Fragment characteristics for the photofission of 238U with 6.1-13.1 MeV bremsstrahlung

The photofission of 238U has been studied experimentally for seven bremsstrahlung endpoint energies between 6.1 and 13.1 MeV. The postneutron yields of the fission products were obtained with γ-spectrometry techniques. Provisional mass and kinetic energy distributions were measured using a double energy detection technique. From the combination of both methods average neutron emission curves and preneutron mass-energy distributions were deduced. The systematic trends of the fragment characteristics have been studied as a function of the compound nucleus excitation energy. The results are compared with expectations from the energy partition model of Ruben et al. They are also discussed in the framework of the scission point model of Wilkins et al., and of the multimode fission with random neck rupture model of Brosa et al. The mass and kinetic energy data can be represented well by the superposition of two dominant mass asymmetric fission modes (standard I and standard II), and one relatively weak mass-symmetric fission mode (superlong). The standard I mode yield diminishes slightly with increasing compound-nucleus excitation energy. Up to an excitation energy of 7.9 MeV, i.e. the fission barrier plus pairing gap, the average fission-fragment total kinetic energy increases, and the average total 'asymptotic' excitation energy and the charge odd-even effect in the element yields are about constant (δP=(29±2)%). These results hint to low energy dissipation. At higher compound nucleus excitation energies the fragment kinetic energy decreases, their excitation energy increases, and the proton odd-even effect decreases exponentially. (orig.)