Search for Monoenergetic Positron Emission from Heavy-Ion Collisions at Coulomb-Barrier Energies

Positron production in ${}^{238}\mathrm{U}{+}^{232}\mathrm{Th}$ and ${}^{238}\mathrm{U}{+}^{181}\mathrm{Ta}$ collisions near the Coulomb barrier has been studied. Earlier experiments reported narrow lines in the spectra of positrons, accumulated without the requirement of electrons detected in coincidence. No evidence of such structure is observed in the present data. The positron energy spectra are compared with estimates from dynamic atomic processes, and from internal pair conversion of electromagnetic transitions from the excited nuclei.