Precision Mass Measurement of the Proton Dripline Halo Candidate 22 Al
Here, we report the first mass measurement of the proton-halo candidate 22 Al performed with the low energy beam ion trap facilityβs 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. 22 Al 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 22 Al , Phys. Rev. Lett. 125, 192503 (2020).]. The measured mass excess value of ME=18 092.5β’(3) keV, corresponding to an exceptionally small proton separation energy of ππ=100.4β’(8) keV, is compatible with the suggested halo structure. Our result agrees well with predictions from π β’π-shell USD Hamiltonians. While USD Hamiltonians predict deformation in the 22 Al ground state with minimal 1β’π 1/2 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.