Five-valence-protonN=82isotone137Cs

Yrast excitations in the $N=82\mathrm{}$ isotone ${}^{137}\mathrm{Cs}$ have been identified for the first time in thick-target $\gamma$-ray coincidence measurements for the system ${}^{232}\mathrm{Th}{+}^{136}\mathrm{Xe}$ using the Gammasphere array. The ${}^{137}\mathrm{Cs}$ nuclei were produced in deep inelastic one-proton-transfer reactions. By-products of the main investigation were two well-developed rotational bands which were identified in heavy reaction partner products complementary to ${}^{137}\mathrm{Cs}$ and are tentatively assigned to the little studied nucleus ${}^{231}\mathrm{Ac}.$ There was no difficulty in placing the 11 observed ${}^{137}\mathrm{Cs}$ transitions in a yrast level scheme extending to an $I^{\pi }{=(31/2\mathrm{}}^{-})$ level at 5494 keV. Interpretation was also straightforward, since the experimental level energies are in close agreement with results of shell model calculations performed earlier using empirical single particle energies and interaction matrix elements derived from experimental data for other $N=82\mathrm{}$ isotones.