Observation of three-proton-decaying resonant states in ²¹Al

The structure of the proton-unbound nucleus Al21 has been studied via proton transfer from a Be9 target to a Mg20 beam at E/A = 103 MeV. Three resonant states in Al21 are identified by the invariant-mass reconstruction of Ne18 + 3p events. The decay-energy spectra for the two subsystems of Al21, i.e. Ne18 + 1p and Ne18 + 2p, indicate that the observed resonant state with the lowest energy has decay branches consisting of three sequential steps of one-proton decay. Based on the ab initio and the distorted-wave Born approximation (DWBA) calculations, this resonant state at E^⁎ = 6.09(11) MeV likely has a spin-parity of (1/2⁺-7/2⁺). Meanwhile, the possibility that the observed resonant peak represents an admixture of several states cannot be ruled out. The good isospin-symmetry of the low-lying states with d_5/2, d_3/2 configurations in the Al21-O21 pair and the ab initio calculations indicate weak configuration mixing between the s- and d-orbital components. The systematics of experimental energy differences between the 5/2⁺ ground state and lowest-lying 3/2⁺ excited state for isotopes (isotones) of Z(N) = 13 suggest a likely semimagic nature of Al21.