학술논문
Microsecond Isomer at the N=20 Island of Shape Inversion Observed at FRIB
Document Type
Working Paper
Author
Gray, T. J.; Allmond, J. M.; Xu, Z.; King, T. T.; Lubna, R. S.; Crawford, H. L.; Tripathi, V.; Crider, B. P.; Grzywacz, R.; Liddick, S. N.; Macchiavelli, A. O.; Miyagi, T.; Poves, A.; Andalib, A.; Argo, E.; Benetti, C.; Bhattacharya, S.; Campbell, C. M.; Carpenter, M. P.; Chan, J.; Chester, A.; Christie, J.; Clark, B. R.; Cox, I.; Doetsch, A. A.; Dopfer, J.; Duarte, J. G.; Fallon, P.; Frotscher, A.; Gaballah, T.; Harke, J. T.; Heideman, J.; Huegen, H.; Holt, J. D.; Jain, R.; Kitamura, N.; Kolos, K.; Kondev, F. G.; Laminack, A.; Longfellow, B.; Luitel, S.; Madurga, M.; Mahajan, R.; Mogannam, M. J.; Morse, C.; Neupane, S.; Nowicki, A.; Ogunbeku, T. H.; Ong, W. -J.; Porzio, C.; Prokop, C. J.; Rasco, B. C.; Ronning, E. K.; Rubino, E.; Ruland, T. J.; Rykaczewski, K. P.; Schaedig, L.; Seweryniak, D.; Siegl, K.; Singh, M.; Stuchbery, A. E.; Tabor, S. L.; Tang, T. L.; Wheeler, T.; Winger, J. A.; Wood, J. L.
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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 $^{32}\textrm{Na}$ nuclei. This is the only known microsecond isomer ($1{\text{ }\mu\text{s}}\leq T_{1/2} < 1\text{ ms}$) 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 $^{32}\textrm{Mg}$, $^{32}\textrm{Mg}+\pi^{-1} + \nu^{+1}$. This odd-odd coupling and isomer formation provides a sensitive measure of the underlying shape degrees of freedom of $^{32}\textrm{Mg}$, 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.
Comment: 7 pages, 5 figures, accepted by Physical Review Letters
Comment: 7 pages, 5 figures, accepted by Physical Review Letters