Two neutron-rich $N=93$ isotones, ${}^{155}\mathrm{Sm}$ and ${}^{153}\mathrm{Nd}$, have been studied by delayed $\gamma$-ray and conversion-electron spectroscopy at the Lohengrin mass spectrometer. A half-life of 2.9(5) $\mu$s has been measured for the $\nu 5/2^{+}$[642] state at 16.5 keV in ${}^{155}\mathrm{Sm}$. The decay of a 1.17(7)-$\mu$s isomer in ${}^{153}\mathrm{Nd}$, at 191.7 keV, has been remeasured and its spin has been reassigned as $(5/2){}^{+}$. This state contains a strong component of the $\nu 5/2^{+}$[642] Nilsson orbital. In addition, a new 1.00(8)-$\mu$s isomeric state at 538.6 keV, with a probable $\nu 11/2^{-}$[505] Nilsson configuration, has been observed in ${}^{155}\mathrm{Sm}$. Triple $\gamma$-ray coincidence data from the spontaneous fission of a ${}^{252}\mathrm{Cf}$ source placed inside the Gammasphere array were used to extend the collective band on top of the $(5/2^{+})$ isomeric state of ${}^{153}\mathrm{Nd}$, and a new band with the same bandhead spin has been observed in ${}^{151}\mathrm{Ce}$. 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 ${}^{151}\mathrm{Ce}$ to $(3/2^{-})$. Calculations using the quasiparticle-rotor model successfully reproduce the majority of the features of the $\gamma$ 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 $\nu 5/2^{+}$[642] states of the three nuclei studied and the $\nu 11/2^{-}$[505] level of ${}^{155}\mathrm{Sm}$.

• Received 18 November 2009

DOI:https://doi.org/10.1103/PhysRevC.81.024313