Here we present new information on the shape evolution of the very neutron-rich Se92,94 nuclei from an isomer-decay spectroscopy experiment at the Radioactive Isotope Beam Factory at RIKEN. High-resolution germanium detectors were used to identify delayed γ rays emitted following the decay of their isomers. New transitions are reported extending the previously known level schemes. The isomeric levels are interpreted as originating from high-K quasineutron states with an oblate deformation of β∼0.25, with the high-K state in Se94 being metastable and K hindered. Following this, Se94 is the lowest-mass neutron-rich nucleus known to date with such a substantial K hindrance. Furthermore, it is the first observation of an oblate K isomer in a deformed nucleus. This opens up the possibility for a new region of K isomers at low Z and at oblate deformation, involving the same neutron orbitals as the prolate orbitals within the classic Z∼72 deformed hafnium region. From an interpretation of the level scheme guided by theoretical calculations, an oblate deformation is also suggested for the Se6094 ground-state band.
Bibliographical noteFunding Information:
This work was carried out at the RIBF operated by 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. The authors thank the RIBF and BigRIPS teams for providing a stable high-intensity uranium beam and operating the secondary beams. We acknowledge support from the German BMBF Grants No. 05P15RDFN1, No. 05P19RDFN1, No. 05P15PKFNA, and No. 05P19PKFNA, the ERC Grant No. MINOS-258567, the Spanish MEC under Contracts No. FPA2014-57196-C5-4-P and No. FIS-2014-53434, the National Key R&D Program of China (Contract No. 2018YFA0404402), the National Natural Science Foundation of China (Grants No. 11961141004, No. 11735017, No. 11675225, No. 11635003), the Vietnam MOST via the Physics Development Program Grant No. ĐTĐLCN.25/18, as well as from the Science and Technology Facilities Council (STFC). We further thank GSI for providing computing facilities.
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