180°-correlated equal-energy photons from 5.9-MeV/nucleon U + Th collisions

We have found a narrow line (intrinsic width ≤2.5 keV) at 1062±1 keV in the summed-energy 180°-c.m.-correlated two-photon spectrum from 5.95-MeV/nucleon U+Th collisions. Including possible systematic uncertainties, the production cross section is found to be 50±25 μb, averaged over the 1-mg/${\mathrm{cm}}^2$ Th target thickness. This line may belong to the decay of a neutral system which also produces correlated electron-positron pairs found by others in the same reaction.