Identification of high spin states in $^{137,138}$Cs nuclei

High spin states of $^{137,138}$Cs have been studied by measuring the $\gamma-\gamma-\gamma$ coincidences from the spontaneous fission of $^{252}$Cf with the Gammasphere detector array. The level scheme of the N=83 neutron-rich Cs (Z=55) isotope, $^{138}$Cs, has been established for the first time up to a 4626keV level assigned ($16^+$) and that of $^{137}$Cs has been expanded up to a 5495keV level assigned ($31/2^-$). Spins, parities and configurations are assigned based on shell model calculations and level systematics. Pronounced differences in low-lying yrast cascade patterns are seen between N=83 isotones $^{138}$Cs (Z=55), $^{134}$Sb (Z=51) and $^{136}$I (Z=53), and between Cs isotopes $^{138}$Cs and $^{137}$Cs. Significant similarity is observed in the N=82 isotones $^{137}$Cs and $^{135}$I up to $17/2^+$ but not above the spin nor with $^{133}$Sb, which indicates the important role played by interactions between the excitation of the $g_ {7/2}$ valence protons outside the Z=50 major shell, and the $f_{7/2}$ valence neutron outside the N=82 major shell.