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

Author:

S. E. Campbell, G. Bollen, B. A. Brown, A. Dockery, K. Fossez, C. M. Ireland, K. Minamisono, D. Puentes, A. Ortiz-Cortez, B. J. Rickey, R. Ringle, S. Schwarz, C. S. Sumithrarachchi, A. C. C. Villari, I. T. Yandow

Keyword:

Nuclear Experiment, Nuclear Experiment (nucl-ex)

journal:

date:

2023-12-18 00:00:00

Abstract

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.

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