Mass and kinetic energy distributions for the photofission of 232Th with 6.44 to 13.15 MeV bremsstrahlung

Abstract

Post- and preneutron-emission mass and kinetic energy distributions of the fragments emitted in the photofission of 232Th with 6.44, 7.33, 8.35, 9.31, 11.13 and 13.15 MeV have been studied. Energy correlation and γ-spectrometric measurements were performed. Sb, Ru and Cd were separated chemically to determine postneutron yields in the symmetric mass region. The 232Th system predominantly splits in an asymmetric way with a maximum yield for heavy fragments in the region of mass 140. An enhanced yield around heavy mass 134 is observed, becoming of increasing importance with increasing compound nucleus excitation energy. For 6.44 and 7.35 MeV bremsstrahlung induced fission no symmetric component in the mass distribution could be observed. For the higher endpoint energies symmetric fission becomes more and more evident. From the symmetric fission yields at different excitation energies, using barrier penetration calculations, the height of the symmetric fission barrier is estimated to be of the order of 7.5 to 7.7 MeV. The total fragment kinetic energy shows a minimum for symmetric splits and a maximum for splits with heavy mass in the vicinity of mass 132. It increases with increasing excitation energy of the 232Th compound nucleus. This effect is especially pronounced in the energy region just above the barrier. It is observed for all masses, but mass splits with heavy mass in the vicinity of mass 132 show the strongest effects. The fragment mass distributions for 232Th(γ, f) show a clear difference when compared with those for α-particle accompanied fission of 235U. Our results are interpreted in the framework of the Brosa fission channels model and in the scission point model. They also provide information concerning the dissipation of collective energy into the intrinsic degrees of freedom during the transition from saddle to scission point.