Metastable States of $^{92,94}\mathrm{Se}$: Identification of an Oblate $K$ Isomer of $^{94}\mathrm{Se}$ and the Ground-State Shape Transition between $N=58\text{ }\text{and}\text{ }60

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.