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

Le résumé fourni par la source

The resonant states in $^{20}\mathrm{Mg}$ and $^{19}\mathrm{Mg}$ have been studied by the invariant-mass reconstruction of $^{18}\mathrm{Ne}+2p$ and $^{17}\mathrm{Ne}+2p$ events, respectively. Two new resolved states (${4}_{1}^{+}$ and ${2}_{2}^{+}$) in $^{20}\mathrm{Mg}$ are found to decay through $^{19}\mathrm{Na}$ intermediate states to the ground state of $^{18}\mathrm{Ne}$ from the examination of the decay-energy spectra for $^{18}\mathrm{Ne}+1p$ subsystems. The deduced ${R}_{42}={E}^{*}({4}_{1}^{+})/{E}^{*}({2}_{1}^{+})$ ratio of 2.10(1) is close to the expected harmonic-vibrator value and reveals a spherical or weakly deformed shape for $^{20}\mathrm{Mg}$. Systematic comparison of ${E}^{*}({2}_{1}^{+})$ and ${R}_{42}$ 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 ${2}_{1}^{+}$ excitation energy in $^{18}\mathrm{Mg}$ relative to $^{20}\mathrm{Mg}$ can be ascribed to different excitation characters of the valence protons. The $1/{2}_{1}^{+}$ state in $^{19}\mathrm{Mg}$ has been observed for the first time. The energy differences between the $1/{2}_{1}^{\ensuremath{-}}$ and $1/{2}_{1}^{+}$ states in $^{19}\mathrm{Mg}$ and its isotones manifest the magnitude of the $N=8$ shell gap on the proton-rich side.

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