Determination of Photofission Fragment Characteristics of 234,238U and 232Th in the Barrier Region

The photofission of 232Th, 234U and 238U at excitation energies in the barrier region has been studied. The goal has been to extract information on the mass, total kinetic energy (TKE) and angular distributions of the fission fragments. The experiments were performed using bremsstrahlung, produced by an electron beam in a copper radiator at the injector of the Superconducting Darmstadt linear accelerator (S-DALINAC). Mass and TKE distributions were determined by means of the double kinetic energy technique using a twin Frisch grid ionization chamber. The angular distributions were derived simultaneously by measuring the drift time of ionization electrons. Results show that this drift-time method is comparable in accuracy to other more commonly used methods, but with a simplified procedure for setting up the experiment. Mass, TKE and angular distributions have been obtained from bremsstrahlung-induced fission of 232Th at average excitation energies = 6.68 MeV and 7.26 MeV, for 234U at = 5.80 MeV, 6.49 MeV and 7.23 MeV, and for 238U at = 5.90 MeV, 6.11 MeV and 6.93 MeV. Results on fission fragment characteristics from 238U(g,f) show good agreement with literature data, which verifies the experimental procedure. The correlated mass and TKE data have been analyzed in terms of fission modes within the multi-modal random-neck-rupture model. The result exhibits a dominant yield of the mass asymmetric standard-2 mode in all the investigated fissioning nuclei, with a relative yield of ~77 % in 232Th, ~75 % in 234U and ~67 % in 238U. No strong fluctuation of the mode yields were found as a function of the excitation energy. Correlations between mass, TKE and angular distributions have been investigated in 232Th and 234U. The correlation takes the form of an increased anisotropy for far-asymmetric masses and low TKE. The dependence of the anisotropy on mass and TKE could simultaneously be described by assuming specific angular distributions for the two mass asymmetric standard fission modes. This analysis results in a larger anisotropy for the standard-2 mode, which suggests a higher outer barrier for this fission mode