Identification and shell model calculation of high spin states inCs137,138nuclei

Abstract

High spin states of $^{137,138}\mathrm{Cs}$ have been studied by measuring $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidences from the spontaneous fission of $^{252}\mathrm{Cf}$ with the LBNL Gammasphere detector array. The high spin level scheme of the $N=83$ neutron-rich Cs ($Z=55$) isotope, $^{138}\mathrm{Cs}$, built on the ${6}^{\ensuremath{-}}$ isomeric state, has been established for the first time up to a 4626 keV level, assigned (${16}^{+}$). The level scheme of $^{137}\mathrm{Cs}$ has been expanded up to a 5495 keV level, assigned ($31/{2}^{\ensuremath{-}}$). Spins, parities, and configurations are assigned based on the agreement between experimental level energies and shell model calculations and level systematics. Similarities are observed in the $N=82$ isotones, $^{137}\mathrm{Cs}$ and $^{135}\mathrm{I}$, up to $17/{2}^{+}$ as well as in the $N=83$ isotones, $^{138}\mathrm{Cs}$ and $^{136}\mathrm{I}$, up to ${12}^{\ensuremath{-}}$. The shell model calculations indicate the important role played by interactions between the excitation of the valence protons outside the $Z=50$ major shell and the ${f}_{7/2}$ valence neutron outside the $N=82$ major shell.