STATISTICAL-MODEL ANALYSIS OF FAST-NEUTRON-INDUCED FISSION OF U-ISOTOPES

We have obtained the first experimental evidence of the washing out of the collective level density enhancement associated with the gamma de formation of the triaxial first barrier in the U isotopes. In order to fit U(n, f) cross sections at neutron energies up to similar to 20 Me V, with a statistical model which uses level densities obtained from N ilsson model single particle levels, we find that it is necessary to ( 1) wash out the triaxial level density enhancement at an excitation en ergy of similar to 7 MeV above the triaxial barriers with a width of s imilar to 1 MeV, and (2) incorporate the effects of preequilibrium emi ssion. These results imply a gamma deformation of the first barriers i n the range 10 degrees-20 degrees. Above an incoming neutron energy of similar to 20 MeV where insufficient data exist to constrain optical model potentials, our statistical model U(n, f) cross sections increas ingly overestimate the experimental data. A satisfactory reproduction of all the available U(a, f) cross sections above similar to 20 MeV is obtained by sealing our calculated compound nucleus formation cross s ections. This scaling factor falls from 1.0 at similar to 20 MeV to 0. 82 at 100 MeV.