Evidence for core excitation in single-particle states of $^{19}$Na

We present an experimental study of $^{19}$Na states in the excitation energy range between 2 and 3 MeV. The presence of $^{19}$Na single-particle levels at these energies was first predicted by a microscopic cluster model and then experimentally confirmed by measuring the elastic and inelastic scattering of a 66 MeV $^{18}$Ne radioactive beam on a $(CH2)_n$ target. The H($^{18}$Ne,p)$^{18}$Ne(g.s.) and H($^{18}$Ne,p')18Ne*($2^+$, 1.887 MeV) cross sections have been obtained in the laboratory angular range $\theta$_lab$ = 6.1°−18.4° and analyzed by using the R-matrix method. Two new states in $^{19}$Na have been observed at centre of mass energies $E_c.m.$ = 2.78 ± 0.01 MeV and 3.09 ± 0.05 MeV. Both resonances exhibit large widths in the $^{18}$Ne($2^+$)+p channel, and low branching ratios into the elastic channel. The reduced proton widths confirm the single-particle nature of these states, with a $^{18}$Ne($2^+$)+p structure