LOW-ENERGY FISSION STUDIES OF NEUTRON-DEFICIENT PROJECTILE FRAGMENTS OF U-238

Low-energy fission of neutron-deficient actinium, thorium. protactiniu m and uranium isotopes has been investigated using a new experimental technique. The isotopes were produced as secondary beams by projectile fragmentation from a 950 A MeV U-238 primary beam. Their fission was induced by electromagnetic excitation and nuclear reactions in a lead and in a plastic target at energies between 510 A MeV and 150 A MeV. T he transition from symmetric to asymmetric fission is shown to take pl ace around N = 138.