Correlated two-photon lines from 6-MeV/nucleon U+Th, U+U, and Th+Th collisions

We find that narrow lines at 1043 and 1062 keV in the summed-energy, 180°-correlated two-photon spectrum from ≊6-MeV/nucleon U+Th collisions can be produced by cascades from high-spin states (${32}^{+}$) in ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$. We see no evidence for narrow two-photon lines in U+Th, Th+Th, and U+U collisions corresponding to the electron-positron lines seen by others: In the summed-energy region between 1.2 and 2.0 MeV, we set an upper limit for the cross section of 6×${10}^{\mathrm{-}31}$ ${\mathrm{cm}}^2$ averaged over a target thickness of 1 mg/${\mathrm{cm}}^2$.