2.3 Interaction of Neutrons

The Primary Mechanism: Nuclear Interactions

Since neutrons have no charge, they are not subject to the Coulomb forces that cause charged particles and photons to interact with atomic electrons. Instead, a neutron must come very close to an atomic nucleus to interact, making its path through matter much longer on average.

The primary ways a neutron interacts with a nucleus are through scattering and absorption.

Scattering

In a scattering event, a neutron collides with a nucleus, and its direction and/or energy are changed. The nucleus remains in its original state.

• Elastic Scattering: The kinetic energy of the neutron and the nucleus is conserved in the collision. This is the primary mechanism for slowing down high-energy neutrons, a process known as **moderation**. It is most effective when the neutron collides with a nucleus of similar mass (e.g., hydrogen).
• Inelastic Scattering: The neutron collides with a nucleus and transfers some of its energy to it, leaving the nucleus in an excited state. The nucleus later de-excites by emitting a gamma ray. This process is only possible if the neutron's energy is higher than the lowest excited state of the nucleus.

Absorption

In an absorption event, the nucleus captures the neutron, resulting in a nuclear reaction. The compound nucleus formed is usually unstable and quickly decays by emitting other particles or photons.

The probability of both scattering and absorption is quantified by the **cross-section** (\(\sigma\)), which is highly dependent on the neutron's energy. In general, the absorption cross-section for many materials increases dramatically as the neutron energy decreases (e.g., thermal neutrons).