Division of Electronics, Detectors and Computing for Physics (DEDIP)

Understanding the limits of existence of the nucleus, especially concerning its mass limit, is one of the major fields of research in contemporary nuclear physics. In the region of heavy nuclei, neutron-deficient actinides are of particular interest. Indeed, pronounced octupole (pear-shaped) deformations are predicted and have even been observed in some nuclei. The aim of this thesis is to study these octupole deformed nuclei using the new-generation detector SEASON, whose detection efficiency and energy resolution are unprecedented for this type of experiment. The thesis work will focus on the installation, testing, experimental data-taking and analysis from an experiment to be carried out in 2025 at the University of Jyväskylä. In this experiment, the proton-induced fusion-evaporation reaction 232Th(p,X)Y will be used to populate neutron-deficient actinide isotopes, whose decay products will be analyzed using SEASON. The thesis will be in cotutelle with the University of Jyväskylä and divided into two parts:
i) a 1-year period at the University of Jyväskylä, during which the experiment will take place
ii) the following two years at CEA Saclay will be devoted to data analysis and preparation of the experimental program with SEASON at the new facility S3-LEB at GANIL-SPIRAL2.

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+ ? 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). Therefore we will make use of a new technique to search for the double-gamma decay in bare (fully-stripped) ions, which are available at the GSI facility in Darmstadt, Germany. The basic idea of our experiment is to produce, select and store exotic nuclei in their excited 0+ state in the GSI storage ring (ESR). 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 would be identified by so-called time-resolved Schottky Mass Spectroscopy. This method allows to distinguish the isomer and the ground state 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. After a first successful experiment establishing the double-gamma decay in 72Ge a new experiment has been accepted by the GSI Programme Committee and its realization is planned for 2024.

The thesis will focus on the experimental study of the nuulear properties of the heaviest stable zirconium isotope (96Zr).
Recently, observation of a low-lying deformed state in this magic nucleus has been explained by a reorganization of nuclear shells in function of their occupation by protons and neutrons. These sophisticated nuclear-structure calculations predict a variety of shapes, both ellipsoidal and pear-like, to appear at low excitation energy in the 96Zr nucleus. We will investigate them using the powerful Coulomb-excitation technique, which is the most direct method to determine the shapes of nuclei in their excited states. The experiment will be performed using AGATA, a new-generation gamma-ray spectrometer, consisting of a large number of finely segmented germanium crystals, which allows us to identify each point where a gamma ray interacts with the detector material and then, using the so-called “gamma-ray tracking” concept, to reconstruct the energies of all emitted gamma rays and their angles of emission with highest precision. A complementary measurement will be performed at TRIUMF (Vancouver, Canada) using the world’s leading setup for beta-decay measurements called GRIFFIN. This project is a part of an extensive experimental program on shape coexistence and evolution of nuclear shapes undertaken by our group.