LOW-SPIN NONYRAST STATES AND COLLECTIVE EXCITATIONS IN OS-174, OS-176, OS-178, OS-180, OS-182 AND OS-184

Excited states in the range of isotopes (OS)-O-174,176,178,180,182,184 , populated in the beta-decay of Ir-174,Ir-176,Ir-178,Ir-180,Ir-182,Ir -184 parent activities have been identified using gamma-ray singles an d coincidence techniques, utilising the high efficiency of a Compton-s uppressed array, and conversion electron techniques. Many new states a t low excitation energies have been identified, complementing the leve l schemes previously established from in-beam studies. The new states include excited 0(+) states in Os-174, Os-176 Os-178 and Os-180. A lar ge body of data on decay properties, spins and parities, and relative E2 and E0 matrix elements has been obtained. The systematics of the qu asi-beta and quasi-gamma bands is discussed. Some detailed analyses in terms of the schematic band-mixing model are presented, incorporating reproduction of the yrare states in the fits. In particular, the prop osed interpretation of the anomalous low-frequency alignment gains in the yrast positive-parity states, as a consequence of mixing with a lo w-lying intruder band, is confronted. The B(EO)/B(E2) ratios are over- estimated by this model although their form (spin and isotope dependen ce) is reproduced. Better agreement is obtained within the IBM, which uses however reduced effective charges to match experiment. Absolute m easurements of the B(EO) and B(E2) values may be necessary to distingu ish between the models and test conjectures of shape differences.