Microscopic analysis of quadrupole collective motion in Cr-Fe nuclei. II. Doorway nature of mixed-symmetry states

Abstract

The mixed-symmetry collective modes are investigated in Cr-Fe nuclei, by analyzing the realistic shell-model wave functions via the ${H}^{n}$-cooling method. It is clarified that the relatively low-lying mixed-symmetry states behave like doorway states. For these nearly spherical nuclei, the lowest mixed-symmetry state is shown to have ${J}^{P}{=2}^{+}.$ An indication of the mixed-symmetry ${3}^{+}$ state is obtained. The sequence of the mixed-symmetry ${2}^{+},$ ${1}^{+},$ and ${3}^{+}$ levels and its nucleus dependence are discussed. Calculated $M1$ and $M3$ transitions in the low-energy region suggest that the mixed-symmetry ${1}^{+}$ and ${3}^{+}$ components are detectable. We investigate the $B(M1)$ distribution in a wider energy range, without breaking the isospin quantum number. It is confirmed that the mixed-symmetry ${1}^{+}$ component is well separated from the peak of the spin excitation. The isospin-raising component has a peak, well separated from the isospin-conserving one. The orbital angular-momentum contributes destructively to the spin excitations.