Determination of binary fission-fragment yields in the reaction 251Cf(nth, f) and Verification of nuclear reaction theory predictions of fission-fragment distributions in the reaction 238U(n, f)

Abstract: Neutron-induced fission has been studied at different excitation energies of the compound nucleus by measurements on the two fissioning systems, 252Cf' and 239U'.For the first time, the light fission fragment yields from the reaction 251Cf(nth, f) have been measured with high resolution. This experiment was performed with the recoil mass spectrometer LOHENGRIN at ILL in Grenoble, France. When the results from this work, where the compound nucleus is at thermal excitation, are compared to the spontaneous fission of 252Cf, enhanced emission yields as well as an increased mean kinetic energy is observed around A = 115. This suggests the existence of an additional super-deformed fission mode in 252Cf.The reaction 238U(n, f) was studied using the 2E-technique with a double Frisch grid ionization chamber. Fission fragment mass, energy and angular distributions were determined for incident neutron energies between 0.9 and 2.0 MeV. The experiments were performed at the Van de Graaff accelerator of IRMM in Geel, Belgium. This is the first measurement of the mass distribution for incident neutron energies around 0.9 MeV. The motivation for studying 238U(n, f) was to verify theoretical predictions of the mass distribution at the vibrational resonance in the fission cross section at 0.9 MeV. However, the predicted changes in fission fragment distributions could not be confirmed.A precise modelling of the fission process for the minor actinides becomes very important for future generation IV and accelerator driven nuclear reactors. Since fission fragment distributions depend on the excitation of the fissioning system, so does the number of delayed neutrons, which are one of the safety parameters in a reactor.