The masses of neutron-deficient and neutron-rich isotopes of rubidium (${}_{}{}^{74-79}{}_{}{}^{}\mathrm{Rb}$, ${}_{}{}^{90-99}{}_{}{}^{}\mathrm{Rb}$) and of cesium (${}_{}{}^{117-124,126}{}_{}{}^{}\mathrm{Cs}$, ${}_{}{}^{138,140-147}{}_{}{}^{}\mathrm{Cs}$) have been determined by direct on-line mass spectrometry. The measured masses are compared to current mass predictions. From the trend of neutron separation energies, experimental neutron pairing energies are deduced and evidence is found for the onset of a deformation at $N=60\mathrm{}$ in the neutronrich rubidium isotopes.

NUCLEAR STRUCTURE ${}_{}{}^{74-79}{}_{}{}^{}\mathrm{Rb}$, ${}_{}{}^{90-99}{}_{}{}^{}\mathrm{Rb}$, ${}_{}{}^{117-124,126}{}_{}{}^{}\mathrm{Cs}$, ${}_{}{}^{138,140-147}{}_{}{}^{}\mathrm{Cs}$; measured masses, on-line mass spectrometry. $S_{2n}$ deduced; evidence for deformation at $N=60\mathrm{}$. Comparison with mass predictions.

• Received 17 July 1978

DOI:https://doi.org/10.1103/PhysRevC.19.1504