Deformed structures in neutron-rich A ≈ 100 nuclei studied by isomer and ß- delayed γ- ray spectroscopy

Abstract

Neutron-rich A∼110 nuclei have been the subject of intensive discussion since the discovery of a rapid onset of ground-state static quadrupole deformation in the A>100 Zr nuclei over 60 years ago. The results presented in this thesis further the experimental knowledge of such deformations towards more exotic isotopes in this region. An experiment was carried out at the Radioactive Isotope Beam Factory, RIKEN, Wako-shi, Japan. Nuclei of interest were produced through the in-flight abrasion fission of a 345 MeV/nucleon 238U beam by a 555 mg/cm2 9Be target and selected by the BigRIPS spectrometer according to their mass-to-charge ratio and atomic charge. They were implanted into the WAS3ABi silicon stack and γ rays emitted following β-decay, or decay of isomeric states were detected in an array of 12 cluster HPGe (EURICA) and 18 LaBr3(Ce) detectors. A newly implemented fast β-γ timing configuration provided access to the lifetimes of excited states in the nanosecond regime. In particular, the lifetime of the 2+ state of 106Zr is reported for the first time, as well as a more precise measurement for 104Zr, compared to the adopted value. These measurements, combined with the energy of the ground-state transition, have been used to deduce the reduced transition probability and from this a model-dependent value for the ground-state deformation. New isomeric states have also been observed in 102Zr, 113Nb and 115Mo. The reduced transition probability of the 2+ + g.s transition in 106Zr shows a marked decrease from that of 104Zr. This reflects the energy systematics, and further confirms the deformed sub-shell closure at N=64. Comparisons of the systematics of the observed reduced transition probabilities to those calculated in the IBM-1 model show poor agreement. However, calculations based on the nuclear-shell model in a deformed basis state, show good agreement with observation, demonstrating the reliability of the employed ground-state wavefunction. The isomeric state in 102Zr is ascribed to a change in the K quantum number, and the isomeric states in 113Nb and 115Mo are proposed to be due to shape stabilisation of excited states.

Date of Award	Jun 2016
Original language	English
Awarding Institution	University of Brighton