Search for correlated narrow-peak structure in the two-photon spectrum from 6-MeV/nucleon U+Th collisions

We have searched for 180/sup 0/ (c.m.) correlated equal-energy two-photon decays produced in 6-MeV/nucleon U+Th collisions. In the summed-energy region between 1.5 and 1.8 MeV, we set an upper limit of 3 x 10/sup -10/ decays per projectile for the yield integrated over a target thickness of 3.6 mg/cm/sup 2/, which corresponds to a cross section of 3 x 10/sup -29/ cm/sup 2/ averaged over the target. This can be compared to a production cross section of (1-2) x 10/sup -28/ cm/sup 2/, averaged over a target thickness of 0.3--0.6 mg/cm/sup 2/ found by others for correlated electron-positron pairs in the same reaction.