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Abstract:
We have searched for three-photon decays produced in 6-MeV/nucleon U+Th, Th+Th, and U+U collisions. We see no evidence for narrow lines 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 3×10-28 cm2 averaged over a target thickness of 1 mg/cm2.