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Photonuclear reactions of actinides in the giant dipole resonance region

Photonuclear reactions at energies covering the giant dipole resonance (GDR) region are analyzed with an approach based on nuclear photoabsorption followed by the process of competition between light particle evaporation and fission for the excited nucleus. The photoabsorption cross section at energies covering the GDR region is contributed by both the Lorentz type GDR cross section and the quasideuteron cross section. The evaporation-fission process of the compound nucleus is simulated in a Monte-Carlo framework. Photofission reaction cross sections are analyzed in a systematic manner in the energy range of $\sim$ 10-20 MeV for the actinides $^{232}$Th, $^{238}$U and $^{237}$Np. Photonuclear cross sections for the medium-mass nuclei $^{63}$Cu and $^{64}$Zn, for which there are no fission events, are also presented. The study reproduces satisfactorily the available experimental data of photofission cross sections at GDR energy region and the increasing trend of nuclear fissility with the fissility parameter $Z^2/A$ for the actinides.