HARD PHOTONS AND NEUTRAL PIONS AS PROBES OF HOT AND DENSE NUCLEAR-MATTER

The dynamics of heavy-ion collisions is studied in an energy domain in the vicinity of the Fermi energy, The early history of the collision is analyzed from the theoretical and experimental point of view in whi ch the message conveyed by bremsstrahlung photons and neutral pions is exploited, The Boltzmann-Uehling-Uhlenbeck model and the Dubna Cascad e Model, both based on similar principles but each adopting different computation technics, are briefly described and their respective predi ctions are discussed, In particular the emission pattern of bremsstrah lung photons is discussed. The photon production has been measured in the systems Kr-86+Ni-58 at 60A MeV, Ta-181+Au-197 at 40A MeV and Pb-20 8+Au-197 at 30A MeV and energy spectra, angular distributions and two- photon correlations have been analyzed. We find that bremsstrahlung ph otons are emitted from two distinct sources that can be correlated wit h nuclear-matter density oscillations. The properties of photon emissi on are discussed in terms of collective properties of nuclear matter. The high energy tail of the photon spectrum is interpreted by pi(0) an d Delta decay but predominantly by radiative capture of pions, The pi( 0) absorption in the nuclear medium is further analyzed by examining t heir emission pattern. (C) 1997 Elsevier Science B.V.