Ju n 19 97 Weak e + e − lines from internal pair conversion observed in collisions of 238 U with heavy nuclei

We present the results of a Doppler-shift correction to the measured ee–sum-energy spectra obtained from ee–coincidence measurements in U + Pb and U + Ta 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) ee pairs from discrete nuclear transitions of a moving emitter have been observed following Coulomb excitation of the 1.844 MeV (E1) transition in Pb and neutron transfer to the 1.770 MeV (M1) transition in Pb . In the collision system U + Ta , IPC transitions were observed from the Ta-like as well as from the U-like nuclei. In all systems the Dopplershift corrected ee–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 ee–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. PACS numbers: 14.60.Cd; 23.20.Ra; 25.70.Bc; 25.70.De; 25.70.Hi; 29.30.Aj