Transition quadrupole moments of high-spin states in 172Os

Abstract

Lifetimes of states in 172Os have been measured by the Doppler-shift recoil-distance method. These states were populated by the reaction 144Nd(32S, 4n) 172Os at a bombarding energy of 162 MeV from the HHIRF tandem accelerator at ORNL. The data were collected in the γγ-coincidence mode in order to reduce the complexities of the γ-ray spectra and to avoid some of the problems associated with side feeding to excited states. The experimental transition quadrupole moments, Qt, do not cluster about a constant value as a function of rotational frequency up through h̵ω = 0.27 MeV as predicted by cranked Hartree-Fock-Bogoliubov (HFB) calculations around N = 96 for the yrast band. A striking feature in the experimental data for the yrast band is an abrupt increase of the Qt values for the 6+ and 8+ states where the first anomaly in the behavior of the moment of inertia occurs. A recent proposal of band-mixing between the yrast band and a quasi-β band to account for this anomaly in the moment of inertia also seems to provide a logical interpretation of the Qt values at low rotational frequencies. The large collectivity inferred for the quasi-β band from our Qt values for the 6+ and 8+ states is an indirect, but plausible, argument that the quasi-β band corresponds to the non-yrast band in the band-mixing model. The E1 transition probabilities for decay of states in the (π, α) = (−, 1) band to states in the ground-state band range between 1.4 × 10−5 to 3.1 × 10−4 W.u. The B(E1, I → I + 1) is an order of magnitude larger than the B(E1, I → I − 1). The origin of these effects can probably be understood in terms of a predominant admixture of the Coriolis-coupled octupole vibrational-state wave function in the (−, 1) band of 172Os at low spin.