Magicity and occurrence of a band with enhancedB(E2)in neutron-rich nucleiNi68andZr90

Abstract

Experimental energy spectrum and $B(E2)$ values in $^{68}\mathrm{Ni}$ and $^{90}\mathrm{Zr}$ indicate a double-magic character in these neutron-rich nuclei with $N$ or $Z=40$. The data nevertheless do not show any pronounced irregularity in two-nucleon separation energy. To understand the underlying physics, we carry out both shell-model and mean-field calculations. The shell-model calculation can reproduce all the observations well. It is understood from the mean-field results for $^{68}\mathrm{Ni}$ that the shell gap at $N=40$ disappears because of dynamical correlations of the isovector $J=0$ pairing interaction. In $^{90}\mathrm{Zr}$, however, such a dynamic process with the $J=0$ pairing appears not important because of the strong contribution of the $J>0$ interaction. We study also level schemes in the Ni isotopes and $N=$ 50 isotones. We predict a new band built on the ${0}_{2}^{+}$ state in both $^{68}\mathrm{Ni}$ and $^{90}\mathrm{Zr}$. The states of this band are dominated by two-particle-two-hole excitations from the $\mathit{fp}$ shell to the intruder ${g}_{9/2}$ orbit.