the USA. CLAS12, a 15-meter-long fixed-target detector with large acceptance, is dedicated to hadronic physics, particularly GPDs extraction. The selected student will study the exclusive photoproduction of the phi meson (gamma p->phi p’), which is sensitive to gluon GPDs, still largely unexplored. The student will develop a framework to study this reaction in the leptonic decay channel (phi -> e+e-) and develop a novel Graph Neural Network-based algorithm to enhance the scattered proton detection efficiency.
The thesis will aim at extracting the cross section of the photoproduction of the phi, and interpret it in term of the proton's internal mass distribution. Hosted at the Laboratory of Nucleon Structure (LSN) at CEA/Irfu in Saclay, this project involves international collaboration within the CLAS collaboration, travel to Jefferson Lab for data collection, and presentations at conferences. Proficiency in particle physics, programming (C++/Python), and English is required. Basic knowledge of particle detectors and Mahine Learning is advantageous but not mandatory.
Near-threshold phenomena in nuclear structure and reactions
It is proposed to study the salient effects of coupling between discrete and continuous states near various particle emission thresholds using the shell model in the complex energy plane. This model provides the unitary formulation of a standard shell model within the framework of the open quantum system for the description of well bound, weakly bound and unbound nuclear states.
Recent studies have demonstrated the importance of the residual correlation energy of coupling to the states of the continuum for the understanding of eigenstates, their energy and decay modes, in the vicinity of the reaction channels. This residual energy has not yet been studied in detail. The studies of this thesis will deepen our understanding of the structural effects induced by coupling to the continuum and will provide support for experimental studies at GANIL and elsewhere.
NEW PATHS TO PRODUCE NEUTRON RICH HEAVY NUCLEI
One of the strongest research projects in recent years has emerged from a critical, unresolved question about the natural origin of nuclei heavier than iron. The closed neutron shell, N = 126, as the final waiting point in the r-process (rapid neutron capture process), plays an essential role in the formation of these nuclei. However, recent efforts to synthesize superheavy elements and explore N = 126 neutron-rich nuclei have faced significant challenges due to extremely low cross sections using traditional fusion-evaporation reactions.
These factors highlight the urgent need for alternative reaction mechanisms. One alternative has been identified in multinucleon transfer (MNT) reactions, which offer a promising route to neutron-rich heavy nuclei. The challenge is to isolate the desired nuclei from the multitude of products generated during the reaction.
We have been working on this reaction mechanism for several years, performing experiments at Argonne National Laboratory and other international laboratories.
The aim of this thesis is to analyse the data collected during the Argonne experiment (end 2023) and to propose a new experiment at the spectrometer Prisma (Legnaro National Lab) coupled with the Agata germanium detector.
Shapes, vibrations and rotations of the 106Cd nucleus studied with gamma-ray spectroscopy with GRIFFIN and AGATA
One of the key questions in the field of nuclear structure concerns the emergence of collectivity, and its link with the microscopic structure of the nucleus. Atomic nuclei can exhibit so-called collective behaviours, which means that their constituents, protons and neutrons, move in a coherent way. The main modes of collective nuclear motion are vibrations and rotations. If a nucleus is not deformed, it cannot undergo rotations when excited, but vibrations around its spherical equilibrium shape are possible.
Even-even isotopes of cadmium have been considered textbook examples of vibrational behaviour. However, this interpretation has been questioned following recent experimental studies, which have, with a guidance from theoretical calculations, led to the reorganization of the level schemes of 110,112Cd in terms of rotational excitations, suggesting the presence of a variety of shapes in these nuclei. Thanks to a recent PhD work in our group, this new interpretation has been extended to the 106Cd nucleus. However, questions remain regarding the nature of certain low-lying excited states in this nucleus. In particular, we obtained indications that some excited states may result from a coupling between the so-called octupole (i.e. the nucleus deforms into a pear shape) and quadrupole (i.e. the nucleus oscillates between elongated and flattened shapes) vibrations. To test this hypothesis, a high-precision beta-decay experiment has been proposed at TRIUMF (Vancouver, Canada) using the world's most advanced spectrometer for beta-decay measurements GRIFFIN, to search for weak decay paths in the 106Cd level scheme, and to unambiguously determine the spins of the excited states through the analysis of gamma-gamma angular correlations. Thanks to this measurement it will be possible to solve multiple puzzles concerning the structure of this nucleus, in particular regarding the possible triaxiality of its ground state and the suspected coexistence of multiple shapes.
The student will be in charge of the analysis of this experiment, which will take place in 2025. Then, based on the results of this analysis, they will proceed to a re-evaluation of the population cross sections of excited levels in 106Cd, which were measured with the new generation gamma-ray spectrometer AGATA at GANIL using the Coulomb excitation technique. From this combination of measurements, we hope to obtain, for the first time in the nuclear chart, the complete set of transition probabilities between the states resulting from the coupling between octupole and quadrupole vibrations. We will then proceed to the interpretation of the results in close collaboration with experts in nuclear-structure theory.
This thesis work will make it possible for the student to follow a research project in its entirety, from the preparation of the experiment to its theoretical interpretation, and to become familiar with several experimental gamma-ray spectroscopy techniques, using the most advanced gamma-ray spectrometers in the world.
Fission yield measurements for decay heat evaluation of used nuclear fuel
The fission process involves the violent splitting of a heavy nucleus into two fission fragments, resulting in over 300 different isotopes. Understanding the distribution and production of these fragments, known as fission yields, is essential for grasping the underlying mechanisms of fission, which are influenced by nuclear structure and dynamics. Accurate measurements of fission yields are crucial for advancing nuclear energy applications, particularly in developing Generation IV reactors and recycling spent nuclear fuel. The VAMOS magnetic spectrometer enables precise fission yield measurements due to its large acceptance and identification capabilities for various isotopes. An experimental campaign at VAMOS in 2024 utilized beams of (^{238})U and (^{232})Th on a carbon target to produce fissioning actinides. The combination of VAMOS with a new Silicon telescope (PISTA) enhances data quality significantly. The candidate will analyze VAMOS data to produce high-resolution fission yields and study uncerta
Systematic study of the neutron scattering reactions on structural materials of interest for nuclear reactor applications
Elastic and inelastic scattering reactions on structural materials have a significant impact on the simulation of neutron transport. The nuclear data of structural materials of interest for nuclear reactors and criticality studies must be known with good precision over a wide incident neutron energy range, from a few tens of meV to several MeV. The thesis proposal aims to carry out a systematic study of the scattering reactions above the resolved resonance range up to 5 MeV. In this energy range, neither the R-Matrix formalism nor the statistical Hauser-Feshbach model are valid for structural materials. A new formalism will be developed by using high-resolution measurements of the scattering angular distributions. This work will focus more precisely on measurements already done at the JRC-Geel facility (sodium [1], iron [2]) and will be extended to other elements studied within the framework of the IAEA/INDEN project, such as copper, chromium and nickel. As part of this thesis, the experimental database will be complemented by new measurements on the copper isotopes (Cu63 and Cu65). The measurements will be carried out at JRC Geel GELINA facility with the ELISA detector. Concerning the copper isotopes, integral benchmarks from the ICSBEP database revealed several issues in the nuclear data libraries, which provide contradictory integral feedbacks on the nuclear data of U235. For example, the ZEUS benchmarks, which is routinely used to study the capture cross section of U235 in the fast neutron energy range, are very sensitive to the nuclear data of copper. This type of benchmark will provide an ideal framework for quantifying the impact of any new formalism developed to evaluate the nuclear data of structural materials.
This study will allow the PhD student to develop skills in experimental and theoretical nuclear physics, as well as in neutron physics. The results will be communicated to the JEFF working group of the Nuclear Energy Agency (OCDE/AEN).
[1] P. Archier, Contribution à l’amélioration des données nucléaires neutroniques du sodium pour le calcul des réacteurs de génération IV, Thèse, Université de Grenoble, 2011.
[2] G. Gkatis, Study of neutron induced reaction cross sections on Fe isotopes at the GELINA facility relevant to reactor applications, Thèse, Université Aix-Marseille, 2024.
Modeling of nuclear charge polarization as part of fission yield evaluation: applications to actinides of interest to the nuclear fuel cycle
Nuclear data is crucial for civil nuclear energy applications, being the bridge between the micoscopic properties of nuclei and the “macroscopic good values” needed for cycle and reactor physics studies. The laboratory of physics studies at CEA/IRESNE Cadarache is involved in the evaluation of these nuclear physics observables, in the framework of the JEFF Group and the Coordinated Research Project (CRP) of IAEA. The recent development of a new methodology for thermal neutrons induced fission product yield evaluation (fission product yields after prompt neutron emission) has improved the accuracy of the evaluations proposed for the JEFF-4.0 Library, together with their covariance matrix. To extend the assessments of fission yields induced by thermal neutrons to the fast neutron spectrum, it is necessary to develop a coupling of current evaluation tools with fission fragment yield models (before prompt neutron emission). This coupling is essential to extrapolate the actual studies on thermal fission of 235U and 239Pu to less experimentally known nuclei (241Pu, 241Am, 245Cm) or to study the incident neutron energy dependence of fission yields. One of the essential missing components is the description of the nuclear charge distribution (Z) as a function of the mass of the fission fragments and the incident neutron energy. These distributions are characterized by a key parameter: the charge polarization. This polarization reflects an excess (respectively deficiency) of proton in light (respectively heavy) fission fragments compared to the average charge density of the fissioning nucleus. If this quantity has been measured for the 235U(nth,f) reaction, it is incomplete for other neutron energies or other fissioning systems. The perspectives of this subject concern as much the impact of these new evaluations on the key quantities for electronuclear applications as well as the validation of the fission mechanisms described by microscopic fission models.