Shape evolution in Ru-116,Ru-118: Triaxiality and transition between the O(6) and U(5) dynamical symmetries

Abstract

Ru-116 and Ru-118 have been studied via beta-delayed gamma-ray spectroscopy of nuclei produced in fragmentation reactions at the Radioactive Ion-Beam Factory (RIBF) facility. Level schemes with positive-parity states up to spin J = 6 have been constructed. The results have been discussed in terms of the interacting boson model, the algebraic collective model, and total Routhian surfaces. We conclude that the very neutron-rich nuclei still show many features associated with triaxial gamma-soft nuclei, represented by the O(6) symmetry, but are approaching a spherical structure, the U(5) symmetry, with increasing neutron number towards the N = 82 shell closure. In Ru-118, hints of a shape transition in the ground state have been observed.

We would like to thank Dr. V. Werner for valuable discussions about the IBM calculations. This work was carried out at the RIBF operated by the RIKEN Nishina Center, RIKEN and CNS, University of Tokyo. We acknowledge the EUROBALL Owners Committee for the loan of germanium detectors and the PreSpec Collaboration for the readout electronics of the cluster detectors. Part of the WAS3ABi was supported by the Rare Isotope Science Project which is funded by the Ministry of Science, ICT & Future Planning (MSIP) and the National Research Foundation (NRF) of Korea. P.A.S. was financed by the Japan Society for the Promotion of Science (JSPS) Kakenhi Grant No. 23.01752. F.G.K. was supported by the US Department of Energy, Office of Nuclear Physics, under Contract No. DE-AC02-06CH11357. Zs.V. was supported by OTKA Contract No. K100835. We acknowledge financial support from the Spanish Ministerio de Ciencia e Innovacion under Contracts No. FPA2009-13377-C02 and No. FPA2011-29854-C04. H.S.J. was supported by the Priority Centers Research Program in Korea (2009-0093817). K.Y.C. was supported by National Research Foundation of Korea Grant No. NRF-2012R1A1A1041763. F.N. was supported by U.S. DOE Grant No. DE-FG02-91ER-40609.