AbstractValues of the ratio of the capture to fission neutron cross section,α, of235U have been determined in the energy range from 200 eV to 15 keV using a lead slowing down time spectrometer. A235U fission chamber was used to determine the fission rate, and a gamma-ray proportional counter determined both the fission and the capture rates. Both detectors were calibrated in a thermal-neutron flux using the well-known thermal energyαvalues. Neutron and gamma-ray self-shielding in the samples and background counting rates have been carefully corrected. The resultingαvalues agree well with recent time-of-flight measurements in the energy range above 0.5 keV. Below 0.5-keV neutron energy, however, large discrepancies were observed. No obvious errors in the experimental method have been found to explain thesev discrepancies.