Average Number of Prompt Neutrons fromU235Fission Induced by Neutrons from Thermal to 8 MeV

A large, loaded-liquid scintillation counter has been used to measure the average number, $\overline{\nu }$, of neutrons emitted in the neutron-induced fission of ${\mathrm{U}}^{235}$ as a function of the incident energy. Time-of-flight techniques were employed where necessary to select fissions induced by neutrons of the desired energy. Values at nineteen energies in the range thermal to 8 MeV were determined; absolute values are based on $\overline{\nu }$ (${\mathrm{Cf}}^{252}$, spontaneous) = 3.782±0.024, relative errors are about 1%. The variation of $\overline{\nu }$ with energy cannot be represented by a single straight line. A satisfactory fit to the experimental points is given either by the relationship $\overline{\nu }\mathrm{}\left(E\right)=(2.423\mathrm{}\pm 0.008)+(0.088\pm 0.008)E+(0.0088\pm 0.0011)E^2$ or by two straight lines $\overline{\nu }\mathrm{}\left(E\right)=(2.418\mathrm{}\pm 0.008)+(0.109\pm 0.006)E \mathrm{from}0\mathrm{to}3\mathrm{MeV}$ $\overline{\nu }\mathrm{}\left(E\right)=(2.200\mathrm{}\pm 0.023)+(0.181\pm 0.005)E \mathrm{from}3\mathrm{to}8\mathrm{MeV}$