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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. A

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 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 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 $\gamma$-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., Comment: LaTeX, 32 pages incl. 10 PS figures; Accepted by Z.Phys.A

We present the first energy and angle resolved measurements of e+e- pairs emitted from heavy nuclei (Z>=40) at rest by internal pair conversion (IPC) of transitions with energies of less than 2MeV as well as recent theoretical results using the DWBA method, which takes full account of relativistic effects, magnetic substates and finite size of the nucleus. The 1.76MeV E0 transition in Zr90 (Sr source) and the 1.77MeV M1 transition in Pb207 (Bi source) have been investigated experimentally using the essentially improved set-up at the double-ORANGE beta-spectrometer of GSI. The measurements prove the capability of the setup to cleanly identify the IPC pairs in the presence of five orders of magnitude higher beta- and gamma background from the same source and to yield essentially background-free sum spectra despite the large background. Using the ability of the ORANGE setup to directly determine the opening angle of the e+e- pairs, the angular correlation of the emitted pairs was measured. In the Zr90 case the correlation could be deduced for a wide range of energy differences of the pairs. The Zr90 results are in good agreement with recent theory. The angular correlation deduced for the M1 transition in Pb207 is in strong disagreement with theoretical predictions derived within the Born approximation and shows almost isotropic character. This is again in agreement with the new theoretical results., Comment: LaTeX, 28 pages incl. 10 PS figures; Accepted by Z.Phys.A

We present the results from e+e- coincidence measurements in 238U + 206Pb collisions at a beam energy of 5.93 MeV/u, using an improved experimental setup at the double-Orange spectrometer of GSI. The capability of our device to detect Internal-Pair-Conversion (IPC) e+e- pairs from discrete nuclear transitions of a moving emitter is demonstrated by investigating the known 1.844 MeV (E1) transition in 206Pb and the 1.770 MeV (M1) transition in 207Pb, excited via Coulomb excitation and a neutron transfer reaction, respectively. The Doppler-shift corrected e+- sum-energy spectra show weak lines at the expected energies with cross sections being consistent with the measured excitation cross sections of the corresponding $\gamma$ lines and the theoretically predicted IPC coefficients. No other e+e- sum-energy lines were found in the measured spectra. The observed 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., Comment: 12 pages, standard LaTeX with 3 included PS figures; Submitted to Physics Letters B

We present new results obtained from a series of follow-up e+e- coincidence measurements in heavy-ion collisions, utilizing an improved experimental set-up at the double-Orange beta-spectrometer of GSI. The collision system U+Ta was reinvestigated in three independent runs at beam energies in the range (6.0-6.4)xA MeV and different target thicknesses, with the objective to reproduce a narrow sum-energy e+e- line at ~635 keV observed previously in this collision system. At improved statistical accuracy, the line could not be found in these new data. For the ''fission'' scenario, an upper limit (1 sigma) on its production probability per collision of 1.3x10^{-8} can be set which has to be compared to the previously reported value of [4.9 +- 0.8 (stat.) +- 1.0 (syst)]x10^{-7}. In the light of the new results, a reanalysis of the old data shows that the continuous part of the spectrum at the line position is significantly higher than previously assumed, thus reducing the production probability of the line by a factor of two and its statistical significance to < 3.4sigma., Comment: 15 pages, standard LaTeX with 3 included PS figures; Submitted to Physics Letters B