High-K states and critical-point symmetries in 138Gd

UoM administered thesis: Phd

  • Authors:
  • Mark Procter

Abstract

This thesis, entitled High-K states and critical-point symmetries in 138Gd was submitted to The University of Manchester by Mr Mark George Procter on 3rd March 2012 for the degree Doctor of Philosophy (PhD).Excited states in 138Gd have been populated with the 106Cd(36Ar,2p2n) reaction at beam energies of 180 and 190 MeV, at the University of Jyväskylä, Finland. In Experiment I, the Recoil-Isomer Tagging technique was utilised to correlate delayed γ-ray decays, detected in the GREAT focal-plane spectrometer, with prompt decays above the known Kpie = 8- isomer measured in the PRE-JUROGAM II spectrometer at the target position. In Experiment II, the Recoil-Distance Doppler-Shift technique was used, with the Köln differential-plunger device located at the target position of the JUROGAM II array, to determine lifetimes in the ground-state sequence of 138Gd.The lifetime of the Kpie = 8- isomeric state has been remeasured as 6.2(2) μs. Two high-lying strongly-coupled bands have been established with Kpie greater than or equal to 12-. Potential-Energy Surface (PES) calculations, in conjunction with g-factor measurements, reveal that they are built upon four-quasiparticle structures comprising two-quasineutron plus two-quasiproton configurations. The short half-life or lack of hindrance for the decays from these four-quasiparticle band-head states is reasoned to be a consequence of increased triaxial deformation and mixing due to the high density of states, relative to the lower two-quasiparticle 6 μs isomeric state.B(E2) values, determined from the experimentally measured lifetimes, have been compared with X(5) critical-point calculations, describing the phase transition between a vibrational and axially-symmetric nuclear shape, as well as Interacting Boson Model (IBM-1) calculations at the critical point. While there is a high degree of correspondence between the measured excitation energies and X(5) predictions, the large uncertainties in the measured B(E2) values cannot preclude contributions to the low-lying structure of 138Gd from vibrational and rotational modes of excitation. PES calculations suggest an increase in γ softness in the ground-state band, where there is currently limited knowledge of the β and γ-vibrational bands.New lifetime measurements in 136Sm, coupled with previous results, allude to a low-lying nuclear structure governed predominantly by axially-symmetric rotation. This result is unexpected given the proximity of the Z = 50 shell closure, which would suggest that vibrational excitations should contribute largely to the low-lying structure. PES calculations show a significant amount of γ softness for each of the excited states, similar to that observed for 138Gd. However, B(E2) values derived from these calculations support the description of 136Sm as an axially-symmetric rotor.

Details

Original languageEnglish
Awarding Institution
Supervisors/Advisors
  • David Cullen (Supervisor)
Award date1 Aug 2012