Precision Mass Measurement of the Proton Dripline Halo Candidate ${}^{22}\mathrm{Al}$

We report the first mass measurement of the proton-halo candidate ${}^{22}\mathrm{Al}$ 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. ${}^{22}\mathrm{Al}$ 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 ${}^{22}\mathrm{Al}$, Phys. Rev. Lett. 125, 192503 (2020).]. The measured mass excess value of $\mathrm{ME}=18092.5(3)\mathrm{keV}$, corresponding to an exceptionally small proton separation energy of $S_p=100.4(8)\mathrm{keV}$, 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 ${}^{22}\mathrm{Al}$ 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.

Figure 1

(a) Observed 75 ms ${}^{22}{\mathrm{Al}}^{+}$ TOF-ICR resonance and fit [23], containing approximately 350 detected ions. (b) Observed 50 ms PI-ICR beam spot resulting from reduced cyclotron motion, containing approximately 50 detected ions. The spot is fit to a 2D Gaussian in polar coordinates. The black arrow shows the direction of motion.

Figure 2

Measured frequency ratios of ${}^{22}{\mathrm{Al}}^{+}$ relative to ${}^{23}{\mathrm{Na}}^{+}$. Blue points and uncertainty region correspond to TOF-ICR measurements, red points and uncertainty region correspond to PI-ICR measurements. Dashed black lines indicate the combined TOF-ICR and PI-ICR result. All uncertainties are shown to $1\sigma$.

Figure 3

Experimental levels of ${}^{22}\mathrm{F}$ up to 5 MeV compared to the results obtained from the USDC and USDI Hamiltonians. The calculated levels of ${}^{22}\mathrm{Al}$ are also shown. The horizontal lines in the figure are proportional to the $J$ value for positive parity states. For the experiment shown on the left-hand side, the levels with unknown $J$ are shown by black dots. Levels with uncertain $J$ are indicated by the red lines ending with a black dot.

Figure 4

Energy of the ${}^{21}\mathrm{Mg}+\mathrm{p}$ system (top panel), and norms of channel wave functions for $j_{\mathrm{PRM}}={5/2}^{+}$ (middle panel) and ${1/2}^{+}$ (lower panel) as a function of the quadrupole deformation parameter ${\beta }_2$.