Photoexcitation mechanisms and photofission cross section for Bi by 100–300 MeV quasi-monochromatic photons

The photofission cross section of natural Bi was measured in the energy range 100–300 MeV by means of a quasi-monochromatic photon beam. The nuclear fissility $P_f$ was calculated using the recently measured total photoabsorption cross sections. A discussion on the dependence of fissility from the excitation energy $E_x$ shows that a linear dependence of ln$P_f$ vs $E_x^{\mathrm{-}1/2}$ can hardly be assumed over all the considered energy range. The analysis of the data confirms this consideration and shows an evident saturation effect at high excitation energy. As a consequence, in disagreement with recent interpretations, inferring that the modified quasi-deuteron model is the only efficient mechanism in inducing fission of Bi is less compelling, and also the pion photoproduction excitation mechanism plays a role.