Kooperative Produktion von neutralen Pionen und hochenergetischen Photonen in Kern-Kern-Stössen bei 60-84 MeV/u

The described experiments which were performed at the CERN synchrocyclotron with different target-projectile combinations at different beam energies allow a systematic of the pion production in nucleus-nucleus collisions below the threshold. Thereby the pion production shows clearly the existence of collective effects in the nucleus-nucleus collision in the energy range of 60-84 MeV/u: 1.) The measured cross sections exceed those predicted in the simple nucleon-nucleon collision with Fermi motion by more than one order of magnitude. 2.) The mean velocity determined from the longitudinal rapidity cannot be explained by single nucleon collisions. 3.) The model of the collective pionic bremsstrahlung describes well both the excitation function, the energy spectra, and the angular distribution. Thereby result slow stopping ranges which indicates, that the pions in the collision are produced only during a short time. For the asymmetric systems in the rapidity distribution for low pion energies a structure in the data is seen which can be interpreted as two source velocity components which can be caused by superposition of two processes. For photons with energies above 50 MeV in the nucleus-nucleus collision a cross section was found which is comparable with that of pions with the same whole energy. The high-energy photons can be described by electromagnetic bremsstrahlung in the slowing down of the charge in the nuclear field.