The probability that a neutron absorbed in a fissile nuclide causes fission is important for each fissile isotope. In terms of cross-sections, this probability is defined as:
σf / (σf + σγ) = 1 / (1 + σγ/σf) = 1 / (1 + α),
where α = σγ/σf is referred to as the capture-to-fission ratio. The capture-to-fission ratio may be used as an indicator of “quality” of fissile isotopes. The lower C/F ratio simply means that an absorption reaction will result in fission rather than radiative capture. The ratio depends strongly on the incident neutron energy. In the fast neutron region, the C/F ratio decreases. It is determined by the steeper decrease in radiative capture cross-section (see chart).
For 235U and 233U, the thermal neutron capture-to-fission ratios are typically lower than fast neutrons (for mean energy of about 100 keV). It must be noted, the neutron flux of most fast reactors tends to peak around 200 keV, but the mean energy is between 100-200 keV depending on certain reactor design.
Further increase in neutron energy causes a decrease in the C/F ratio conversely. This is not the case with 239Pu. For 100 keV neutrons, the C/F ratio is lower than for thermal neutrons. A small capture-to-fission ratio is advantageous for the fissile isotopes (233U, 235U, and 239Pu) because neutrons captured onto them are lost.