Identification and shell model calculation of high spin states in Cs137,138 nuclei

High spin states of Cs-137,Cs-138 have been studied by measuring gamma-gamma-gamma coincidences from the spontaneous fission of Cf-252 with the LBNL Gammasphere detector array. The high spin level scheme of the N=83 neutron-rich Cs (Z=55) isotope, Cs-138, built on the 6(-) isomeric state, has been established for the first time up to a 4626 keV level, assigned (16(+)). The level scheme of Cs-137 has been expanded up to a 5495 keV level, assigned (31/2(-)). 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, Cs-137 and I-135, up to 17/2(+) as well as in the N=83 isotones, Cs-138 and I-136, up to 12(-). 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.