NEUTRON-INDUCED FISSION OF URANIUM ISOTOPES UP TO 100 MEV

The statistical-model description of the neutron-induced fission of U isotopes has been developed using densities of intrinsic states and sp in cutoff parameters obtained directly from appropriate Nilsson model single-particle levels. The first-chance fission cross sections are re produced well when the rotational contributions to the nuclear level d ensities are taken into account. In order to fit the U(n,f) cross sect ions above the threshold of second-chance fission, we must: (1) assume that the triaxial level-density enhancement is washed out at an excit ation energy of approximately 7 MeV above the triaxial barriers with a width of approximately 1 MeV, implying a gamma deformation for the fi rst barriers where 10 < gamma < 20-degrees, and (2) include preequilib rium particle emission in the calculations. Above an incoming-neutron kinetic energy of approximately 17 MeV, our statistical model U(n,f) o f cross sections increasingly overestimates the experimental data. Thi s is not surprising since, at these high energies, little data exist o n the scattering of neutrons to help guide the choice of optical-model parameters. A satisfactory reproduction of all of the available U(n,f ) cross sections above 17 MeV is obtained by scaling our calculated co mpound-nucleus formation cross sections. This scaling factor falls fro m 1.0 at 17 MeV to 0.82 at 100 MeV.