Investigation of the fission fragment properties of the reaction 238U(n,f) at incident neutron energies up to 5.8 MeV

Abstract

The fission fragment properties of the reaction 238U(n,f) have been studied, at different incident neutron energies ranging from E n=1.2 to 5.8 MeV. The pre-neutron emission mass, kinetic energy and fission fragment angular distributions have been investigated with a double Frisch-gridded ionization chamber. The influence of the subthreshold vibrational resonances and of the proton pairing effect on the fission fragment properties is clearly visible. The total kinetic energy averaged over all fission fragment masses ( TKE ) shows an increasing trend up to E n=3.5 MeV with a sudden drop at roughly E n=3.8 MeV which has been attributed to the onset of pair breaking at the barrier. Above E n=3.8 MeV, the TKE is again continuously increasing. The changes in the mass yield and TKE ( A) distributions have been studied as a function of the compound nuclear excitation energy and their contribution to the observed variations in the TKE have been determined. The two-dimensional mass-TKE distributions have been described in terms of fission modes and compared with theoretical calculations performed recently in the frame of the multi-modal random neck-rupture model. Although theoretically six asymmetric fission modes are predicted which all surpass individual outer barriers, an interpretation in terms of only two asymmetric modes has physical meaning. This points to an influence of shell structure effects to the observed distributions. In any case, the super-long symmetric mode has to be included, in order to explain the dip in TKE ( A) distribution close to symmetry.