Photoexcitation mechanisms and fission ofBi209from threshold to theΔregion

The absolute electrofission cross section of ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}$ was measured in the energy range 43–250 MeV; the photofission cross section was deduced by means of the virtual-photon technique in the same energy range. The analysis of the fission probability, which was estimated from the photofission cross section and photoabsorption cross sections available in the literature, showed that ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}$ undergoes fission through compound nucleus formation for energies between the threshold and the Δ region. It was found that Levinger’s modified quasi-deuteron model cross section accounts for all the compound nucleus cross section, even at energies well above the pion threshold. The ineffectiveness of the pion photoproduction excitation mechanism in inducing fission of ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}$ could be explained in terms of the role played by the pion mean free path inside the nucleus.