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Abstract:
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 238U. 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 6×10-31 cm2 averaged over a target thickness of 1 mg/cm2.