Abstract
The fission fragment properties of 234 U(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 234 U(n,f) is relevant, since it involves the same compound nucleus as formed after neutron evaporation from highly excited 236 U*, the so-called second-chance fission of 235 U. 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, 234 U is also an important isotope in the Thorium-based fuel cycle. The strong anisotropy of the angular distribution around the vibrational resonance at En = 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 En = 0.77 MeV. The present results are in contradiction with corresponding literature values. Changes in the mass yield around the vibrational resonance and at En = 5 MeV relative to En = 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 235,238 U(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 En . The decrease in mean TKE overrules the increase in S1 yield, so the mean TKE is dropping as a function of En above 2.5 MeV.
Highlights
The neutron induced fission of 234U is interesting in several respects
The decrease in mean total kinetic energy (TKE) overrules the increase in standard 1 (S1) yield, so the mean TKE is dropping as a function of En above 2.5 MeV
We looked at changes in the mass yield distribution going from one incident neutron energy to the other
Summary
The neutron induced fission of 234U is interesting in several respects. First of all it is a threshold reaction with a (or even several) strong vibrational resonances at the threshold of the fission cross section. EPJ Web of Conferences observed fluctuations in literature of the fission fragment properties in the vibrational resonances in the fission cross section threshold could not be verified [2] This isotope is the daughter of the 235U(n,f) reaction after second chance fission, setting in from about 6 MeV incident neutron energy. Further improvements of the available database and especially the knowledge of the fission fragment mass and total kinetic energy (TKE) distributions will help to improve on the prompt neutron emission properties. These data are input data for model calculations e.g. with the Point by Point (PbP) model [4]. Whether this has a consequence on other fission fragment properties like mass and kinetic energy needs to be verified
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