Measurement of the neutrons emitted in the proton induced fission of Bi-209 and U-238 at 475-MeV

We have measured the number of prefission and postfission neutrons emitted in the fission of {sup 209}Bi and {sup 238}U targets bombarded with 475 MeV protons as a function of the total kinetic energy of the two fission fragments, the fission fragment mass, and the recoil velocity of the excited nucleus prior to fission. The average number of prefission neutrons is approximately equal to the average number of postfission neutrons for both targets and the total number equals {similar to}17. The number of prefission neutrons increases with increasing recoil velocity and their average number is substantially larger than that predicted by the Bohr-Wheeler theory.