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

Le résumé fourni par la source

The structure of the proton-unbound nucleus Al 21 has been studied via proton transfer from a Be 9 target to a Mg 20 beam at E / A = 103 MeV. Three resonant states in Al 21 are identified by the invariant-mass reconstruction of Ne 18 + 3 p events. The decay-energy spectra for the two subsystems of Al 21 , i.e. Ne 18 + 1 p and Ne 18 + 2 p , 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 Al 21 - O 21 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 Al 21 .

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