4. Recent HRIBF Research - Study of the 124,132Sn + 96Zr Reactions
(W. Loveland, Spokesperson)

One of the interesting aspects of the study of nuclear reactions induced by radioactive ion beams is the possibility of using neutron-rich projectiles to synthesize new, neutron-rich heavy nuclei. Also, large fusion cross section enhancement has been predicted for fusion reactions with massive neutron-rich radioactive nuclei by different authors. This is due to the lowering of the fusion barrier, excitation of the soft dipole modes and lowering of reaction Q values. However, previous experimental data with stable beams indicates fusion hindrance in the case of massive neutron-rich nuclei. We have measured the fission excitation functions of 96Zr reactions with neutron-rich short-lived 132Sn and stable 124Sn projectiles near the Coulomb barrier. The measurement was carried out at ORNL. The coincident fission fragments were detected using four Si strip detectors. The time of flight of the beam as well as the fragments were measured using two upstream timing MCP's and timing signal from strip detectors. Using different energy, time and position conditions, the fission events were separated from other reaction processes.

Preliminary excitation functions obtained for both systems (124Sn + 96Zr and 132Sn + 96Zr) are shown in Fig. 4-1. The results show drastic differences between stable beam and the radioactive beam data. At energies above the Coulomb barrier, both systems show some sort of cross section suppression, and below barrier show enhancement with respect to one-dimensional barrier penetration models(1D-BPM) and coupled channel calculations. However, neither differences between the two excitation functions nor fusion hindrance observed with respect to 1D-BPM are understood. At below barrier energies, more data is needed for the 132Sn reaction. The capture cross section calculations shown by Fazio et al. using the DNS model for the reaction of  96Zr with  124Sn, give better agreement with our measurements and suggest a large quasi fission component in the reaction.