ELECTRON- AND BREMSSTRAHLUNG-INDUCED FISSION OF HEAVY AND MEDIUM-HEAVY NUCLEI

The electron-and bremsstrahlung-induced fission cross sections of the isotopes U92238, Bi83209, Pb82208, Yb70174, and Sm62154 have been measured over the energy range 60-1000 MeV. The consistency of the theoretical expressions for the bremsstrahlung spectrum and the virtual-photon spectrum associated with the electron has been tested using the experimental data. Good agreement was obtained on the assumption of E1 or M1 transitions. The photofission cross sections obtained on the basis of this procedure were analyzed in terms of the photon-interaction cross section and the fission probability. It was found that the rapid increase in the photofission cross section with increasing energy for Bi209 and the lighter isotopes is due to the normal increase in fission probability with energy, and not to the onset of π-meson photoproduction as was previously suggested. The photon-interaction cross section predicted by the quasideuteron model seems to account for all of the interaction leading to fission in these elements, even at energies above the threshold for π-meson production. In the case of U238, however, since the fission probability is approximately constant as a function of energy, the fission cross-section behavior reflects the characteristics of the total photon-interaction cross section. In the latter case it was found that the π-meson photoproduction and the quasideuteron interaction are both involved in producing excitation leading to fission. The difference between the behavior of U238 and the lighter isotopes is understood in terms of differences in fission barriers and differences in energy deposition associated with the two interaction mechanisms. The asymmetric fission of U238 at high electron or bremsstrahlung energies is also explained on this basis. © 1969 The American Physical Society.