Search for narrow-peak structure in the three-photon spectrum from 6-MeV/nucleon U+Th, U+U, and Th+Th collisions

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×${10}^{\mathrm{-}28}$ ${\mathrm{cm}}^2$ averaged over a target thickness of 1 mg/${\mathrm{cm}}^2$.