Threshold Cross Sections

With higher incident neutron energies—and as a result higher excitation energies—additional reactions become possible. These we refer to as threshold reactions because the cross section is zero below the threshold energy. Inelastic scattering cross sections exhibit threshold behavior because for such scattering to occur the incident neutron must have enough kinetic energy both to raise the target nucleus to an excited quantum state and to overcome the binding energy and be reemitted. Referring again to the examples of Fig. 2.5 we note that the lowest excited state of a nucleus generally decreases with increasing atomic weight. As a result the threshold for inelastic scattering also decreases with increasing atomic number. For the lighter nuclides, inelastic scattering thresholds are so high that the reaction is insignificant in reactors: The threshold for carbon-12 is 4.8 MeV, whereas that for oxygen-16 is 6.4 MeV. However, for heavier elements the threshold is lower; in uranium-238 is it at 0.04 MeV. Fertile materials, such as uranium-238, also have thresholds above which fission becomes possible; the threshold for uranium-238 fission is approximately 1.0 MeV. Figure 2.8 depicts the threshold cross section for both the inelastic scattering and fission in uranium-238

A third class of threshold reaction that emits neutrons is (n, 2n) in
which the incident neutron ejects two neutrons from a nuclide.
However, the threshold for this reaction is sufficiently high and the
cross section small enough that generally it can be ignored in elementary
treatments of reactor physics.

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