Particle-γ spectroscopy of the (p,d-γ)Gd155 reaction: Neutron single-quasiparticle states at N=91

A segmented Si telescope and HPGe array is used to study the Gd156(p,d-γ)Gd155 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=0-32^-[521] bandhead) and several new γ-ray transitions (particularly for states with excitation energies >1 MeV) are presented. Most notably, the previous ν72⁺[404] systematics at and around the N=90 transition region are brought into question and reassigned as ν52⁺[402]. This reassignment makes the ν12⁺[400], ν32⁺[402], and ν52⁺[402] orbitals, which originate from the 3s1/2, 2d3/2, and 2d5/2 spherical states, respectively, responsible for the three largest cross sections to positive-parity states in the (p,d)Gd155 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 ν2p-2h 0⁺ states below 2Δ. Indeed, this could account for a large number of the low-lying 0⁺ states populated in the (p,t)Gd154 direct reaction.