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

Abstract

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.

Three-quasiparticle isomers in odd-even Pm159,161 : Calling for modified spin-orbit interaction for the neutron-rich region

“…In the calculation, we used deformation parameters ε 2 = 0.30 and ε 4 = 0.03 to construct the deformed basis for 159 Pm, and ε 2 = 0.30 and ε 4 = 0.01 for 161 Pm, which are consistent with those for other nuclei in this well-deformed mass region [11]. All other parameters including the monopolepairing strength and quadrupole-pairing strength appearing in the Hamiltonian are the same as those used in previous works [11,32].…”

“…A 13/2 + state is calculated at slightly higher energy, which is consistent with the fact that it is not observed as an isomer. It is noteworthy that this good agreement can only be achieved by using the newly proposed Nilsson parameters for the N = 6 neutron shell [11]. If we use the SP states generated by the standard Nilsson parameters in Ref.…”

“…In order to make a minimal modification for the spin-orbit force to describe existing data for the neutron-rich A ≈ 160 mass region, Liu et al recently proposed an isotope-dependent spin-orbit term [11] for the "standard" Nilsson model suggested by Bengtsson and Ragnarsson in 1985 [12]. The new formula for the Nilsson parameters κ and μ of the neutron n = 6 shell,…”

“…The new Nilsson parametrization was tested thoroughly using available experimental data from ground and low-lying states in odd-mass nuclei in the neutron-rich region. It was emphasized [11] that, to understand the change of deformed SP orbitals around the neutron numbers N = 98 and 100, the study of isomeric states is of great importance because the excitation energies of two quasineutron isomers involving two different orbitals from the N = 5 shell of normal parity and the N = 6 intruder with unique parity are sensitive to the spin-orbit interaction and allow us to investigate its variation. Experimental studies of N ≈ 100 rare-earth region became available recently with the advent of new generation radioactive isotope (RI) beam facilities.…”

Sensitivity of Neutron-Rich Nuclear Isomer Behavior to Uncertainties in Direct Transitions

“…A good understanding of the single-particle states is essential for the theoretical study of isomers, and these new results demand a careful reconsideration of mean-field models. In a first attempt [80], the traditional Nilsson model was extended in order to describe the deformed rare-earth nuclei in the very neutron-rich region.…”