Variations in the photofission cross section near ''threshold'' with initial kind of nucleus were investigated for seven spheroidal nuclei in the region of atomic numbers 90, 92, 93. The photofission cross sections at the two discrete energies 6.14 and 7.0 Mev were determined for the following even-even nuclide . Th/sup 232/, U/sup 234/, U/;sup 236/, and U/sup 238/; and also the odd- A nuclides. U/sup 233/, U/sup 235/ and Np/sup 237/. Nuclear excitation of the target nuclei was achieved via gamma rays from nuclear capture of protons on fluorine. Protons of 1.380- and 3.645-Mev energy impinged on a thick CaF/sub 2/ target. The highenergy quanta of 5.14, 6.91, and 7.12 Mev so obtained represent the well-known ground state transitions in O/sup 16/, whose relative intensities change with bombarding energy of the proton. The fission counting was done in a double-region ionization chamber of 2 pi geometry. The heavy-element samples were nearly uniform oxide deposits of about one mg/cm/sup 2/, on platinum. Employing the known thick target relative gamma-ray intensity ratio, I(6.91 + 7.12)/I(6.14), plus the measured gamma flux and fission counting yield at each of the two above proton energies, the ( gamma , fission) cross sections at 6.14 snd 7.0 Mev were calculated. The ( gamma , fission) cross sections for Th/sup 232/ and U/sup 238/ are greater at 6.0 Mev than at 7.0 Mev, as indicated by earlier betatron results. U/sup 236/ also exhibits a strong resonance at about 6.0 Mev. However, the other even-even uranium isotope investigated, U/sup 234/, does not show this behavior. Its ( gamma , fission) cross section appears to be increasing monotonically with energy between 6.0 and 7.0 Mev. The odd-A uranium isotopes U/sup 233/ and U/sup 235/ also do not show any unusual behavior in this energy interval. The ( gamma , fission) cross section for U/sup 234/ appears to be essentially constant, which again is a confirmation of earlier betatron studies. For U/sup 233/ the cross section is increasing with energy. The one oddproton nuclide examined, Np/sup 237/ appears also to exhibit a resonance behavior at or about 6.0 Mev. Experimental results are correlated with existing information on low-energy photofission and other nuclear data to explain tentatively the small bumps or resonances in the photofission excitation curves. (auth)
