Positron spectra from internal pair conversion observed in {238}U + {181}Ta collisions

We present new results from measurements and simulations of positron spectra, originating from 238U + 181Ta collisions at beam energies close to the Coulomb barrier. The measurements were performed using an improved experimental setup at the double-Orange spectrometer of GSI. Particular emphasis is put on the signature of positrons from Internal-Pair-Conversion (IPC) processes in the measured e+ energy spectra, following the de-excitation of electromagnetic transitions in the moving Ta-like nucleus. It is shown by Monte Carlo simulations that, for the chosen current sweeping procedure used in the present experiments, positron emission from discrete IPC transitions can lead to rather narrow line structures in the measured energy spectra. The measured positron spectra do not show evidence for line structures within the statistical accuracy achieved, although expected from the intensities of the observed $\gamma$ transitions (E$_{\gamma}~1250-1600$ keV) and theoretical conversion coefficients. This is due to the reduced detection efficiency for IPC positrons, caused by the limited spatial and momentum acceptance of the spectrometer. A comparison with previous results, in which lines have been observed, is presented and the implications are discussed.Comment: LaTeX, 20 pages including 5 EPS figures; Accepted by Eur. Phys.Jour.

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