A ground state $^{22}$Al halo is unlikely

TL;DR

This study determines the ground state spin and parity of $^{22}$Al, revealing that its proton configuration and Coulomb barrier prevent halo formation despite low proton separation energy.

Contribution

First $eta$-delayed charged particle emission experiment at FRIB that precisely identifies the ground state of $^{22}$Al and investigates its halo potential.

Findings

Ground state of $^{22}$Al is $4^+$.

Proton is confined by a $d$-wave barrier, hindering halo formation.

Weak $eta$-delayed $eta$-delayed $ extalpha$ transition observed.

Abstract

We report the decisive resolution of the ground state spin and parity of the proton-dripline nucleus $^{22}$Al, a prime candidate for a proton halo. The resolution stems from the first $\beta$-delayed charged particle emission experiment in the Gas Stopping Area at the Facility for Rare Isotope Beams (FRIB), leveraging high-intensity, low-energy beams extracted from the Advanced Cryogenic Gas Stopper (ACGS). The pristine beam quality from FRIB and the ACGS enabled a sensitive particle identification technique using thin silicon detectors, allowing for the suppression of the dominant proton background and the first observation of the weak $\beta$-delayed $\alpha$ transition from the Isobaric Analog State in $^{22}$Mg to the $^{18}$Ne ground state. This observation uniquely fixes the $^{22}$Al ground state as $4^+$. The valence proton is confined by a dominant $d$-wave centrifugal barrier which, combined with the Coulomb repulsion, hinders the tunneling required for halo formation despite the exceptionally low proton separation energy of $^{22}$Al.