Microsecond Isomer at the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>N</mml:mi><mml:mo>=</mml:mo><mml:mn>20</mml:mn></mml:math> Island of Shape Inversion Observed at FRIB
T. J. Gray, J. M. Allmond, Z. Y. Xu, T. King, R. S. Lubna, H. L. Crawford, Vandana Tripathi, B. P. Crider, R. Grzywacz, S. N. Liddick, A. O. Macchiavelli, T. Miyagi, A. Poves, A. Andalib, E. Argo, C. Benetti, S. Bhattacharya, C. M. Campbell, M. P. Carpenter, J. Chan, A. Chester, J. Christie, B. R. Clark, I. Cox, A. A. Doetsch, J. Dopfer, J. G. Duarte, P. Fallon, A. Frotscher, T. Gaballah, J. T. Harke, J. Heideman, H. Huegen, J. D. Holt, R. Jain, N. Kitamura, K. Kolos, F. G. Kondev, A. Laminack, B. Longfellow, S. Luitel, M. Madurga, Ruchi Mahajan, M. J. Mogannam, C. Morse, S. Neupane, A. Nowicki, T. H. Ogunbeku, W.-J. Ong, C. Porzio, C. J. Prokop, B. C. Rasco, E. K. Ronning, E. Rubino, Thomas Ruland, K. P. Rykaczewski, L. Schaedig, D. Seweryniak, K. Siegl, M. Singh, A. E. Stuchbery, S. L. Tabor, T. L. Tang, T. Wheeler, J. A. Winger, John L. Wood
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 ^{32}Na nuclei. This is the only known microsecond isomer (1 μs≤T_{1/2}<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 ^{32}Mg, ^{32}Mg+π^{-1}+ν^{+1}. This odd-odd coupling and isomer formation provides a sensitive measure of the underlying shape degrees of freedom of ^{32}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.