Search for Correlated Narrow-Peak Structure in the Two-Photon Spectrum from 6-MeV/ Nucleon U + Th Collisions

We have searched for 180° (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 × ${10}^{-10\mathrm{}}$ decay per projectile for the yield integrated over a target thickness of 3.6 mg/${\mathrm{cm}}^2$, which corresponds to a cross section of 3 × ${10}^{-29\mathrm{}}$ ${\mathrm{cm}}^2$ averaged over the target. This can be compared to a production cross section of (1-2) × ${10}^{-28\mathrm{}}$ ${\mathrm{cm}}^2$, averaged over a target thickness of 0.3-0.6 mg/${\mathrm{cm}}^2$ found by others for correlated electron-positron pairs in the same reaction.