Weak e$^+$e$^-$ lines from internal pair conversion observed in collisions of $^{238}$U with heavy nuclei

Abstract

We present the results of a Doppler-shift correction to the measured e$^+$e$^-$–sum-energy spectra obtained from e$^+$e$^-$–coincidence measurements in $^{238}$U + $^{206}$Pb and $^{238}$U + $^{181}$Ta  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 $^{206}$Pb and neutron transfer to the 1.770 MeV (M1) transition in $^{207}$Pb . In the collision system $^{238}$U + $^{181}$Ta , 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