Nuclear structure at the neutron emission threshold and below explored via beta-decays of 82,83Ga and 86As.

Abstract
The work presented in this Ph.D. thesis focuses on the β-decay studies of neutron-rich nuclei around the N=50 shell closure, with a particular interest in the population of high energy states around the neutron emission threshold. What is the structure of these states, are they Pygmy Dipole States (PDR), and how does beta-decay connect to PDR states are some of the questions that motivated this study. The research was conducted through two complementary experiments. The first was performed at the ALTO facility (Accélérateur Linéaire et Tandem d’Orsay) and aimed at studying β-delayed gamma spectroscopy of 82,83Ge using PARIS (Photon Array for studies with Radioactive Ion and Stable beams) scintillators and HPGe detectors. The second experiment, aiming at the study of β-delayed gamma spectroscopy of 86Se, was carried out at the Ion Guide Isotope Separator On-Line (IGISOL) facility of the JYFL Accelerator Laboratory at the University of Jyväskylä. For the latter experiment, the JYFLTRAP Penning trap was used to provide pure samples of 86As and the β-delayed gamma spectroscopy was performed with high-resolution germanium detectors. A detailed analysis of the two experiments is presented. The level schemes of the β-decay daughters, 82Ge (N=50), 83Ge (N=51), and 86Se (N=52), were updated, and more than 80 new γ-ray transitions were observed. γ-ray transitions with energies higher than the neutron separation energy were detected for the first time in 82Ge. In addition, new gamma-ray transitions following the beta-delayed neutron emission were observed in both 81Ge and 85Se. The half-lives of 82,83Ga and 86As were measured as well as the neutron emission probabilities, Pn, of both 83Ga and 86As. The high-energy states populated in the β-decay of 82Ga lie in a region where PDR is expected to appear. QRPA and pnQRPA calculations have been performed for the electric dipole B(E1) and Gamow-Teller strength B(GT) distributions for 82Ge. Calculated B(E1) strength distributions show a low-lying dipole strength compatible with PDR. The composition of the calculated PDR states can be accessed in terms of quasi-particle (qp) excitations and neutron and proton transition densities. Comparing the experimental and calculated B(E1) and B(GT) distributions and keeping in mind that β-decay can only connect with final states through particular selection rules, this work discusses the structure of states populated by beta-decay lying at high energy around the neutron separation energy that connect directly to the ground or the 2+ state in 82Ge and their eventual dipole character.
Main Author
Format
Theses Doctoral thesis
Published
2023
Series
ISBN
978-951-39-9534-8
Publisher
Jyväskylän yliopisto
The permanent address of the publication
https://urn.fi/URN:ISBN:978-951-39-9534-8Use this for linking
ISSN
2489-9003
Language
English
Published in
JYU Dissertations
License
In CopyrightOpen Access
Copyright© The Author & University of Jyväskylä

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