Measurement of the Prompt Neutrons Emitted in the Fission ofBi209andU238Induced by 155-MeV Protons

Abstract

The energy distribution of the neutrons emitted in the fission of $^{209}\mathrm{Bi}$ and $^{238}\mathrm{U}$ induced by 155-MeV protons was measured at 0 and 90\ifmmode^\circ\else\textdegree\fi{} with respect to the fission fragment direction. The neutron velocity was measured by the time-of-flight method in coincidence with the measurement of the kinetic energy of the two fission fragments. The number of postfission neutrons as a function of the fragment mass and the energy distribution of the postfission neutrons were determined without reference to any theoretical model. The number of prefission neutrons was determined on the assumption of isotropic emission from the excited nucleus. With this assumption we find that 5.8\ifmmode\pm\else\textpm\fi{}1.0 prefission neutrons are emitted from U and 6.9\ifmmode\pm\else\textpm\fi{}1.0 from Bi. The number of postfission neutrons is 5.1\ifmmode\pm\else\textpm\fi{}0.5 for U and 4.2\ifmmode\pm\else\textpm\fi{}0.5 for Bi. We compare our experimental results with the results of calculations based on the usual model of a prompt intranuclear cascade followed by neutron evaporation-fission competition with several assumptions for $\frac{{\ensuremath{\Gamma}}_{f}}{{\ensuremath{\Gamma}}_{n}}$ (ratio of fission width to neutron emission width). We are unable to find a consistent set of parameters which would give good agreement between the calculated results, our experimental results, radiochemical results for spallation residues, and total fission cross-section measurements. Our experimental results indicate that $\frac{{\ensuremath{\Gamma}}_{f}}{{\ensuremath{\Gamma}}_{n}}$ decreases at high excitation energies. We find that the number of postfission neutrons increases rapidly with increasing fragment mass for both Bi and U. When our results for U are compared with low-energy fission data for this element, it is found that almost all the additional excitation energy available in high-energy fission is concentrated in the heavy fragment.