Precision Mass Measurement of the Proton Dripline Halo Candidate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>Al</mml:mi></mml:mrow><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>22</mml:mn></mml:mrow></mml:mmultiscripts></mml:mrow></mml:math>
Scott E. Campbell, G. Bollen, B. A. Brown, Adam Dockery, Chris M. Ireland, K. Minamisono, D. Puentes, Brooke J. Rickey, R. Ringle, I. T. Yandow, K. Fossez, A. Ortiz-Cortes, S. Schwarz, C. Sumithrarachchi, A. C. C. Villari
Abstract
We report the first mass measurement of the proton-halo candidate ^{22}Al 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. ^{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).PRLTAO0031-900710.1103/PhysRevLett.125.192503]. The measured mass excess value of ME=18 092.5(3) keV, corresponding to an exceptionally small proton separation energy of S_{p}=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 ^{22}Al ground state with minimal 1s_{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.