  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.
