Energy dependence of 238U fission yields investigated in inverse kinematics

The production cross sections of neutron-rich fission residues produced in reactions induced by a 238U beam of 950 A*MeV impinging onto Pb and Be targets were investigated at the Fragment Separator (FRS) at GSI using the inverse kinematic technique providing a full identification of the final products. We have measured production cross sections of neutron-rich nuclei from Z=36 to Z=60 down to 100 pb for both targets. The data clearly show that the mass distribution of fission fragments at low excitation energy (lead target) is asymmetric, becoming more symmetric at higher excitation energies (beryllium target). The neutron excess distribution broadens with excitation energy although part of the possible gain in excitation energy is lost by neutron evaporation. The new data clearly indicates that fission at moderate excitation energies (20-50 MeV) increases the production of extremely neutron-rich nuclei, compared to the fission at lower excitation energy, even for asymmetric cases