Half-life measurements in Dy 164,166 using γ-γ fast-timing spectroscopy with the ν -Ball spectrometer

We report on the measurement of lifetimes of excited states in the near-mid-shell nuclei Dy164,166 using the gamma-ray coincidence fast-timing method. The nuclei of interest were populated using reactions between an O18 beam and a gold-backed isotopically enriched Dy164 target of thickness 6.3mg/cm2 at primary beam energies of 71, 76, and 80 MeV from the IPN-Orsay laboratory, France. Excited states were populated in Dy164, Dy166, and W178,179 following Coulomb excitation, inelastic nuclear scattering, two-neutron transfer, and fusion-evaporation reaction channels respectively. Gamma rays from excited states were measured using the ν-Ball high-purity germanium (HPGe)-LaBr3 hybrid γ-ray spectrometer with the excited state lifetimes extracted using the fast-timing coincidence method using HPGe-gated LaBr3-LaBr3 triple coincident events. The lifetime of the first Iπ=2+ excited state in Dy166 was used to determine the transition quadrupole deformation of this neutron-rich nucleus for the first time. The experimental methodology was validated by showing consistency with previously determined excited state lifetimes in Dy164. The half-lives of the yrast 2+ states in Dy164 and Dy166 were 2.35(6) and 2.3(2) ns, respectively, corresponding to transition quadrupole moment values of Q0=7.58(9) and 7.5(4) eb, respectively. The lifetime of the yrast 2+ state in Dy166 is consistent with a quenching of nuclear quadrupole deformation at β≈0.35 as the N=104 mid-shell is approached.