Departmental Seminar

Neutron-rich rhenium isotopes and the progression into triaxiality

Dr Matthew Reed
Department Nuclear Physics
Australian National University

The region with Z=76 and N=116 has been predicted to exhibit changes in nuclear deformation and our recent results on neutron-rich isotopes, such as 188,190W and 192Os, show corresponding signatures of a transition to triaxial shapes. These signatures are observed through changes in the quantum numbers associated with the deformed nucleus. However, the presence of an odd particle can distort and change the transition from prolate to triaxial shapes by filling deformation-driving nuclear orbitals. The focus of this talk is on odd-A neutron-rich rhenium isotopes (187Re, 189Re and 191Re). Since these nuclei cannot be produced by conventional fusion-evaporation reactions, our approach has been to access these nuclei via multinucleon transfer or deep inelastic reactions, specifically using a pulsed or chopped 136Xe beam from the ATLAS accelerator at Argonne National Laboratory, incident on gold-backed 187Re and 192Os targets. The γ rays from excited reaction products were measured using the Gammasphere detector array.

Previous experiments identified delayed γ rays from isomeric states in 187Re and 191Re, although a partial decay scheme is only known for 187Re. In addition to γ-ray spectroscopic studies, low spin states in 187Re, 189Re and 191Re have also been the subject of particle transfer experiments. In the current measurement, the 9/2-[514] proton orbital and its associated rotational band were observed, populated in the decay of 3-quasiparticle isomers, in all three isotopes and for the first time in 189Re and 191Re. Progression towards higher neutron numbers shows a decrease in K-hindrances for the isomeric decays, and an increase in the signature splitting of the 9/2-[514] rotational band. These properties suggest the development of triaxiality which will be explored within three theoretical approaches: potential energy-surface, triaxial particle-rotor and total Routhian surface calculations.

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