Deformation and mixing of coexisting shapes in neutron-deficient polonium isotopes

Abstract

Coulomb-excitation experiments are performed with postaccelerated beams of neutron-deficient $^{196,198,200,202}\mathrm{Po}$ 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, $^{200,202}\mathrm{Po}$, the transitional and diagonal matrix elements of the ${2}_{1}^{+}$ state are determined. In $^{196,198}\mathrm{Po}$ multistep Coulomb excitation is observed, populating the ${4}_{1}^{+},\phantom{\rule{0.28em}{0ex}}{0}_{2}^{+}$, 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 $^{196}\mathrm{Po}$ 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.