Identification of an oblate K-isomer of 94Se60 and implications for a ground-stateshape transition between N = 58 - 60

In this letter we report on new information on the shape evolution of the very neutron-rich 92;94Se nuclei from an isomer-decay spectroscopy experiment at the Radioactive Isotope Beam Factory at RIKEN. High-resolution germanium detectors were used to identify delayed -rays emitted following the decay of these nuclei. New transitions are reported extending the previously known level schemes. The isomeric levels are interpreted as originating from high-K quasi-neutron states with an oblate deformation of B ~ - 0:25, with the high-K state in 94Se being metastable and K hindered. Following this, 94Se is the lowest-mass nucleus known to date with K-forbidden decay and a substantial K hindrance. Furthermore, it is the first observation of an oblate K isomer in a deformed nucleus. From the decay patterns, an oblate deformation is suggested for the 94Se60 ground-state band, in line with the predictions of recent beyond-mean-field calculations.