Precision Mass Measurement of the Proton Dripline Halo Candidate Al22

We report the first mass measurement of the proton-halo candidate ^22Al performed with the low energy beam ion trap facility's 9. 4 T Penning trap mass spectrometer at facility for rare isotope beams. This measurement completes the mass information for the lightest remaining proton-dripline nucleus achievable with Penning traps. ^22Al has been the subject of recent interest regarding a possible halo structure from the observation of an exceptionally large isospin asymmetry J. Lee et al. , Large isospin asymmetry in Si22/O22 Mirror Gamow-Teller transitions reveals the halo structure of ^22Al, Phys. Rev. Lett. 125, 192503 (2020). . The measured mass excess value of ME=18 092. 5 (3) keV, corresponding to an exceptionally small proton separation energy of S₏=100. 4 (8) keV, is compatible with the suggested halo structure. Our result agrees well with predictions from sd-shell USD Hamiltonians. While USD Hamiltonians predict deformation in the ^22Al ground state with minimal 1s₁/₂ occupation in the proton shell, a particle-plus-rotor model in the continuum suggests that a proton halo could form at large quadrupole deformation. These results emphasize the need for a charge radius measurement to conclusively determine the halo nature.