We present the results of a Doppler-shift correction to
the measured e+e−–sum-energy spectra obtained from e+e−–coincidence
measurements in 238U + 206Pb and 238U + 181Ta col li sions at beam energies close to the
Coulomb barrier, using an improved experimental setup at the double-Orange
spectrometer of GSI. Internal-Pair-Conversion (IPC) e+e− pairs from discrete
nuclear transitions of a moving emitter have been observed following Coulomb
excitation of the 1.844 MeV (E1) transition in 206Pb and neutron transfer to
the 1.770 MeV (M1) transition in 207Pb . In the collision system 238U + 181Ta , IPC
transitions were observed from the Ta-like as well as from the U-like nuclei.
In all systems the Doppler-shift corrected e+e−–sum-energy spectra show
weak lines at the energies expected from the corresponding γ–ray
spectra with cross sections being consistent with the measured excitation
cross sections of the γ lines and the theoretically predicted IPC
coefficients. No other than IPC e+e−–sum-energy lines were found in the
measured spectra. The transfer cross sections show a strong dependence on the
distance of closest approach (Rmin), thus signaling also a strong
dependence on the bombarding energy close to the Coulomb barrier