Experimental fissility data from photofission reactions of ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$, ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$, and ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ nuclei at 52 MeV have been obtained using monochromatic and polarized photon beams and dielectric fission-track detectors. These data, along with literature data for ${}_{}{}^{237}{}_{}{}^{}\mathrm{Np}$, ${}_{}{}^{235,238}{}_{}{}^{}\mathrm{U}$, ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$, ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}$, ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$, and ${}_{}{}^{174}{}_{}{}^{}\mathrm{Yb}$ nuclei have been analyzed within the framework of a simple two-step model for photofission reactions, i.e., absorption of the incident photon by a neutron-proton pair followed by an evaporation-fission competition mechanism for the excited nucleus. For nuclei in the Ta-Np region the trend of calculated fissilities clearly shows shell effects in the vicinity of ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$. In the case of ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ the effect of photon polarization on fission direction has also been studied, and isotropy was observed in the fragment azimuthal distribution.

• Received 20 May 1991

DOI:https://doi.org/10.1103/PhysRevC.44.1683