Two-quasiparticle K-isomers and pairing strengths in the neutron-rich isotopes 174Er and 172Er

Abstract

Isomeric two-quasiparticle states have been identified in the neutron-rich isotopes 172Er and 174Er using multi-nucleon transfer reactions with 136Xe beams incident on various targets, and γ-ray spectroscopy with Gammasphere. A candidate for the Kπ=6+ two-quasineutron state in 172Er is found at 1500 keV. In 174Er, a nuclide whose level scheme was previously unknown, a long-lived isomer is identified at 1112 keV decaying via an inhibited E1 transition and revealing the yrast sequence of 174Er. This isomer is proposed to be a Kπ=8− two-quasineutron state, defining a sequence in the N=106 isotones extending from the well-deformed neutron-rich isotope 174Er to the neutron-deficient isotope 188Pb, where the presence of the isomer signifies a prolate minimum in an otherwise spherical well. Configuration-constrained potential-energy surface calculations are used to predict the excitation energies of the 6+ and 8− intrinsic states and as a basis for extracting the pairing force strength, Gn, in the N=104 and N=106 isotones.