Abstract
Coulomb-excitation experiments are performed with postaccelerated beams of neutron-deficient Po-196,Po-198,Po-200,Po-202 isotopes at the REX-ISOLDE facility. A set of matrix elements, coupling the low-lying states in these isotopes, is extracted. In the two heaviest isotopes, Po-196,Po-198, the transitional and diagonal matrix elements of the 2(1)(+) state are determined. In Po-196,Po-198 multistep Coulomb excitation is observed, populating the 4(1)(+), 0(2)(+), and 2(2)(+) states. The experimental results are compared to the results from the measurement of mean-square charge radii in polonium isotopes, confirming the onset of deformation from Po-196 onwards. Three model descriptions are used to compare to the data. Calculations with the beyond-mean-field model, the interacting boson model, and the general Bohr Hamiltonian model show partial agreement with the experimental data. Finally, calculations with a phenomenological two-level mixing model hint at the mixing of a spherical structure with a weakly deformed rotational structure.
Highlights
Nuclear shape coexistence is the remarkable phenomenon in which states at similar excitation energies exhibit different intrinsic deformations
We report on two Coulomb-excitation experiments with neutron-deficient 196−202Po beams, which were performed at the REX-ISOLDE facility at CERN
A set of matrix elements coupling the low-lying states in the even-even neutron-deficient 196−202Po isotopes was extracted in two Coulomb-excitation campaigns, which were performed at the REX-ISOLDE facility at CERN
Summary
Nuclear shape coexistence is the remarkable phenomenon in which states at similar excitation energies exhibit different intrinsic deformations. The ground states of the neutron-deficient lead isotopes are found to stay essentially spherical while different shapes appear at low excitation energies [7,8]. Recent beyond-mean-field (BMF) studies of polonium isotopes result in potential-energy surfaces that are soft for heavier polonium isotopes (A > 198), pointing toward the possibility of triaxial structures [19] Theoretical descriptions, such as phenomenological shapemixing calculations [23,24,25,26], contemporary symmetry-guided models [4], and BMF approaches [19], can reproduce the global trends that are deduced from experiments in the lightlead region.
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