E Collective Behavior in the Even-Even Osmium and Platinum Nuclei.

E2 collective properties in ('186,188,190,192)Os and ('194)Pt have been investigated using heavy-ion Coulomb excitation with 3.3-4.8 MeV/nucleon ('40)Ca, ('58)Ni, ('136)Xe, and ('208) Pb projectiles. The particle-(gamma) (for ('40)Ca, ('58)Ni, and ('136)Xe projectiles) and particle-particle -(gamma) (for ('58)Ni and ('208)Pb projectiles) coincidence techniques were employed to measure the de-excitation (gamma) -ray yields as a function of the projectile Z value and the scattering angle. The experimental (gamma)-ray yields were analyzed using a new semiclassical Coulomb-excitation least-squares search--code GOSIA. For each nucleus studied, the unique and almost complete set of E2 matrix elements up to the backbending region has been determined, which includes the B(E2) values, the relative signs between the transitional matrix elements, and the static quadrupole moments. These data were compared with the predictions of various nuclear collective models such as the asymmetric rigid rotor model, the (gamma)-soft model of Leander, and the IBA-2 model. These particular models do not reproduce the data satisfactorily. However, in view of the fact that the (gamma)-soft type of models more or less reproduce the general trends of the data, we believe that the E2 properties for the low-lying excited states in these nuclei most probably still can be described in terms of quadrupole collective degrees of freedom. The almost complete set of experimental E2 matrix elements extracted from the present work for each nucleus studied makes it possible, for the first time, to determine the nuclear quadrupole shape degrees of freedom directly using the model-independent sum-rule method. The results from this sum rule are generally consistent with the above conclusion. The backbending anomalies were observed in all nuclei studied. The observed anomalies can be understood in the framework of the two-band-mixing model by assuming that the occurrence of this phenomenon is due to the intersection between a rotation-aligned band and the ground-state band. The results for the deorientation effect are a by-product of this work and were found to be in good agreement with the predictions of the two-state model proposed by Brenn et al.