Inelastic scattering of neutron-rich Ni and Zn isotopes off a proton target

Abstract

Proton inelastic scattering of $^{72,74}\mathrm{Ni}$ and $^{76,80}\mathrm{Zn}$ ions at energies around 235 MeV/nucleon was performed at the Radioactive Isotope Beam Factory and studied using $\ensuremath{\gamma}$-ray spectroscopy. Angular integrated cross sections for direct inelastic scattering to the ${2}_{1}^{+}$ and ${4}_{1}^{+}$ states were measured. The Jeukenne-Lejeune-Mahaux folding model, extended beyond 200 MeV, was used together with neutron and proton densities stemming from quasiparticle random-phase approximation (QRPA) calculations to interpret the experimental cross sections and to infer neutron to proton matrix element ratios. In addition, coupled-channels calculations with a phenomenological potential were used to determine deformation lengths. For the Ni isotopes, correlations favor neutron excitations, thus conserving the $Z=28$ gap. A dominance of proton excitation, on the other hand, is observed in the Zn isotopes, pointing to the conservation of the $N=50$ gap approaching $^{78}\mathrm{Ni}$. These results are in agreement with QRPA and large-scale shell-model calculations.