The ratio $\alpha =\frac{{\sigma }_c}{{\sigma }_f}$, where ${\sigma }_c$ is the neutron capture cross section and ${\sigma }_f$ the neutron-induced fission cross section, has been measured for ${\mathrm{U}}^{235}$ as a function of neutron energy. A pulsed and collimated neutron beam is passed through a ${\mathrm{U}}^{235}$ sample at the center of a large liquid scintillator. Captures and fissions are detected by means of their prompt gamma rays; elastic and inelastic scattering events are ignored because of smaller pulse heights. Fissions are distinguished from captures by means of delayed pulses from the capture of thermalized fission neutrons. It is found that in the neutron energy range $E_n=0.175\mathrm{} \mathrm{to} 1.0$ Mev the value of $\alpha$ is given approximately by $\alpha =0.190-0.116\mathrm{}{E}_n$. The accuracy of the determination of $\alpha$ is 10 to 15% in terms of the standard deviation of individual points.

• Received 13 September 1957

DOI:https://doi.org/10.1103/PhysRev.109.144