Description du sujet de thèse

Heavy and superheavy nuclei (SHE) are found at the top of the nuclei chart. They only exist due to a fragile balance between the strong Coulombian repulsion between the many protons in the nucleus, which tends to cause the system to fission, and the strong interaction between the different nucleons, which tends to bind it together. Fundamental yet unanswered questions include: What is the nature of the interactions in the nucleus? What is the maximum number of protons, Z, a nucleus can sustain? Is there an ultimate island of enhanced stability for Super Heavy Elements?
A class of mean-field calculations for these elements [1] predict a substantial gap in the spectrum of individual nucleon energy levels for neutron number N=126 and proton number Z=92 (218U). This contradicts an extrapolation of the energies of individual particles obtained from the energy levels measured in the lightest known N=126 isotones up to 216Th [2] and raises the question of the double magicity of 218U. It also raises questions about the validity of predictions concerning the structure of superheavy nuclei.
To better understand the organisation and correlations of nucleons near 218U, an experiment to study 217Pa at the Cyclotron Laboratory of the University of Jyvävskylä in Finland has recently been accepted. The experiment is scheduled at the end of 2025. Only two states are known in the 217Pa nucleus: the ground state and a metastable alpha-emitting state. Using alpha, gamma and internal conversion electron spectroscopy techniques at the focal plane of the RITU recoil ion separator, the aim will be to identify other states and study the sequence and properties of the first excited states of 217Pa.
Recent SHE discoveries involve only a few events. The assignment of their mass and charge relies on calculations of the excitation function for the reaction and the measurement of their alpha decay, with cross-bombardments as a consistency check. However, a systematic and undetected charged particle evaporation channel [3] would change the assignment. So, while the elements with atomic numbers Z=113, 115, 117 [4] and 118 [5] have been assigned to the periodic table of the chemical elements, completing the seventh row, there is no direct proof of their atomic number, Z. X-rays are a fingerprint of the atomic number of a nucleus since the X-ray energy is proportional to the atomic charge Z. Our group is therefore involved in the development and upgrade of the SIRIUS detection system [6] dedicated to the spectroscopy of superheavy elements. SIRIUS will be used at the focal plane of the S3 recoil separator [7] at the new facility SPIRAL2 at GANIL, France. The first commissioning experiments are expected to take place in 2026. The PhD student will participate in various optical tests of the separator and the first online tests of SIRIUS. The SHEXI (SuperHeavy Element X-ray Identification) project [8] aims to improve the detection efficiency by a factor of ten at L X-ray energies and the intrinsic resolution by a factor of twenty compared to the current performance of SIRIUS. The proposed upgrade will be capable of unambiguously identifying the atomic number of the newly discovered SHE through L X-ray measurements. The doctoral student will be required to participate in testing prototype detectors and qualifying their performance.

The proposed thesis has three parts:
1. Preparing and carrying out the experiment at JYFL and analyzing and interpreting the spectroscopic data of 217Pa.
2. Participation in the commissioning of SIRIUS and S3 at GANIL. Commissioning should occur during 2026 using known fusion evaporation reactions [9] and similar analysis techniques that will be used for the 217Pa experiment.
3. Participation in the SHEXI developments to upgrade SIRIUS.

[1] K. Rutz et al., Nucl. Phys. A 634, 67 (1998)
[2] K. Hauschild et al., Phys. Rev. Lett. 87, 072501 (2001)
[3] A. Lopez-Martens, et al., Phy. Lett. B 795 (2019) 271
[4] P.J. Karol, et al., Pure and Applied Chemistry 88 (2016) 139
[5] P.J. Karol, et al., Pure and Applied Chemistry 88 (2016) 155
[6] B.J.P. Gall, et al., Acta Phys. Pol. B 42 (2011)
[7] F. Déchery, et al., Nucl. Instr. and Meth. B 376 (2016) 125
[8] K. Hauschild, Acta Phys. Pol. B 18 (2025) 2-A28
[9] K. Hauschild, et al., Nucl. Instr. and Meth. A 608 (2006) 388, R. Chakma, et al., Eur. Phys. J. A 56 (2020) 245

Contexte de travail

The Irène Joliot-Curie Laboratory (IJCLab), managed by the CNRS, Paris-Saclay University and the University of Paris, is located on the Orsay campus of Paris-Saclay University, 30 km south of Paris and accessible by public transport. The staff consists of more than 500 permanent members (researchers, teachers, engineers, technicians and administrative staff) and 200 non-permanent members (including 100 doctoral students). The laboratory's research topics include nuclear physics, high-energy physics, theoretical physics, astroparticle physics, cosmology, particle accelerators, energy and health. The IJCLab has all the technical capabilities to design, develop and implement the experimental devices necessary for its scientific activity.
The doctoral student will work in the Spectroscopy-Decay-Fission group of the laboratory's Nuclear Physics Division, which has around ten physicists (researchers, post-docs and doctoral students) who conduct experiments on the structure of the atomic nucleus in various laboratories around the world. The work will be carried out in collaboration with the IPHC, GANIL and IRFU-CEA.
IJCLAB bases its recruitment policy on the promotion of equality, diversity and inclusion. Essential values, they allow the professional development of agents, who are real actors in a collective success, but also the development of the laboratory itself.

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.

Contraintes et risques

Several weeks of travel to Jyväskylä (Finland) and Caen will be required during the thesis.