Neutron induced fission of ²³⁴U

¹ EC-JRC-Institute for Reference Materials and Measurements, Retieseweg 111, B-2440 Geel, Belgium
² Uppsala University, Uppsala, Sweden

Abstract

The fission fragment properties of ²³⁴U(n,f) were investigated as a function of incident neutron energy from 0.2 MeV up to 5 MeV. The fission fragment mass, angular distribution and kinetic energy were measured with a double Frisch-grid ionization chamber using both analogue and digital data acquisition techniques. The reaction ²³⁴U(n,f) is relevant, since it involves the same compound nucleus as formed after neutron evaporation from highly excited ²³⁶U*, the so-called second-chance fission of ²³⁵U. Experimental data on fission fragment properties like fission fragment mass and total kinetic energy (TKE) as a function of incident neutron energy are rather scarce for this reaction. For the theoretical modelling of the reaction cross sections for Uranium isotopes this information is a crucial input parameter. In addition, ²³⁴U is also an important isotope in the Thorium-based fuel cycle. The strong anisotropy of the angular distribution around the vibrational resonance at E_n = 0.77 MeV could be confirmed using the full angular range. Fluctuations in the fragment TKE have been observed in the threshold region around the strong vibrational resonance at E_n = 0.77 MeV. The present results are in contradiction with corresponding literature values. Changes in the mass yield around the vibrational resonance and at E_n = 5 MeV relative to E_n = 2 MeV show a different signature. The drop in mean TKE around 2.5 to 3 MeV points to pair breaking as also observed in ^235,238U(n,f). The measured two-dimensional mass yield and TKE distribution have been described in terms of fission modes. The yield of the standard 1 (S1) mode shows fluctuations in the threshold of the fission cross section due to the influence of the resonance and levels off at about 20% yield for higher incident neutron energies. The S2 mode shows the respective opposite behaviour. The mean TKE of both modes decreases with E_n. The decrease in mean TKE overrules the increase in S1 yield, so the mean TKE is dropping as a function of E_n above 2.5 MeV.