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-2- UCRL-18535 photoproduction at energies near 140 MeV. The:n: meson produced inside the nucleus was thought to be reabsorbed producing high excitation energy.Since this paper was published the same effect was observed in other isotopes and was always attributed to the onset of :n: meSon photoproduction. 5 ,8,9 Attempts we~e not usually made to consider separately the behavior of the fiss:1.onprobability as distinct from the interaction cross section, nor to take into account the energy deposition associated with each interaction process.In the present work these effects are taken into account separately and it is found that the variation in the photofission cross section as a function of energy for elements in the region of bismuth or lighter is essentially accounted for by the increase in fission probability with increasing excitation energy. On the other hand, since the fission prob..; ability in uranium is practically constant as a function of energy, the energy dependence of its photofission cross section is a reflection of the interaction cross section.· It seems also that the interaction described by the quasi-deuteron model lO is on the average much more efficient in transferring energy to the nucleus than is the mechanism involving :n: meson photoproduction. Consequently the former interaction process appears to be the dominant one in the excitations of sufficient energy to cause fission of lighter nuclei where the fission barriers are of large magnitude. On the other hand both processes are important in the fiSSion of heavier nuclei, such as uranium, which have small fission barriers.In the present :work we have measured the electron and Bremsstrahlung induced fission cross sections of the -3- UCRL-18535 cal expressions for the eBergy distributicn of the virtua~ photon spectrum associated with electrons, we have calculated the photofission cross sections from the electron-induced fission cross section data. Then the photofission cross sections calculated above have been integrated over the Bremsstrahlung spectrum and found to be in agreement with the measured Bremsstrahlung induced fission cross sections. This procedure has been followed assuming the virtual photon spectra corresponding to El, Ml, and E2 transitions, and some information about multipolarity has been obtained.The photofission cross sections have been subsequently analyzed in terms of , .the contributions due to photon interaction and to the fission.probability.The nature of the energy dependence of the photofission cross section has been establiShed. II. THEORETICAL RELATIONS A. Bremsstrahlung-Induced Reaction'Cross SectionsThe Bremsstrahlung-induced reaction cross sections are related to the photon-induced reaction cross sections through~the following expressionwhereO'B is the Bremsstrahlung induced reaction cross section, 0' (E) is y the photo reaction cross'section and KB(Eo:,E) is the energy distribution of the Bremsstrahlung from a thin radiator. This last q~antity is given by the following relation 11(1) -4-, UCRL-18535wher...

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