Interplay of quasiparticle and vibrational excitations: First observation of isomeric states in 168Dy and 169Dy

The neutron-rich dysprosium isotopes Dy-168(102) and Dy-169(103) have been investigated using the EURICA gamma-ray spectrometer, following production via in-flight fission of a high-intensity uranium beam in conjunction with isotope separation through the BigRIPS separator at RIBF in RIKEN Nishina Center. For Dy-168, a previously unreported isomer with a half-life of 0.57(7) us has been identified at an excitation energy of 1378 keV, and its presence affirmed independently using gamma-gamma-gamma coincidence data taken with Gammasphere via two-proton transfer from an enriched Er-170 target performed at Argonne National Laboratory. This isomer is assigned J(pi) = K-pi = (4(-)) based on the measured transition strengths, decay patterns, and the energy systematics for two-quasiparticle states in N = 102 isotones. The underlying mechanism of two-quasiparticle excitations in the doubly midshell region is discussed in comparison with the deformed QRPA and multi-quasiparticle calculations. In Dy-169, the B(E2) value for the transition de-exciting the previously unreported K-pi = (1/2(-)) isomeric state at 166 keV to the K-pi = (5/2(-)) ground state is approximately two orders of magnitude larger than the E2 strength for the corresponding isomeric-decay transition in the N = 103 isotone Yb-173, suggesting the presence of a significant gamma-vibrational admixture with a dominant neutron one-quasiparticle component in the isomeric state. (C) 2019 The Authors. Published by Elsevier B.V.