The capture-to-fission ratio $\alpha =\frac{{\sigma }_c}{{\sigma }_f}$ of ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ has been determined in the neutron energy range from 10 to 500 keV with an accuracy of typically 8 to 10%. The measurement was carried out on a 3 MV pulsed Van de Graaff accelerator using an $800 l$ liquid scintillator tank for the detection of capture and fission gamma rays. A fission neutron counter in coincidence with the tank served to distinguish between capture and fission events. With the good energy resolution of 2 nsec/m achieved in this experiment, intermediate structure in the capture-to-fission ratio could be resolved up to ∼50 keV neutron energy. Interpreted in terms of the double humped fission barrier, the experimental average level spacing $D_{\mathrm{II}}=1000\mathrm{}\pm 300$ eV led to an energy difference between the first and second well of $E_{\mathrm{II}}=3.26\mathrm{}\pm 0.14$ MeV.

NUCLEAR REACTIONS ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$($n$,$f$), ($n$,$\gamma$), $E=10-500\mathrm{}$ keV; measured $\frac{\sigma (n,\gamma )}{\sigma \mathrm{}(n,f)\mathrm{}}$, deduced average level spacing of class II states and energy difference between first and second well, enriched target.

• Received 18 October 1978

DOI:https://doi.org/10.1103/PhysRevC.20.201