Electromagnetic properties of the low-lying states in heavy (A≃190) transitional nuclei

In a particle-$\gamma$ spectroscopy experiment, ${}_{}{}^{188,190,192}{}_{}{}^{}\mathrm{Os}$ nuclei were Coulomb excited by 14.5 MeV ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, 48.0 MeV ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, and 62.0 MeV ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$ beams, successively. Using both semiclassical and quantum mechanical coupled channels codes, the coincidence $\gamma$-ray yields were analyzed to obtain the relative quadrupole moments of the $2_1^{+}$ state, as well as branching ratios and the spectroscopic quadrupole moments of the $2_2^{+}$ states in these nuclei. As an independent check, the spectroscopic quadrupole moments of the $2_1^{+}$ and $2_2^{+}$ states in ${}_{}{}^{188}{}_{}{}^{}\mathrm{Os}$ were measured by particle spectroscopy techniques using ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ projectiles. The results of the two experiments are in good agreement. These results, together with the corresponding values for ${}_{}{}^{184,186}{}_{}{}^{}\mathrm{W}$ and ${}_{}{}^{194}{}_{}{}^{}\mathrm{Pt}$, are compared with the predictions of several models, including (i) microscopic pairing-plus-quadrupole and boson expansion theory, and (ii) phenomenological interacting boson approximation and asymmetric rigid rotor models. The general experimental trends are best described by the above microscopic models. Both phenomenological models fail to reproduce the systematics of the experimental quadrupole moments.