Collective rotational bands at low excitation energy in Os186 : Vibrational and rotational degrees of freedom

Collective structures in $^{186}\mathrm{Os}$ have been investigated through the $^{186}\mathrm{W}(^{4}\mathrm{He},4n)^{186}\mathrm{Os}$ reaction, at a beam energy of 48 MeV. The low-lying bands built on the excited $0{}_{2}^{+}, 2{}_{2}^{+}$, and $4{}_{3}^{+}$ states have been extended up to spins of ${12}^{+}, {15}^{+}$, and ${8}^{+}$, respectively, and a number of new linking transitions were identified. The features of the collective bands in $^{186}\mathrm{Os}$, such as level energies, are presented in the context of a systematic study of the neighboring even-even $^{182--192}\mathrm{Os}$ isotopes. In addition, the validity of the $K$-selection rule, stemming from a description based on axial symmetry of the nuclear shape, is examined. The observed decays between the rotational bands support a description where $K$ is conserved. However, some $K$-forbidden decays were also identified, suggesting that a model allowing for small $K$ admixtures is probably required. The experimental data are further compared with calculations using a five-dimensional collective Hamiltonian based on covariant density functional theory. The calculations predict that the collective bands are associated with different nuclear shapes, varying in quadrupole deformation, triaxiality, and softness.