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 Na32 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 Mg32, Mg32+π-1+ν+1. This odd-odd coupling and isomer formation provides a sensitive measure of the underlying shape degrees of freedom of Mg32, 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 Na32: 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.
Microsecond Isomer at the N=20 Island of Shape Inversion Observed at FRIB
Garcia Duarte, J.;
2023
Abstract
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 Na32 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 Mg32, Mg32+π-1+ν+1. This odd-odd coupling and isomer formation provides a sensitive measure of the underlying shape degrees of freedom of Mg32, 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 Na32: 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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.