Search

Measurements of photofission fragment angular distributions of238U between 5.1 and 6.0 MeV are presented. As γ-source the intense bremsstrahlung beam from the Catania University Microtron is used. The results are analysed within the framework of the double-humped barrier model with a potential constructed with three smoothly joined parabolae and a damping term in the second well. The values of the fission barrier parameters for the involved 2+0, 1−0 and 1−11 channels are extracted and a comparison with the theoretical predictions and experimental results in the literature is made. Delayed-fission contributions for the involved channels are taken into account in the 1−1 channel parameter determination.

The formalism of the doorway state model for fission is extended to excitation energy as low as the pairing gap region in the intermediate well of the potential barrier. Expressions for the wave function amplitudes which describe the various possible admixtures of the vibrational modes into the compound-state sets are generalized and can be used to calculate prompt- and delayed-fission probabilities in the energy range between the isomer level and the top of the barrier. The occurrence of different coupling situations for the238U l−0 channel is discussed.