Capture and fission $\gamma$ rays from epithermal neutron irradiation of ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ have been identified. The capture $\gamma$ rays were used to construct a level diagram for ${}_{}{}^{236}{}_{}{}^{}\mathrm{U}$ and to determine the spins of the ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ resonances. Coincidence $\gamma$-ray spectra, taken with a large $4\pi$ fission neutron detector and Ge(Li) crystal were used to identify prompt-fission $\gamma$ rays. This apparatus was also used to suppress the background of prompt-fission $\gamma$ rays in the resonance spectra and to increase the probability of seeing capture $\gamma$ rays.

Energies of high-energy $\gamma$ rays identified as due to resonant neutron capture in ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ are 6396, 5588, 5546, 5487, 5417, and 5292 keV; these populate excited states in ${}_{}{}^{236}{}_{}{}^{}\mathrm{U}$ at 149, 957, 999, 1058, 1128, and 1253 keV. Many of the 14 low-energy $\gamma$ rays identified as capture have been assigned as those representing transitions between these low-lying excited states. On the basis of observing the 5588-keV $\gamma$ ray, which populates a $2^{+}$ final state in ${}_{}{}^{236}{}_{}{}^{}\mathrm{U}$, the spin-parity of the 2.040-, 12.39-, 14.53-, and 32.05-eV resonances have been assigned as $3^{-}$. We assigned $4^{-}$ to resonances at 11.67 and 16.10 eV by placing a $5^{+}$ state at 1128 keV. From the work of Schermer et al. and our $3^{-}$ assignment for the 2.040-eV resonance, we conclude that the 0.290- and 1.14-eV resonances are $3^{-}$ and $4^{-}$, respectively.

On the basis of level systematics in even uranium isotopes and our high- and low-energy capture $\gamma$ rays, we propose that the 1128-keV level in ${}_{}{}^{236}{}_{}{}^{}\mathrm{U}$ is most likely the $5^{+}$ member of the $K^{\pi }$, $J^{\pi }=2^{+}$, $2^{+}$ $\gamma$-vibrational band.

[NUCLEAR REACTIONS ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ measured $I_{\gamma }$, slow neutrons $(n,\gamma ) \mathrm{}(n,F)\mathrm{}$; deduced $J$ ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ resonances 0-30 eV, ${}_{}{}^{236}{}_{}{}^{}\mathrm{U}$ levels to 1300 keV, delayed-, prompt-fission $\gamma$'s.]

• Received 5 March 1973

DOI:https://doi.org/10.1103/PhysRevC.8.781