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Title: High-K states and critical-point symmetries in 138Gd
Author: Procter, Mark George
ISNI:       0000 0004 2720 3692
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2012
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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 Kπ = 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 Kπ = 8− isomeric state has been remeasured as 6.2(2) μs. Two high-lying strongly-coupled bands have been established with Kπ ≥ 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.
Supervisor: Cullen, David Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available