Electron- and Bremsstrahlung-Induced Fission of Heavy and Medium-Heavy Nuclei

The electron-and bremsstrahlung-induced fission cross sections of the isotopes ₉₂²³⁸U, ₈₃²⁰⁹Bi, ₈₂²⁰⁸Pb, ₇₀¹⁷⁴Yb, and ₆₂¹⁵⁴Sm 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 ²⁰⁹Bi 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 ²³⁸U, 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 ²³⁸U 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 ²³⁸U at high electron or bremsstrahlung energies is also explained on this basis.