Analysis of 209Bi and 238U photofission cross section in the quasi-deuteron region of photonuclear absorption

An analysis of the photofission reactions in the quasi-deuteron energy range of photonuclear absorption (∝30-140 MeV) has been performed for 209Bi and 238U nuclei. Experimental cross section data available in the literature have been compared with calculated values obtained from a model in which the incoming photon is assumed to be absorbed by a neutron-proton pair (Levinger's quasi-deuteron photoabsorption), followed by a mechanism of evaporation-fission competition for the excited residual nuclei. The model has been shown to reproduce the main experimental features of 209Bi and 238U photofission cross section, although unexplained differences still remain in the case of 238U-fission by 30-50 MeV incident photons. (orig.)