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

She present new results from measurements and simulations of positron spect ra; originating from U-238 + Ta-181 collisions at beam energies close to th e Coulomb barrier. The measurements were performed using an improved experi mental setup at the double-Orange spectrometer of GSI. Particular emphasis is put on the signature of positrons from Internal-Pair-Conversion (IPC) pr ocesses in the measured e(+)- energy spectra, following the de-excitation o f electromagnetic transitions in the moving Ta-like nucleus. It is shown by Monte Carlo simulations that, for the chosen current sweeping procedure us ed in the present experiments, positron emission from discrete IPC transiti ons can lead to rather narrow line structures in the measured energy spectr a. The measured positron spectra do not show evidence for line structures w ithin the statistical accuracy achieved, although expected from the intensi ties of the observed gamma -transitions (E-gamma similar to 1250-1600 keV) and theoretical conversion coefficients. This is due to the reduced detecti on efficiency for IPC positrons, caused by the limited spatial and momentum acceptance of the spectrometer. A comparison with previous results, in whi ch lines have been observed, is presented and the implications are discusse d.