Institute of research into the fundamental laws of the Universe

Although known and studied for more than 80 years, nuclear fission remains a very active research topic both for its fundamental aspect of the study of nuclear matter and for its applications, including nuclear energy.

Prompt gamma rays allow probing the structure and properties of fragments emitted during the fission process. Their use therefore offers a new perspective for its study. In particular, it makes it possible to explore effects that have not yet been studied experimentally, such as the influence of the shape of the fragments on the fission process or the sharing of excitation energy between the fragments. On the other hand, the measurement of fission-prompt gamma rays also provides valuable data for the simulation of gamma heating in nuclear reactors.

The thesis work will consist in the analysis of the data from the new gamma spectrometer, FIPPS, installed at the Grenoble research reactor. This spectrometer is composed of an array of Germanium detectors arranged around a fissile target placed in an intense flux of thermal neutrons. Experimental results will allow the student to test recent models on the fission and fragment de-excitation processes.

The Quark Gluon Plasma is a state of matter created under extreme conditions at temperatures of the few hundreds MeV where nucleon constituants are deconfined long enough to produce sizeable effects when interacting with the plasma.

Thermodynamical conditions required to form this plasma can be reproduced in ultra relativistic heavy-ion collisions at the LHC (CERN).

The goal of the proposed thesis is the first measurement in the ALICE experiment of hadrons made of bottom quarks down to zero transverse momentum in Pb-Pb collisions from their decay into J/psi resonances. This measurement adds a very important constraint in the understanding of the initial state of heavy-ion collisions and in the heavy-quarks transport within the plasma, in particular their hadronisation.

The proposed thesis will be initially concentrated on the commissioning of a new high-precision silicon detector, called Muon Forward Tracker, necessary for the detection at forward rapidities of muon pairs coming from the J/psi decay. In a second stage, the analysis of the first Pb-Pb data that will be taken in 2021 with this new detector will allow studying the separation between prompt J/psi produced during the collisions from those coming from the B-mesons decay. This study, novel within the ALICE experiment, represents a very important test of heavy quarks properties inside the Quark Gluon Plasma.

One of the most fundamental properties of the atomic nucleus is its shape, which is governed by the interplay of macroscopic, liquid-drop like properties of the nuclear matter and microscopic shell effects, which reflect the underlying nuclear interaction. In some cases, configurations corresponding to different shapes may coexist at similar excitation energies, which results in the wave functions of these states mixing. Experimental observables such as quadrupole moments and the electromagnetic transition rates between states are closely related to the nuclear shape. The experimental determination of these observables, therefore, represents a stringent test for theoretical models. This thesis is integrated in our ongoing programme to study nuclear shapes by means of Coulomb excitation and more specifically such an experiment is planned on 100Zr. This method allows to extract the excitation probability for each excited state and to extract a set of electro magnetic matrix elements, and in particular the quadrupole moment which determines the shape of the nucleus. The radioactive 100Zr beam is provided by the ATLAS-CARIBU facility at Argonne National Laboratory (ANL), which is currently the only facility world-wide able to deliver beams of such refractory elements. The programme advisory committee has already accepted the experiment with high priority and we expect it to be scheduled in Q4/2020. The PhD student will participate in the preparation and setting-up of the experiment. It would be advantageous if he/she started already working on the subject already during the stage M2. He/she will be responsible for the data analysis, the presentation of the scientific results (at conferences or workshops) and their publication in a scientific journal. During the thesis work the PhD student may also participate in other experiments of the research group. All experiment(s) take place in international collaborations and may require prolonged stays at foreign laboratories (e.g. 4-6 weeks at ANL, USA).

Most of the visible mass of the universe is contained in nucleons. However, the origin of this mass remains mysterious, with the portion from the Higgs mechanism in standard renormalization schemes corresponding to only a few percent of the total mass. The answer is to be found in the dynamics of strong interaction, described by the theory of quantum chromodynamics (QCD) in terms of quarks and gluons. Thus, the interaction between quarks and gluons is responsible for the emergence of known and measured properties of hadrons such as their masses or spins.

There is now a strong theoretical and experimental dynamic to determine the 3D structure of hadrons in terms of quarks and gluons. From a theoretical point of view, the classical tools of quantum field theory, namely perturbative expansion, do not allow the study of the emerging properties of hadrons. The latter are inherently non- disruptive.

The aim of this thesis is to develop and use a non-perturbative formalism based on the Dyson-Schwinger and Bethe-Salpeter equations to determine the 3D structure of hadrons, in particular the nucleon. Different dynamic assumptions will be used to obtain a 3D mapping of the charge, mass and orbital angular momentum effects. A comparison of the results obtained with the experimental data will be carried out in collaboration with the other LSN members.

In nuclear physics, atomic nuclei are described in the framework of shell models, with formal analogies to atomic physics. Orbitals characterized with angular momentum are successively filled from the bottom of the nuclear well. « Magic nuclei » correspond to to the full filling of orbitals before a major energy gap is crossed, with a net gain of stability. Not only stable nuclei were successfully described this way, but it also provides a framework for the evolution of nuclear properties versus the neutron/proton asymmetry parameter. Evolution of orbitals and associated magic numbers is under study with increasing neutron number for exotic nuclei far from stability. For calcium isotopes with a magic proton number Z=20, the relative ordering of proton orbitals 0d3/2 and 1s1/2 evolves from 40Ca (N=20) to 48Ca (N=28) and filling the neutron 0f7/2 orbital as an effect of the neutron-proton interaction in nuclei [1]. It was evidenced by the measurement of energies and spins of ground states and first excited states of potassium isotopes Z=19, which can be described as a proton hole in a calcium core. We propose to extend this study of low-energy spectroscopy to the neighbor chlorine isotopes Z=17 and a valence proton in 0d3/2 and 1s1/2 orbitals.

This study will be the aim of an experiment to be done in 2020 at the RIBF facility (Tokyo). An intense secondary beam including 46,48Ar isotopes will be obtained from fragmentation of a primary 70Zn beam at 345 MeV/u. In beam spectroscopy will be studied from one proton removal reaction in inverse kinematics 46,48Ar (p,2p) 45,47Cl and a cryogenic thick liquid hydrogen target surrounded by a time-projection chamber used for proton tracking and reconstruction of the reaction vertex in the target [2]. In flight emitted photons from 45,47Cl at target will be detected with the high resolution HiCARI array of Ge detectors, dedicated to this 2020 experimental campaign. We will measure the momentum distribution of 45,47Cl fragments to determine the angular momentum (s or d wave) of the knocked proton, giving access to the spin of the final state.

The experiment will be examined in 2019 December for an expected realization in 2020. A close collaboration with nuclear structure theoreticians will be an opportunity to analyze the results and develop the theoretical skills of the student.

Antiproton-nucleus reactions can occur at rest or in flight. The reactions at rest, or almost (~100 eV - 1 keV), are notably used at the Antiproton Decelerator (AD) at Cern by different experiments (GBAR, ASACUSA, AEgIS, ALPHA, ATRAP,). At FAIR, the PANDA project (antiProton ANnihilation at DArmstadt) aims to study hypernuclei with in-flight reactions (~GeV). In both cases, reliable simulations are necessary for a good analysis of the results. This is where we propose to make our contribution.



The INCL calculation code (IntraNuclear Cascade Liège) developed at the CEA (Irfu/DPhN) processes hadron-nucleus reactions for energies up to 20 GeV. Recognized for its reliability, it has recently been extended to the production of strange particles and hypernuclei (with the help of the ABLA de-excitation code), and can also be used within the Geant4 transport code. The extension to antiproton-nucleus reactions will therefore make it possible to participate in PANDA’s studies on hypernuclei, with a first step of tests on data already available (obtained at LEAR), as well as in the experiments of the Antiproton Decelerator where the behaviour of anti-hydrogen atoms is studied.



In addition, INCL is able to treat nucleus-nucleus reactions when one of the nuclei is light (A <= 18). This could also be used to treat by extension reactions with anti-deuteron and anti-helium. The GAPS experiment (General Anti-Particle Spectrometer) aims precisely to measure the fluxes of these particles in cosmic radiation. Simulations are obviously necessary in this case and INCL could thus contribute to this.

Understanding the limits of the nucleus cohesion, and in particular its mass limit, is currently one of the major areas of research in nuclear physics. The study of heavy nuclei, located in the upper part of the nuclear chart, has recently benefited from a new experimental approach: the laser spectroscopy. This is a method from atomic physics, allowing the properties of the nucleus to be deduced through the study of atomic levels, independent of nuclear models.

In this region of heavy nuclei, neutron-deficient actinides are of particular interest. Indeed, several theoretical calculations predict strong octupole deformations (pear shape).

The objective of the thesis is to study octupole deformations in neutron-deficient actinides. It will be done in collaboration with the University of Jyväskylä. The thesis is divided into two parts: i) an experiment at Jyväskylä to study the production of neutron-deficient actinides by a new reaction mechanism, ii) the development of a detector that will couple laser spectroscopy and delayed spectroscopy at S3/LEB at GANIL-Spiral2.

Micromegas-based gas detectors are used in several nuclear physics projects. One of these applications is in connection with pulsed neutron beams, making it possible to use the time-of-flight technique to determine the energy of the neutron. Using an electrode with a thin layer containing an isotope to be studied, charged particles from a neutron-induced reaction are detected. In this way it is possible to measure probabilities of reaction. The low probability of interaction of the beam with the deposits can make it possible to put several detectors in series to make relative measurements. By using orthogonal tracks for the Micromegas detector, it is possible to extract the coincidences and thus reconstruct the image of the incident beam, which is also done as a function of the energy of the neutrons. A more in-depth analysis will make it possible to reconstruct the tracks from the point of impact of the neutron and to determine in this way the angular distribution of the reaction studied. The thesis project will be to develop a Micromegas time projection chamber (TPC) for measurements in combination with time-of-flight of neutrons, having excellent resolution in position and in time, and by integrating the associated electronics.