1 research outputs found
Microsecond Isomer at the N=20 Island of Shape Inversion Observed at FRIB
Excited-state spectroscopy from the first Facility for Rare Isotope Beams
(FRIB) experiment is reported. A 24(2)-s isomer was observed with the FRIB
Decay Station initiator (FDSi) through a cascade of 224- and 401-keV
rays in coincidence with nuclei. This is the only known
microsecond isomer () in the
region. This nucleus is at the heart of the 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 , .
This odd-odd coupling and isomer formation provides a sensitive measure of the
underlying shape degrees of freedom of , where the onset of
spherical-to-deformed shape inversion begins with a low-lying deformed
state at 885 keV and a low-lying shape-coexisting state at 1058 keV. We
suggest two possible explanations for the 625-keV isomer in Na: a
spherical shape isomer that decays by or a deformed spin isomer that
decays by . 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