ROLE OF EFFECTIVE DISTANCE IN THE FISSION MECHANISM STUDY BY THE DOUBLE-ENERGY MEASUREMENT FOR URANIUM ISOTOPES

Fission product kinetic energies were measured by the double-energy me thod for thermal-neutron fission of U-235,U-233 and proton-induced fis sion of U-238 at the 15.8-MeV excitation. From the obtained energy-mas s correlation data, the kinetic-energy distribution was constructed fr om each mass bin to evaluate the first moment of the kinetic energy fo r a given fragment mass. The resulting kinetic energy was then convert ed to the effective distance between the charge centers at the moment of scission. The effective distances deduced for the proton-induced fi ssion was concluded to be classified into two constant values, one for asymmetric and the other for symmetric mode, irrespective of the mass though an additional component was further extracted in the asymmetri c mass region. This indicates that the fission takes place via tyro we ll-defined saddles, followed by the random neck rupture. On the contra ry, the effective distances obtained for thermal-neutron induced fissi on turned out to lie along the contour line at the same level as the e quilibrium deformation in the two-dimensional potential map. This stro ngly suggests that it is essentially a barrier-penetrating type of fis sion rather than the over-barrier fission.