Neutron continued energy dependence

Thus, Ij. is the contribution to the effective group cross section for group / due to process x from all resonances of sequence (i). It is shown in Section IV that, if < ( ), the energy-dependent neutron flux, is known exactly, then, as is well known, this definition of the multigroup cross sections leads to exactly the same value of the multiplication constant as the continuous energy diffusion theory. 

The fission neutrons produced in nuclear reactors have a continuous kinetic-energy spectrum, mostly in the range of 1-10 MeV. Since (n, y) reactions are of more widespread analytical use, fission neutrons must be slowed to thermal energies by passing them through HjO, D2O, or graphite, which act as moderators. Depending on the type of nuclear reactor and the irradiation position in the reactor, the neutron spectrum may vary widely. Therefore, both (n, y) and threshold reactions can occur in samples placed in nuclear reactors. Threshold reactions may produce interferences, of which the experimenter should be aware. 

Graham s law has practical application in the preparation of fuel rods for nuclear fission reactors. Such reactors depend on the fact that the uranium-235 nucleus undergoes fission (splits) when bombarded with neutrons. When the nucleus splits, several neutrons are emitted and a large amount of energy is liberated. These neutrons bombard more uranium-235 nuclei, and the process continues with the evolution of more energy. However, natural uranium consists of 99.27% uranium-238 (which does not undergo fission) and only 0.72% uranium-235 (which does undergo fission). A uranium fuel rod must contain about 3% uranium-235 to sustain the nuclear reaction. 

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