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Microsecond Isomer at the N=20 Island of Shape Inversion Observed at FRIB

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Physical Review Letters
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Excited-state spectroscopy from the first experiment at the Facility for Rare Isotope Beams (FRIB) is reported. A 24(2)−μs isomer was observed with the FRIB Decay Station initiator (FDSi) through a cascade of 224- and 401-keV γ rays in coincidence with 32Na nuclei. This is the only known microsecond isomer (1  μs≤T1/2<1  ms) in the region. This nucleus is at the heart of the N=20 island of shape inversion and is at the crossroads of the spherical shell-model, deformed shell-model, and ab initio theories. It can be represented as the coupling of a proton hole and neutron particle to 32Mg, 32Mg+π−1+ν+1. This odd-odd coupling and isomer formation provides a sensitive measure of the underlying shape degrees of freedom of 32Mg, where the onset of spherical-to-deformed shape inversion begins with a low-lying deformed 2+ state at 885 keV and a low-lying shape-coexisting 0+2 state at 1058 keV. We suggest two possible explanations for the 625-keV isomer in 32Na: a 6− spherical shape isomer that decays by E2 or a 0+ deformed spin isomer that decays by M2. The present results and calculations are most consistent with the latter, indicating that the low-lying states are dominated by deformation.