Sections for Neutron Reactions

In a nuclear reactor we have to deal with neutrons covering a wide range of kinetic energies, from about 10 MeV down to 0.001 eV. The neutrons are in continuous collision with the nuclei of the fuel and other materials present in the reactor. As the result of the collision of a neutron with an atomic nucleus, a variety of possible reactions may take place. We may illustrate by considering the collision of a neutron with a nucleus of when the possible reactions are as follows (see Fig. 1.8)  

It is possible that, after ejection of the neutron from the compound nucleus, the residual nucleus, in this case AP, may be left in an excited state, rather than in the ground state from which it was raised by the neutron absorption. The excited nucleus, denoted by AP, will then decay by y-ray emission. Since the y-ray energy is obtained at the expense of the scattered neutron, the latter will emerge with a lower kinetic energy than if the collision had been elastic. This process is known as resonance inelastic scattering (n, n ), and is the second listed above. 

Example 1.4. Calculate the energy release in the reaction above, using the masses given in Table A.3. 

Subtracting the two latter masses from the sum of the masses of the boron nucleus and the neutron gives a mass loss of 

For all but light nuclei, the potential barrier is high enough that the compound nucleus formed by the capture of a slow neutron nearly always de-excites by the emission of a y ray or of an elastically scattered neutron. 

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