138 Chapter 2. Interaction of Radiation with Matter
A.1 ElasticScattering........................
Elastic scattering is the principal mode of interaction of neutrons with atomic nuclei.
In this process the target nucleus remains in the same state after interaction. The
reaction is written asA(n, n)Aor
n+Xpn+p→n+Xnp+p. (2.6.1)The elastic scattering process of neutrons with nuclei can occur in two different
modes: potential elastic and resonance elastic. Potential elastic scattering refers to
the process in which the neutron is acted on by the short range nuclear forces of
the nucleus and as a result scatters off of it without touching the particles inside.
In the resonance mode, a neutron with the right amount of energy is absorbed by
the nucleus with the subsequent emission of another neutron such that the kinetic
energy is conserved.
The cross section for elastic cross section for uranium-238 as computed from
different models is shown in Fig.2.6.1 (21).
Figure 2.6.1: Neutron elastic cross section for uranium-238 (21).A.2 InelasticScattering.......................
Unlike elastic scattering, the inelastic scattering leaves the target nucleus in an
excited state. The reaction is written asA(n, n)A∗or
n+Xnp+p→n+[
Xpn+p]∗
. (2.6.2)
In such a process the incoming neutron is absorbed by the nucleus forming a
compound nucleus. The compound nucleus is unstable and quickly emits a neutron
of lower kinetic energy. Since it still has some excess energy it goes through one or
moreγ-decays to return to the ground state.