LOW-ENERGY FISSION INVESTIGATED IN REACTIONS OF 750AMEV U-238 IONS ONPB-208 - 2 - ISOTOPIC DISTRIBUTIONS

Projectile fission of 750 AMeV U-238-ions interacting with a Pb target was studied by means of the spectrometer FRS, GSI-Darmstadt. One of t he two fission fragments was detected with a transmission of few perce nt and identified in mass and charge. Low-energy fission (E <25 MeV) events were selected by their magnetic rigidity. Whereas the productio n of asymmetric fission events is dominated by the GDR excitation, ver y asymmetric fission and symmetric fission take place after a GQR or D GDR excitation or after a nuclear interaction. Cross sections of more than 250 isotopes were measured. Isotopic distributions of low-energy fission were reconstructed for elements from Se to Te. The fission mod es SI, SII and SL were clearly shown in these distributions and in the mass and TKE distributions. Charge polarization and mass dispersion w ere deduced for each fission mode. Finally, the characteristics of the low-energy fission process explain the production rates of neutron-ri ch species.