Heavy ion reactions

In the energy range from a few MeV/nucleon to 20-30 MeV/nucleon there are rapid changes in the nature of nuclear reactions induced by heavy ions. The complete fusion of projectile and target becomes less probable while the cross sections for emission of small fragments such as α-particles and Li isotopes with velocities close to that of the beam increase. At the upper end of the range, these cross sections are strikingly similar to those measured in the fragmentation of relativistic heavy ions. The group's goal is to gain a detailed understanding of heavy ion reactions in this transition energy range and, if possible, to arrive at a description of them that will tie together the phenomena that are observed from the lowest to the highest beam energies. The Plastic Box, a major new 4π detector for the study of heavy ion reactions, became fully operational during the period under review. It consists of six 8'' x 8'' double plastic paddles arranged as a cube around the target. Several combinations of projectile and target have been studied: 20Ne + 197Au, 16O + 60Ni, 16O + 208Pb and 16O + 12C. The results for 20Ne + 197Au at 11 and 17 MeV/nucleon have been fully analyzed and complementary inclusive measurements for the same system have been made at six energies between 5.4 and 11 MeV/nucleon. The Plastic Box experiments show that the transfer of at least 14 projectile nucleons can occur in events with no associated charged particles. The most likely number of associated particles is just one. This, combined with the observation that the energy spectra of fragments in coincidence with one charged particle are shifted down in energy by comparison with fragments without an associated particle, is entirely consistent with the production of excited fragments that subsequently decay by α or proton emission