First we need to look at the reacti

First we need to look at the reactions that neutrons induce on various isotopes. This will determine the design of our spectroscopy tools and the elements that they can measure.Inelastic reactions involve the internal structure of a target nucleus getting excited via transfer of some of the incident neutron’s original kinetic energy. The neutron can scatter off with reduced energy, or it can be absorbed and cause the ejection of other particles. The excited nucleus immediately de-excites by emitting one or more gamma-rays (energetic photons), of characteristic energies. This process of exciting the nuclear structure of the target is only possible with high-energy (“fast”) neutrons.Neutron capture occurs when the incident neutron is simply absorbed by the target nucleus. This new nucleus+neutron system is also in an excited state, which quickly de-excites by emitting gamma-rays of different characteristic energies. This process can occur at any energy but is much more likely once the neutron has slowed down to the low energies near ambient temperature. We say that these “slow” neutrons have “thermalized”.Finally, elastic scattering is just like one billiard ball bouncing off another one (though possibly a much bigger ball). The neutron scatters off the target with reduced energy, and the target nucleus recoils away, but its nuclear structure has not been excited so it does not emit any gamma rays.