Neutron magicity in the proton drip-line nucleus Mg 20: First invariant-mass reconstruction of Mg 19,20

The resonant states in Mg20 and Mg19 have been studied by the invariant-mass reconstruction of Ne18+2p and Ne17+2p events, respectively. Two new resolved states (41+ and 22+) in Mg20 are found to decay through Na19 intermediate states to the ground state of Ne18 from the examination of the decay-energy spectra for Ne18+1p subsystems. The deduced R42=E∗(41+)/E∗(21+) ratio of 2.10(1) is close to the expected harmonic-vibrator value and reveals a spherical or weakly deformed shape for Mg20. Systematic comparison of E∗(21+) and R42 along isotopic and isotonic chains with shell model calculations, using the YSOX interaction, suggest the persistence of N=8 shell around the proton drip-line. The relatively higher 21+ excitation energy in Mg18 relative to Mg20 can be ascribed to different excitation characters of the valence protons. The 1/21+ state in Mg19 has been observed for the first time. The energy differences between the 1/21- and 1/21+ states in Mg19 and its isotones manifest the magnitude of the N=8 shell gap on the proton-rich side.