Excited-state spectroscopy from the first Facility for Rare Isotope Beams
(FRIB) experiment 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 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 02+​ 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.Comment: 7 pages, 5 figures, accepted by Physical Review Letter