title: Gamma-ray spectroscopy of neutron-rich cerium isotopes following beta-decay of mass-separated caesium ions
Creators: name: Satrazani, Maria
Publisher: name: University of Liverpool Repository
Year: 2025

description: The structure of the atomic nucleus is a result of the interactions between its fundamental constituents, the protons and the neutrons, and the interplay between the strong and the electromagnetic force. When nucleons collectively interact and move in a coordinated manner, they can arrange in energy states in different ways which result in spherical and non-spherical (deformed) configurations. 
Nuclear structure studies offer insights into the various arrangements of nuclei by investigating the underlying principles that govern the organisation of nucleons within the nucleus.
 
Octupole deformation is a form of nuclear collectivity that results from the long-range octupole-octupole interaction between nucleons occupying pairs of orbitals which differ by 3 units in both orbital and total angular momentum. This interaction gives rise to an asymmetric pear-shaped form that can be manifested by the appearance of low-lying negative-parity states. This type of deformation has only been observed in localised regions of the nuclear chart and more specifically in the mass region around Z=88, N=134 with another promising region around Z=56, N=88. Isotopes with their Fermi surfaces close to these particle numbers are predicted, by theoretical calculations and experimental observations, to possess octupole collectivity. Parameters indicative of collectivity, such as transition strengths (B(Eλ)), can be measured either directly or indirectly through a variety of experimental techniques, and can be used to characterise the deformation shape.

The beta-decay experiment presented in this thesis was performed at the radioactive ion beam facility ISAC-I at the TRIUMF particle accelerator center in Vancouver, Canada in 2018, aiming to look for low-lying negative-parity states in neutron-rich cerium isotopes. Beams of Cs were initially produced from a UC{x} target and implanted on an aluminised mylar moving tape collector. The population of states in 146,148,150Ce was possible through the decay of the grand-daughter La isotopes. Spectroscopy measurements have been performed using the GRIFFIN spectrometer and its ancillary detectors. GRIFFIN is a high-purity germanium clover detector array, meant for high-precision gamma-ray spectroscopy studies. In the experiment presented in this thesis, GRIFFIN operated with the following ancillary detectors: 5 scintillators for conversion electron spectroscopy, 8 LaBr(3} detectors to allow for fast-timing measurements of excited-state lifetimes and a Zero Degree Scintillator for the detection of beta-particles. Studying the electromagnetic properties of excited states is essential in our understanding of the magnitude and mode of deformation, whether it is static or vibrational in nature. 

Through the spectroscopic analysis of the 146,148,150Ce isotopes presented in this thesis, the level scheme of 150Ce was extended with the observation of new gamma-ray transitions. Furthermore, the proposed low-lying excited 1- and 3- states in 150Ce were identified and compared to the energy systematics of the 1- and 3- states of other neutron-rich nuclei in the lanthanide region, for the first time. Low-spin negative-parity states that had been previously observed in 146,148Ce, were also confirmed. The characteristic B(E1; (1- → 2(1)+))/B(E1; (1^- → 0(g.s)}+)) and B(E1; (3- → 2(1)+))/B(E1; (3- → 4(1)+)) ratios were measured both experimentally and theoretically. They were then compared to the expected theoretical values predicted by the Bohr-Mottelson collective model, demonstrating consistency with them and contributing to the expansion of our understanding of pear-shaped nuclei in this region of the nuclide chart. An angular correlation analysis was also performed; however, it did not yield reliable results.