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Deformation and mixing of co-existing shapes in the neutron-deficient Polonium isotopes

Date: 18/03/2015
Author: Kesteloot, N.
Subject: Deformation and mixing of co-existing shapes in the neutron-deficient Polonium isotopes
University: KULeuven
Promotor: Van Duppen, P.
SCK CEN Mentor: Popescu, L

The neutron-deficient polonium isotopes, with only 2 protons outside the Z = 82 shell closure, are situated in an interesting region of the nuclear chart. In the neighboring lead (Z = 82) and mercury (Z = 80) isotopes, experimental and theoretical efforts identified evidence of shape coexistence. Shape coexistence is the remarkable phenomenon in which two or more distinct types of deformation occur in states of the same angular momentum and similar excitation energy in a nucleus. The neutron-deficient polonium isotopes have also been studied intensively, experimentally as well as theoretically. The closed neutron-shell nucleus 210Po (N = 126) manifests itself as a two-particle nucleus where most of the excited states can be explained by considering the degrees of freedom of the two valence protons outside of 208Pb. The near-constant behavior of the yrast 2+1 and 4+1 states in the isotopes with mass 200 A 208 can be explained by coupling the two valence protons to a vibrating lead core. 200Po seems to mark the end of this regular seniority-based character, with a sudden downsloping trend of almost all the excited states in the lighter isotopes with mass A <200. The observed characteristics in the lightest polonium isotopes have been interpreted as evidence for an interplay between intruder structures and the regular structure.The transitional region between the regular seniority-based behavior in the heavier polonium isotopes and the shape coexistence regime in the lighter isotopes is studied in this thesis with the technique of Coulomb excitation.This powerful technique allows to extract information about the deformation and mixing of co-existing shapes in a model-independent way. Recently, the Coulomb-excitation study of 182−188Hg led to the interpretation of mixing between a weakly-deformed oblate-like band and a more-deformed prolate-like band. A reaction is classified as Coulomb excitation when the collision between an incident projectile beam and a target nucleus leads to the excitation of one of the collision partners. At beam energies below the so-called “safe value”,the reaction is purely electromagnetic. Semi-classical perturbation theory can then be applied to describe the Coulomb-excitation process. The cross section for Coulomb excitation is related to the reduced electric quadrupole matrix elements coupling the populated states in the excited nucleus.  Two Coulomb-excitation experimental campaigns with neutron-deficient 196,198,200,202Po beams were performed at the REX-ISOLDE facility in CERN (Geneva, Switzerland). Beams were produced and post-accelerated to an energy of 2.85MeV per nucleon and made to collide with a 104Pd and a 94Mo target,both of 2.0 mg/cm2 thickness. A double-sided silicon-strip detector was placed inside the collision chamber to detect the scattered particles. Surrounding the target chamber was a position-sensitive germanium detector array to detect the de-excitation rays. Conditions related to timing and kinematic properties were applied to distinguish the Coulomb-excitation events from the background events. The background subtracted and Doppler-corrected -ray spectra showed that the 2+1 state was populated in all isotopes. Furthermore, in 196,198Po multi-step Coulomb excitation was observed and populated the 4+1 , 0+2 and 2+2 states. The relatively large uncertainties on the de-excitation yields of the 0+2 and 2+2 states are due to the indirect observation of the E0 transitions through characteristic polonium X rays. For future experiments, a direct way of observing E0 transitions by the electron spectrometer SPEDE is discussed.The extracted results have been interpreted in the framework of three different nuclear models: the beyond-mean-field model, the interacting-boson model and the general Bohr Hamiltonian model. Next to that, a deformation parameter that can be extracted from the Coulomb-excitation results was compared to a deformation parameter, deduced from charge radii measurements. Finally,a phenomenological two-state mixing model was applied and hinted towards the spin-independent mixing of a spherical with a more deformed structure. Overall, the comparison to theory could benefit from improved uncertainty of the experimental data. This could be achieved with the higher-energy beams that will be available at the HIE-ISOLDE facility.

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