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

Abstract

Excited-state spectroscopy from the first Facility for Rare Isotope Beams (FRIB) experiment 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 ${}^{32}\text{Na}$ nuclei. This is the only known microsecond isomer ($1\mu \text{s}\le T_{1/2}<1\text{ ms}$) 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 ${}^{32}\text{Mg}$, ${}^{32}\text{Mg}+{\pi }^{-1}+{\nu }^{+1}$. This odd-odd coupling and isomer formation provides a sensitive measure of the underlying shape degrees of freedom of ${}^{32}\text{Mg}$, 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 ${}^{32}$Na: 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.

7 pages, 5 figures, accepted by Physical Review Letters

References in corpus (10)

• Nuclear magic numbers: new features far from stability
• Improved nuclear matter calculations from chiral low-momentum interactions
• A nucleus-dependent valence-space approach to nuclear structure
• A new effective interaction for $0\hslash \omega$ shell model calculations in the sdpf valence space
• Discovery of the shape coexisting 0+ state in 32Mg by a two neutron transfer reaction
• Converged ab initio calculations of heavy nuclei
• Spectroscopy of 32Ne and the Island of Inversion
• Unveiling the intruder deformed 0${}_2^{+}$ state in ${}^{34}$Si
• Segmented YSO scintillation detectors as a new $\beta$-implant detection tool for decay spectroscopy in fragmentation facilities
• Electric monopole transition from the superdeformed band in ${}^{40}$Ca

Cited by in corpus (6)

• Further evidence for shape coexistence in ${}^{79}$Zn${}^m$ near doubly-magic ${}^{78}$Ni
• Identifying the spin trapped character of the ${}^{32}$Si isomeric state
• Refined topology of the N = 20 island of inversion with high precision mass measurements of ${}^{31-33}$Na and ${}^{31-35}$Mg
• Structure of odd-mass Ne, Na, and Mg nuclei
• Universal Effective Charges in the $sd$ and $fp$ Shells
• Commissioning and Full Realization of the PLASEN System at BRIF