Near-yrast structure of N=93 neutron-rich lanthanide nuclei

Two neutron-rich N=93 isotones, 155Sm and 153Nd, have been studied by delayed γ-ray and conversion-electron spectroscopy at the Lohengrin mass spectrometer. A half-life of 2.9(5) μs has been measured for the ν5/2+[642] state at 16.5 keV in 155Sm. The decay of a 1.17(7)-μs isomer in 153Nd, at 191.7 keV, has been remeasured and its spin has been reassigned as (5/2)+. This state contains a strong component of the ν5/2+[642] Nilsson orbital. In addition, a new 1.00(8)-μs isomeric state at 538.6 keV, with a probable ν11/2-[505] Nilsson configuration, has been observed in 155Sm. Triple γ-ray coincidence data from the spontaneous fission of a 252Cf source placed inside the Gammasphere array were used to extend the collective band on top of the (5/2+) isomeric state of 153Nd, and a new band with the same bandhead spin has been observed in 151Ce. The observation of this new band and an additional new transition in the ground-state band has led us to change the ground-state spin of 151Ce to (3/2-). Calculations using the quasiparticle-rotor model successfully reproduce the majority of the features of the γ decays of these nuclei, including branching ratios and isomeric half-lives. Because this model uses a reflection-symmetric core, we conclude that the polarizing effect of the odd particle is responsible for the dipole moment present in the ν5/2+[642] states of the three nuclei studied and the ν11/2-[505] level of 155Sm.