Fission modes in 235U (n,f)

Fission fragment energies and emission angles with respect to the incident neutron beam were measured for 235U(n,f) using a twin Frisch gridden ionization chamber. The neutron energy ranges from thermal to 0.5 MeV and from 0.5 MeV to 6 MeV were covered to experiments at the Geel Electron Linear Accelerator (CELINA) and at the 7 MV Van de Graaf, respectively. The experimental set-up will be briefly described. Fission fragment mass and total kinetic energy distributions were evaluated for thermal neutrons, for resolved resonances below 40 eV, and in certain energy bins at higher neutron energy. Fluctuations in the average total kinetic energy with respect to the thermal value of up to ∼ 450 keV were observed for the resolved resonances below 40 eV. Recently Brosa et al predicted on theoretical grounds the existence of three fission channels for 235U(n,f). An analysis of the fragment mass yield, the average total kinetic energy versus fragment mass, and of the variance of TKE as a function of mass for thermal neutrion induced fission of 235U supports this three channel fission model. The vaiation of (TKE) from resonance to resonance can be related to changes in the population of the three fission modes. 19 refs.; 12 figs