Changes of deformed shell gaps at N ∼ 100 in light rare-earth, neutron-rich nuclei

There has been compelling evidence indicating that deformed single-particle (SP) states in neutron-rich regions are different from those in the stable region. It was pointed out that for the light rare-earth (₆₀Nd, ₆₂Sm, and ₆₄Gd), neutron-rich (N = 98 − 102) nuclei, the Woods–Saxon potential, the Nilsson modified oscillator potential with 'universal' parameters, and the folded Yukawa potential all failed to describe the correct ordering of neutron SP states. The location and size of deformed shell gaps in this mass region are under current debate. We propose a modification for the 'standard' Nilsson parameters of Bengtsson and Ragnarsson introduced in 1985. The proposed N-dependent spin–orbit interaction causes directly changes in deformed shell gaps with neutron number and deformation. By applying the modified Nilsson parameters to generate deformed bases for the projected shell model, we demonstrate that the calculation can explain consistently the current experimental data, including the ground state configuration in odd-neutron nuclei, upbending of the yrast moment of inertia at higher spins and the energies of 2-quasineutron 6⁻ and 4⁻ isomers in even–even nuclei. (paper)