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

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.