5.3 Neutron interactions and moderation

Neutron interactions are crucial in understanding radiation's behavior with matter. From elastic scattering to absorption, these processes shape how neutrons move and react. Knowing these interactions helps us grasp nuclear physics and its applications.

Neutron moderation is key in controlling nuclear reactions. By slowing down fast neutrons, moderators like water or graphite help maintain chain reactions in reactors. This process is vital for nuclear power and research applications.

Neutron Scattering

Elastic and Inelastic Scattering

Top images from around the web for Elastic and Inelastic Scattering

• 

  A quasielastic and inelastic neutron scattering study of the alkaline and alkaline-earth ... View original

  Is this image relevant?

• 

  Energy dependent cross sections for neutrons - Physics Stack Exchange View original

  Is this image relevant?

• 

  A quasielastic and inelastic neutron scattering study of the alkaline and alkaline-earth ... View original

  Is this image relevant?

• 

  A quasielastic and inelastic neutron scattering study of the alkaline and alkaline-earth ... View original

  Is this image relevant?

• 

  Energy dependent cross sections for neutrons - Physics Stack Exchange View original

  Is this image relevant?

1 of 3

Top images from around the web for Elastic and Inelastic Scattering

• 

  A quasielastic and inelastic neutron scattering study of the alkaline and alkaline-earth ... View original

  Is this image relevant?

• 

  Energy dependent cross sections for neutrons - Physics Stack Exchange View original

  Is this image relevant?

• 

  A quasielastic and inelastic neutron scattering study of the alkaline and alkaline-earth ... View original

  Is this image relevant?

• 

  A quasielastic and inelastic neutron scattering study of the alkaline and alkaline-earth ... View original

  Is this image relevant?

• 

  Energy dependent cross sections for neutrons - Physics Stack Exchange View original

  Is this image relevant?

1 of 3

• Elastic scattering occurs when a neutron collides with a nucleus and bounces off without losing energy
• The direction of the neutron changes, but its speed remains the same after an elastic scattering event
• Inelastic scattering happens when a neutron collides with a nucleus and transfers some of its energy to the nucleus
• In inelastic scattering, the neutron's speed decreases, and the nucleus is left in an excited state
• The probability of scattering depends on the scattering cross-section, which varies with the target nucleus and neutron energy
• Scattering cross-sections are measured in units of barns (1 barn = $10^{-24} cm^2$)
• Examples of materials with high scattering cross-sections include hydrogen, deuterium, and graphite

Scattering Applications

• Neutron scattering is used in various applications, such as neutron diffraction and neutron spectroscopy
• Neutron diffraction helps determine the atomic and magnetic structure of materials by analyzing the scattered neutron patterns
• Neutron spectroscopy investigates the dynamics and excitations in materials by measuring the energy and momentum transfer during inelastic scattering
• Scattering is also important in nuclear reactor design, as it affects neutron transport and moderation

Neutron Absorption

Capture and Fission Reactions

• Neutron absorption occurs when a neutron is captured by a nucleus, forming a heavier isotope
• Neutron capture can lead to the formation of stable or radioactive isotopes, depending on the target nucleus
• Some heavy nuclei, such as uranium-235 and plutonium-239, can undergo fission upon neutron absorption
• In fission reactions, the heavy nucleus splits into two lighter nuclei, releasing energy and additional neutrons
• The released neutrons can trigger further fission reactions, leading to a chain reaction if the conditions are suitable

Thermal Neutron Absorption

• Thermal neutrons have energies around 0.025 eV, corresponding to the average energy of atoms at room temperature
• Many nuclei have higher absorption cross-sections for thermal neutrons compared to fast neutrons
• Examples of strong thermal neutron absorbers include boron-10, cadmium-113, and gadolinium-157
• Thermal neutron absorption is crucial in nuclear reactor control, as control rods containing strong absorbers are used to regulate the reactor's power output
• Neutron absorption is also used in neutron activation analysis, a technique for determining the elemental composition of materials

Neutron Moderation

Moderator Materials

• Neutron moderators are materials that slow down fast neutrons through multiple collisions, reducing their energy to the thermal range
• Effective moderators have low atomic mass, high scattering cross-sections, and low absorption cross-sections
• Common moderator materials include light water (H2O), heavy water (D2O), graphite, and beryllium
• The choice of moderator depends on factors such as neutron economy, reactor design, and desired neutron spectrum

Neutron Flux and Moderation

• Neutron flux is the number of neutrons passing through a unit area per unit time (neutrons/cm^2/s)
• Fast neutrons have energies above 0.1 MeV and are produced by fission reactions or accelerators
• Moderation reduces the energy of fast neutrons through elastic scattering collisions with the moderator nuclei
• The moderating power of a material depends on its scattering cross-section and the average logarithmic energy decrement per collision
• A higher neutron flux in the thermal energy range is desirable for many applications, such as neutron scattering experiments and nuclear power generation
• The design of a moderator system must balance the need for efficient moderation with minimizing neutron absorption losses