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Measurement of the Ground State Spin and Parity of Al22 Disfavors Halo Formation

E. A. M. Jensen1,2,*, J. S. Nielsen1, B. S. O. Johansson1, A. Adams3,4, J. Dopfer3,4, C. S. Sumithrarachchi4, L. J. Sun4, L. E. Weghorn3,4, T. Wheeler3,4 et al.

C. Wrede3,4, M. J. G. Borge5, O. Tengblad5, M. Madurga6, B. Jonson2, K. Riisager1, and H. O. U. Fynbo1

  • *Contact author: erik.jensen@chalmers.se
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Phys. Rev. Lett. 136, 202503 – Published 20 May, 2026

DOI: https://doi.org/10.1103/3lpm-sy41

Abstract

We report the decisive resolution of the ground state spin and parity of the proton–drip line nucleus Al22, a prime candidate for a proton halo. The resolution stems from the first β-delayed charged particle emission experiment in the gas stopping area at the Facility for Rare Isotope Beams (FRIB), leveraging high-intensity, low-energy beams extracted from the Advanced Cryogenic Gas Stopper (ACGS). The pristine beam quality from FRIB and the ACGS enabled a sensitive particle identification technique using thin silicon detectors, allowing for the suppression of the dominant proton background and the first observation of the weak β-delayed α transition from the isobaric analog state in Mg22 to the Ne18 ground state. This observation uniquely fixes the Al22 ground state as 4+. The valence proton is confined by a dominant d-wave centrifugal barrier which, combined with the Coulomb repulsion, hinders the tunneling required for halo formation despite the exceptionally low proton separation energy of Al22.

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Phys. Rev. Lett. 136, 202503– Published 20 May, 2026
  • Received 8 January 2026
  • Revised 20 March 2026
  • Accepted 27 April 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by Bibsam.

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