Franz-Joseph Hambsch, Hans-Hermann J. Knitter, Carl Budtz-Jørgensen

Commission of the European Communities,
Joint Research Centre - Geel-Establishment,
Central Bureau for Nuclear Measurements, 2440 Geel, Belgium

Jürgen P. Theobald

Physikalisches Institut,Technische Hochschule
D-6100 Darmstadt, W-Germany

Fission fragment energies and emission on angles with respect to the incident neutron beam were measured for 235U(n,f) using a twin Frisch-gridded ionization chamber. The neutron energy range from 0.006 eV to 0.5 MeV was covered in a time-of-flight experiment at the Geel Electron Linear Accelerator (GELINA). 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 <TKE> with respect to the thermal value of up to ~450 keV are observed for resonances below 40 eV. The variations of <TKE> from resonance to resonance can be related to changes in the population of the three fission exit channels. Also the symmetric fission yields or valley to peak ratios of the mass distributions fluctuate from resonance to resonance up to a factor 2. Both fluctuations are not correlated with the resonance spin of the compound nucleus. An explanation for the fluctuations is proposed.

KEYWORDS: 235U(n,f), resolved resonances, fission fragment mass- and total kinetic energy distributions, multi-exit channel model