Nuclear charge and mass yields for 235U(nth, f) as a function of the kinetic energy of the fission products

Abstract

The mass spectrometer “Lohengrin” of the Institut Laue-Langevin in Grenoble was used to determine the yields of the light fission products as a function of A, Z, the kinetic energy E and the ionic charge state q. The nuclear charge and mass distributions summed over all ionic charge states are given for different kinetic energies between 83.6 MeV and 112.0 MeV. The proton pairing causes fine structures in both the mass and nuclear charge distributions at all kinetic energies. Additional fine structures due to shell effects are apparent in the yields at high kinetic energies. With the aid of Monte Carlo calculations the isobaric nuclear charge and energy distributions before neutron emission were determined. The mean nuclear charge Z of the fragments before neutron emission depends on the kinetic energy of the fragments. The pre-neutron emission variance σz2 of the isobaric Z-distributions is found to be independent of the total excitation energy Extot in the region 12 MeV ≦ Extot ≦ 37 MeV. This may be an indication for quantum-mechanical zero-point motion. The kinetic energy distributions for odd-Z elements are shifted toward lower kinetic energies by 0.4 MeV with respect to even-Z elements. The weak dependence of the proton odd-even effect on the kinetic energy of the fragments leads to the conclusion that the energy dissipation between saddle and scission is almost independent of the asymptotic kinetic energy. Furthermore it can be estimated that in about 25% of all fission events all protons remain in a paired state. The mean total excitation energy of the fragments shows no influence of the proton shell Z = 50 and the neutron shells N = 50 and N = 82.