Measurement of the Ground State Spin and Parity of Al 22 Disfavors Halo Formation

We report the decisive resolution of the ground state spin and parity of the proton-drip line nucleus ²²Al, 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 ²²Mg to the ¹⁸Ne ground state. This observation uniquely fixes the ²²Al 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 ²²Al.