The production of positron-electron pairs in collisions of ${}^{238}\mathrm{U}{+}^{232}\mathrm{Th}$ at 5.95 MeV/nucleon, and of ${}^{238}\mathrm{U}{+}^{181}\mathrm{Ta}$ at 5.95, 6.1, and 6.3 MeV/nucleon, has been studied with the APEX spectrometer at Argonne National Laboratory. Several analyses have been performed to search for sharp structures in sum-energy spectra for positron-electron pairs. Such features have been reported in previous experiments. No statistically convincing evidence for such behavior is observed in the present data.

• Received 25 November 1998

DOI:https://doi.org/10.1103/PhysRevC.60.064601