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Monochromatic photon beams have been used to investigate the photofission of 27Al at 69 MeV with plastic nuclear-track detectors. The measured fissility was found to be (5.7+or-1.1)*10-2. This result, along with the available data for 154Sm and 174Yb nuclei taken at the same energy, has been interpreted within the framework of a two-step model for the photofission reaction. According to this model the incoming photon is considered to be absorbed by a neutron-proton pair, and then followed by an evaporation-fission competition for the excited residual nucleus. The measured fissility values are seen to be in satisfactory agreement with the calculated ones, and the predictions from the model are confirmed, specially in the region of light fissioning systems.

A study of photofission on natTa, natW and natPt nuclei induced by monochromatic photons of 69 MeV has been performed. The fission yields have been measured by using makrofoil sheets as fission-track detectors, in thick-target 2 pi geometry. The absolute photofission cross sections for the tantalum, tungsten and platinum targets are found, respectively, to be 4.8+or-1.0 mu b, 5.2+or-1.2 mu b and 10+or-2 mu b, and the corresponding deduced fissilities are (3.2+or-0.7)*10-4, (3.4+or-0.8)*10-4 and (6.4+or-1.3)*104-4.

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.