High-spin states, lifetime measurements and isomers in ¹⁸¹Os

Abstract

The level scheme of ¹⁸¹₇₆Os has been investigated with the ¹⁵⁰Nd(³⁶S,5n) reaction. The low-K rotational bands built upon the 9/2⁺[624], 7/2⁻[514] and 1/2⁻[521] neutron configurations have been extended and other new bands established. The configurations of these low-K bands are discussed within the framework of the cranked-shell model. The lifetimes for some of the states in the 9/2⁺[624] and 1/2⁻[521] collective rotational bands were also measured using the Doppler Shift Attenuation method. The large deformations deduced are found to be consistent with those predicted from theoretical Total Routhian Surface calculations. These results support the idea that for these low-K states the nuclear shape is axially symmetric and allows the K quantum number to be defined and the associated K-selection rule to be upheld. This behaviour apparently contrasts with that of the higher-K states in ¹⁸¹Os.

In the higher-spin regime, two new high-K intrinsic states, with K^π=37/2⁺ and K^π=41/2⁺, were established, along with the fragmented decay of a K^π=33/2⁻ intrinsic state. The configurations and excitation energies of these experimentally determined intrinsic states are found to be in excellent agreement with theoretical calculations based on a fixed shape Nilsson model plus BCS pairing. The structures on top of these intrinsic states do, however, show very different behaviour. A relatively regular high-K rotational band was observed on top of the K^π=41/2⁺ state but not for the other newly-established intrinsic states. Theoretical configuration-constrained potential energy surface calculations suggest that the irregular transition sequence above the K^π=37/2⁺ intrinsic bandhead state, the limited excitations observed above the other intrinsic states and the observation of fragmented and non-hindered decays, are due to these configurations being subject to an appreciable γ softness. These calculations reveal that the K^π=41/2⁺ configuration is less susceptible to distortions in the γ plane than any of the other high-K states.