Generalization of the $N_pN_n$ scheme to non-yrast levels of even-even nuclei

Abstract

In this paper we present the systematics of excitation energies for even-even nuclei in two regions: the 50$<$Z$\le$66, 82$<$N$\le$104 region, and the 66$<$Z$<$82, 82$<$N$\le$104 region. Using the $N_pN_n$ scheme, we obtain compact trajectories for the ground band as well as quasi-$\beta$ and quasi-$\gamma$ bands. This suggests that the $N_pN_n$ scheme is useful even if one extends it to non-yrast levels, and thus can serve as a general tool to disclose new types of structural evolution for higher excitations, besides the yrast states which have been investigated extensively. It is highlighted that deformations in non-yrast quasibands of nuclei with $Z \sim 80$ and $N \sim 104$ are often very different from those in the ground bands.