MASS AND KINETIC-ENERGY DISTRIBUTIONS FOR THE PHOTOFISSION OF TH-232 WITH 6.44 TO 13.15 MEV BREMSSTRAHLUNG

Post- and preneutron-emission mass and kinetic energy distributions of the fragments emitted in the photofission of Th-232 with 6.44, 7.33, 8.35, 9.31, 11.13 and 13.15 MeV have been studied. Energy correlation and gamma-spectrometric measurements were performed. Sb, Ru and Cd wer e separated chemically to determine postneutron yields in the symmetri c mass region. The Th-232 system predominantly splits in an asymmetric way with a maximum yield for heavy fragments in the region of mass 14 0. An enhanced yield around heavy mass 134 is observed, becoming of in creasing importance with increasing compound nucleus excitation energy . For 6.44 and 7.35 MeV bremsstrahlung induced fission no symmetric co mponent in the mass distribution could be observed. For the higher end point energies symmetric fission becomes more and more evident. From t he symmetric fission yields at different excitation energies, using ba rrier penetration calculations, the height of the symmetric fission ba rrier is estimated to be of the order of 7.5 to 7.7 MeV. The total fra gment kinetic energy shows a minimum for symmetric splits and a maximu m for splits with heavy mass in the vicinity of mass 132. It increases with increasing excitation energy of the Th-232 compound nucleus. Thi s effect is especially pronounced in the energy region just above the barrier. It is observed for all masses, but mass splits with heavy mas s in the vicinity of mass 132 show the strongest effects. The fragment mass distributions for Th-232(gamma,f) show a clear difference when c ompared with those for alpha-particle accompanied fission of U-235. Ou r results are interpreted in the framework of the Brosa fission channe ls model and in the scission point model. They also provide informatio n concerning the dissipation of collective energy into the intrinsic d egrees of freedom during the transition from saddle to scission point.