Excited states in the ${}^{138}\mathrm{I}$ nucleus, including $T_{1/2}=1.3$ $\mu s$ isomer decaying by a stretched $E2$ transition of 68 keV, were observed for the first time. The ${}^{138}\mathrm{I}$ nucleus was populated in the spontaneous fission of ${}^{248}\mathrm{Cm}$ and studied by means of prompt γ-ray spectroscopy using the EUROGAM 2 array. The microsecond isomer was populated in the neutron-induced fission of ${}^{235}\mathrm{U}$ and observed at the LOHENGRIN separator. Excitation scheme consists of a low-spin part and a medium-spin, $\Delta I=1$, band based on the $7^{-}$ state with the $(\pi g_{7/2}\nu f_{7/2}){}_{7^{-}}$ dominating configuration, as predicted by the shell model. The shell-model calculations of ${}^{138}\mathrm{I}$ provide the optimum reproduction of the experimental scheme when the $\pi d_{5/2}$ orbital is lowered by 600 keV relative to its position in ${}^{133}\mathrm{Sb}$. In the calculation the isomeric level has spin and parity $3^{-}$ and deexcites by an $E2$ isomeric transition to the $1^{-}$ level, located only 9 keV above the predicted $0^{-}$ ground state. Considering additional information on the ground-state spin from the literature, we propose that in ${}^{138}\mathrm{I}$ the $1^{-}$ level corresponds to the ground state and the $0^{-}$ is located above. We note, however, that additional measurements are required to resolve this problem.

• Received 30 January 2007

DOI:https://doi.org/10.1103/PhysRevC.75.054319