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

Excited-state spectroscopy from the first experiment at the Facility for Rare Isotope Beams (FRIB) is reported. A 24(2)-& mu;s isomer was observed with the FRIB Decay Station initiator (FDSi) through a cascade of 224-and 401-keV & gamma; rays in coincidence with 32Na nuclei. This is the only known microsecond isomer (1 & mu;s & LE; 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 + & pi;-1 + & nu;+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.

Automated summary

Excited-state spectroscopy of a 24(2)-μs isomer in 32Na at the FRIB Decay Station initiator (FDSi) was investigated through a cascade of γ rays in coincidence with 32Na nuclei. This is the only known microsecond isomer in the region and sheds light on the coupling and formation of isomers in 32Na, providing insights into the shape degrees of freedom in 32Mg.