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

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

C. Lizarazo et al.

Phys. Rev. Lett. 124, 222501 – Published 1 June 2020

Abstract

Here we present new information on the shape evolution of the very neutron-rich ${}^{92, 94}{Se}$ 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 $β \sim 0.25$ , with the high- $K$ state in ${}^{94}{Se}$ being metastable and $K$ hindered. Following this, ${}^{94}{Se}$ 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 $Z \sim 72$ deformed hafnium region. From an interpretation of the level scheme guided by theoretical calculations, an oblate deformation is also suggested for the ${}^{94}{Se}_{60}$ ground-state band.