Precision Mass Measurement of Proton-Dripline Halo Candidate $^{22}$Al

• Main Authors: Campbell, S. E, Bollen, G, Brown, B. A, Dockery, A, Fossez, K, Ireland, C. M, Minamisono, K, Puentes, D, Ortiz-Cortez, A, Rickey, B. J, Ringle, R, Schwarz, S, Sumithrarachchi, C. S, Villari, A. C. C, Yandow, I. T
• Format: Journal Article
• Language: English
• Published: 18.12.2023
• Subjects: Physics - Nuclear Experiment
• DOI: 10.48550/arxiv.2312.11366

We report the first mass measurement of the proton-halo candidate $^{22}$Al performed with the LEBIT facility's 9.4~T Penning trap mass spectrometer at FRIB. 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 [Phys. Rev. Lett. \textbf{125} 192503 (2020)]. The measured mass excess value of $\text{ME}=18\;093.6(7)$~keV, corresponding to an exceptionally small proton separation energy of $S_p = 99.2(1.0)$~keV, is compatible with the suggested halo structure. Our result agrees well with predictions from \textit{sd}-shell USD Hamiltonians. While USD Hamiltonians predict deformation in $^{22}$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.