Observation of the [formula omitted] and γ bands in 98Ru, and shape coexistence in the Ru isotopes

P E Garrett ,T Faestermann ,C Burbadge ,J N Orce ,J C Nzobadila Ondze ,E A Lawrie ,R Dubey ,V Bildstein ,Christian Mehl ,S Triambak ,B Singh ,E C Vyfers ,R A Bark ,T D Bucher ,S H Mthembu ,C Ngwetsheni ,M Kamil ,N J Mukwevho ,S Valbuena ,L Makhathini ,R Hertenberger ,T Rodrı́guez ,A D Maclean ,S S Ntshangase ,B Rebeiro ,K G Leach ,H.‐F Wirth

doi:10.1016/j.physletb.2020.135762

Abstract

Excited states in 98Ru were investigated using γ-ray spectroscopy following the β-decay of 98Rh, and via the 100Ru(p,t) reaction. Combining the results from the two experiments, two states were revised to have spin-parity of 4+ and subsequently assigned to the 02+ and “γ” bands, respectively. The observed structures in 98Ru are suggested to be deformed and rotational, rather than spherical and vibrational, and fit well into the systematics of these excitations in the Ru isotopes. The 02+ excitation is suggested as a shape coexisting configuration. This observation eliminates some of the last remaining candidates for nearly harmonic vibrational nuclei in the Z≈50 region. Beyond-mean-field calculations are presented that support shape coexistence throughout the Ru isotopes with N=52–62, and suggest a smooth evolution of the shapes.

Highlights

The Ru isotopes are in a region where an abundance of examples of shape coexistence exists
With Z = 44, they are located midway between the Sr/Zr isotopes, for which the most rapid change in ground state shape across the nuclear chart is observed between N = 58 and N = 60, and the Cd/Sn isotopes that may be considered as iconic examples of nuclei possessing shape coexistence
The source activity was produced using the 12C+89Y reaction at the iThemba LABS Tape Station, and the present work represents the first results from this new facility

Summary

Introduction

The Ru isotopes are in a region where an abundance of examples of shape coexistence exists. The recent Coulomb excitation study of 96,98Sr [2,3] provided firm evidence for configuration inversion in these nuclei, demonstrating important similarities in terms of transition probabilities and spectroscopic quadrupole moments between the ground-state band in 96Sr and the structure built on the 0+2 state in 98Sr. The local character of the shape change suggests that specific proton and neutron orbitals are responsible for this effect. A recent update, using an expanded set of criteria [12], of the previous survey [11] found that few candidates remained It was only 98,100Ru that might be considered as possible spherical vibrational nuclei up to the excitation energy scale of the two-phonon states. As will be shown below, these states are key to unravelling the structure of 98Ru

Experimental details and results

Theoretical calculations and discussion

Conclusions