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arXiv · 2601.03961

A ground state $^{22}$Al halo is unlikely

E. A. M. Jensen·J. S. Nielsen·B. S. O. Johansson·A. Adams·J. Dopfer·C. S. Sumithrarachchi·L. J. Sun·L. E. Weghorn·T. Wheeler·C. Wrede·M. J. G. Borge·O. Tengblad·M. Madurga·B. Jonson·K. Riisager·H. O. U. Fynbo
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Abstract

We report the decisive resolution of the ground state spin and parity of the proton-dripline nucleus $^{22}$Al, 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 $^{22}$Mg to the $^{18}$Ne ground state. This observation uniquely fixes the $^{22}$Al 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 $^{22}$Al.

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E. A. M. Jensen, J. S. Nielsen, B. S. O. Johansson, A. Adams, J. Dopfer, C. S. Sumithrarachchi, L. J. Sun, L. E. Weghorn, T. Wheeler, C. Wrede, M. J. G. Borge, O. Tengblad, M. Madurga, B. Jonson, K. Riisager, H. O. U. Fynbo. 2026-01-07. A ground state $^{22}$Al halo is unlikely. https://doi.org/10.1103/3lpm-sy41

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