Experimental fissility data from photofission reactions of Th-232, U-238, and U-235 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 Np-237, U-235,U-238 Th-232, Bi-209, Pb-208, and Yb-174 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 Pb-208. In the case of U-238 the effect of photon polarization on fission direction has also been studied, and isotropy was observed in the fragment azimuthal distribution

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A study of photofission on " T a , ""W and ""Pt nuclei induced by monochromatic photons of 69MeV has been performed. The fission yields have been measured by using makrofoil sheets as fission-track detectors, in thick-target 277 geometry. The absolute photofission cross sections for the tantalum, tungsten and platinum targets are found, respectively, to be 4.8-t l.Opb, 5.2-t 1 I p b and 1O-t2pb, and the corresponding deduced fissilities are (3.2 -t 0.7) X (3.4 j: 0.8) X and (6.4 * 1.3) x NUCLEAR REACTIONS ""Ta(y, fission), ""W (7. Iission), and ""'Pt(7, fission) E? = 69 MeV; measured o(fission), deduced fissility.

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