Particle-γ Spectroscopy of the (p,d-γ) 155 Gd Reaction: Neutron Single-quasiparticle States at N=91

A segmented Si telescope and HPGe array is used to study the 156 Gd(p, d-γ) 155 Gd direct reaction by d-γ and d-γ-γ coincidence measurements using 25-MeV protons. The present investigation is the first time that this N = 91 nucleus and the N = 90 region—which is known for a rapid change from vibrational to rotational character, several low-lying 0 + states in the even-even nuclei, and large Coriolis (ΔΩ = 1) plus ΔN = 2 mixing in the even-odd nuclei—have been studied by particle-γ coincidence following a direct reaction with light ions. Gamma-ray energies and branches, excitation energies, angular distributions, and cross sections are measured for states directly populated in the (p, d) reaction. A new low-energy doublet state at 592.46 keV (previously associated with the K = 0 ⊗ 3/2 - [521] bandhead) and several new γ-ray transitions (particularly for states with excitation energies > 1 MeV) are presented. Most notably, the previous ν 7/2 + [404] systematics at and around the N = 90 transition region are brought into question and reassigned as ν 5/2 + [402]. This reassignment makes the ν 1/2 + [400], ν 3/2 + [402], and ν 5/2 + [402] orbitals, which originate from the 3s 1/2 , 2d 3/2, and 2d 5/2 spherical states, respectively, responsible for the three largest cross sections to positive-parity states in the (p, d) 155 Gd direct reaction. These three steeply upsloping orbitals undergo ΔN = 2 mixing with their N = 6 orbital partners, which are oppositely sloped with respect to deformation. The presence of these steeply sloped and crossing orbitals near the Fermi surface could weaken the monopole pairing strength and increase the quadrupole pairing strength of neighboring even-even nuclei, which would bring ν 2p-2h 0 + states below 2Δ. Indeed, this could account for a large number of the low-lying 0 + states populated in the (p, t) 154 Gd direct reaction.