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要旨:
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 decay per projectile for the yield integrated over a target thickness of 3.6 mg/cm2, which corresponds to a cross section of 3 × 10-29 cm2 averaged over the target. This can be compared to a production cross section of (1-2) × 10-28 cm2, averaged over a target thickness of 0.3-0.6 mg/cm2 found by others for correlated electron-positron pairs in the same reaction.
