Study of the positron production in uranium-uranium collisions

In the present thesis the positron production in the high Coulomb field of two 238U nuclei in the heavy ion collision for projectile energies in the range of the Coulomb threshold (5.7-6.2 MeV/u) was studied. The energy spectra of the positrons which were taken up in coincidence to the scattered particles exhibit unexpectedly lines. These are superposed to a continuous spectrum which can be well described by the calculated pair production in a time-dependent Coulomb field and from nuclear decays. The energy of the positron line varies in dependence on the incident energy and the particle scattering angle between 311±7 keV (Ep=5.73 MeV/u; θlab=380-520) and 226±17 keV (Ep=6.2 MeV/u; θlab=25.50-290). At fixed scattering angle around 420 in the laboratory system the line width grows with increasing incident energy from 56±9 keV (5.73 MeV/u) to 219±100 keV (6.2 MeV/u). In this angular range at 5.73 MeV/u and 5.9 MeV/u a maximum in the cross section is observed. For smaller scattering angles of 260< or approx.θlab< or approx.380 the positron spectrum shows at the lowest incident energy a smooth distribution while at the higher projectile energies also for these angles a line occurs. (orig./HSI)