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

"The neutron-deficient polonium isotopes, with only 2 protons outside the Z = 82shell closure, are situated in an interesting region of the nuclear chart. In theneighboring lead (Z = 82) and mercury (Z = 80) isotopes, experimental andtheoretical efforts identified evidence of shape coexistence. Shape coexistence isthe remarkable phenomenon in which two or more distinct types of deformationoccur in states of the same angular momentum and similar excitation energyin a nucleus. The neutron-deficient polonium isotopes have also been studiedintensively, experimentally as well as theoretically. The closed neutron-shellnucleus 210Po (N = 126) manifests itself as a two-particle nucleus where mostof the excited states can be explained by considering the degrees of freedomof the two valence protons outside of 208Pb. The near-constant behavior ofthe yrast 2+1 and 4+1 states in the isotopes with mass 200 A 208 can beexplained by coupling the two valence protons to a vibrating lead core. 200Poseems to mark the end of this regular seniority-based character, with a suddendownsloping trend of almost all the excited states in the lighter isotopes withmass A <200. The observed characteristics in the lightest polonium isotopeshave been interpreted as evidence for an interplay between intruder structuresand the regular structure.The transitional region between the regular seniority-based behavior in theheavier polonium isotopes and the shape coexistence regime in the lighterisotopes is studied in this thesis with the technique of Coulomb excitation.This powerful technique allows to extract information about the deformationand mixing of co-existing shapes in a model-independent way. Recently, theCoulomb-excitation study of 182−188Hg led to the interpretation of mixingbetween a weakly-deformed oblate-like band and a more-deformed prolate-likeband.A reaction is classified as Coulomb excitation when the collision between anincident projectile beam and a target nucleus leads to the excitation of oneof the collision partners. At beam energies below the so-called “safe value”,the reaction is purely electromagnetic. Semi-classical perturbation theory canthen be applied to describe the Coulomb-excitation process. The cross sectionfor Coulomb excitation is related to the reduced electric quadrupole matrixelements coupling the populated states in the excited nucleus.Two Coulomb-excitation experimental campaigns with neutron-deficient196,198,200,202Po beams were performed at the REX-ISOLDE facility in CERN(Geneva, Switzerland). Beams were produced and post-accelerated to an energyof 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 placedinside the collision chamber to detect the scattered particles. Surrounding thetarget chamber was a position-sensitive germanium detector array to detect thede-excitation rays.Conditions related to timing and kinematic properties were applied to distinguishthe Coulomb-excitation events from the background events. The backgroundsubtractedand Doppler-corrected -ray spectra showed that the 2+1 statewas populated in all isotopes. Furthermore, in 196,198Po multi-step Coulombexcitation was observed and populated the 4+1 , 0+2 and 2+2 states. The relativelylarge uncertainties on the de-excitation yields of the 0+2 and 2+2 states are due tothe indirect observation of the E0 transitions through characteristic poloniumX rays. For future experiments, a direct way of observing E0 transitions by theelectron spectrometer SPEDE is discussed.The extracted results have been interpreted in the framework of three differentnuclear models: the beyond-mean-field model, the interacting-boson model andthe general Bohr Hamiltonian model. Next to that, a deformation parameterthat can be extracted from the Coulomb-excitation results was compared toa deformation parameter, deduced from charge radii measurements. Finally,a phenomenological two-state mixing model was applied and hinted towardsthe spin-independent mixing of a spherical with a more deformed structure.Overall, the comparison to theory could benefit from improved uncertainty ofthe experimental data. This could be achieved with the higher-energy beamsthat will be available at the HIE-ISOLDE facility."