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Neutron magicity in the proton drip-line nucleus Mg20: First invariant-mass reconstruction of Mg19,20

L. Ni1,*, Y. Jin1,*, Z. H. Li1,†, K. W. Brown2,3,‡, C. X. Yuan4, H. Hua1,§, C. Y. Niu2, A. K. Anthony2,5,∥, J. Barney2,5 et al.

R. J. Charity6, D. Dell'Aquila2,¶, J. M. Elson6, J. Estee2,5, G. Jhang2, J. G. Li7,8, W. G. Lynch2,5, N. Michel7,8, L. G. Sobotka6,9, S. Sweany2,5, F. C. E. Teh2,5, A. Thomas6, C. Y. Tsang2,5, M. B. Tsang2,5, S. M. Wang10,11, H. Y. Wu1, and K. Zhu2,5

  • *These authors contributed equally to this work.
  • †Contact author: zhli@pku.edu.cn
  • ‡Contact author: brownk@frib.msu.edu
  • §Contact author: hhua@pku.edu.cn
  • ∥Present address: Department of Physics and Astronomy, High Point University, High Point, NC 27268, USA.
  • Present address: Department of Physics “Ettore Pancini”, University of Naples “Federico II” & INFN-Naples, Naples 80138, Italy.

Phys. Rev. C 110, L061301 – Published 5 December, 2024

DOI: https://doi.org/10.1103/PhysRevC.110.L061301

Abstract

The resonant states in Mg20 and Mg19 have been studied by the invariant-mass reconstruction of Ne18+2p and Ne17+2p events, respectively. Two new resolved states (41+ and 22+) in Mg20 are found to decay through Na19 intermediate states to the ground state of Ne18 from the examination of the decay-energy spectra for Ne18+1p subsystems. The deduced R42=E*(41+)/E*(21+) ratio of 2.10(1) is close to the expected harmonic-vibrator value and reveals a spherical or weakly deformed shape for Mg20. Systematic comparison of E*(21+) and R42 along isotopic and isotonic chains with shell model calculations, using the YSOX interaction, suggest the persistence of N=8 shell around the proton drip-line. The relatively higher 21+ excitation energy in Mg18 relative to Mg20 can be ascribed to different excitation characters of the valence protons. The 1/21+ state in Mg19 has been observed for the first time. The energy differences between the 1/21− and 1/21+ states in Mg19 and its isotones manifest the magnitude of the N=8 shell gap on the proton-rich side.

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