Structure of collective states built on the 11/2(+) isomer in Os-187: Quasiparticle-plus-triaxial-rotor model and interpretation as tilted-precession bands

Background: The shape of most nuclei is described by its quadrupole deformation (showing the deviation from spherical shape) and its triaxiality (showing the deviation from axial symmetry). Nuclei affected by triaxiality show additional collective rotational bands, called gamma bands, that appear at low excitation energy. The gamma bands can be caused by the precession of a nucleus with triaxial shape, but can also arise from small gamma vibrations around an axially symmetric shape. Purpose: The aim of this work is to search for new collective excitations in 187Os in particular related with the gamma degree of freedom. Methods: The rotational structures of 187Os were populated by the 186W(4He, 3n)187Os reaction at a beam energy of 37 MeV. The measurements of gamma - gamma coincidences, angular distribution ratios and gamma -ray intensities were performed using eleven Compton-suppressed Ge clover detectors. Results: The previously known positive-parity band built on the 11/2+ isomer has been extended up and a new excited positive-parity band built above a 15/2+ state has been observed. The 11/2+ band was assigned a nu i13/2 configuration while the new 15/2+ band was associated with a coupling of the valence i13/2 neutron with the gamma band of the even-even core. The quasiparticle-plus-triaxial-rotor model calculations provide a good agreement with the experimental data for both bands. They describe the 15/2+ band as a collective excitation with respect to the 11/2+ band that corresponds to a precession of the intermediate nuclear axis similarly to the precession of a rotating top. Conclusions: As shown by the calculations, the new rotational band can be understood as resulting from the three-dimensional rotation of a triaxially-deformed nucleus. However, a description based on the vibrations of a gamma -soft nuclear shape should also be investigated in order to firmly establish the nature of the excited positiveparity band. Further studies able to distinguish between these alternative descriptions will be beneficial.