Weak e(+)e(-) lines from internal pair conversion observed in collisions of U-238 with heavy nuclei

We present the results of a Doppler-shift correction to the measured e(+)e(-)-sum-energy spectra obtained from e(+)e(-)-coincidence measurements in U-238 + Pb-206 and U-238 + Ta-181 collisions 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 Pb-206 and neutron transfer to the 1.770 MeV (M1) transition in Pb-207 . In the collision system U-238 + Ta-181. 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 gamma-ray spectra with cross sections being consistent with the measured excitation cross sections of the gamma 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 (R-min), thus signaling also a strong dependence on the bombarding energy close to the Coulomb barrier.