Neutron Cross Section
Overview
Neutron cross section is a measure of the probability that a neutron will interact with an atomic nucleus. It’s expressed in units called “barns” (1 barn = 10⁻²⁴ cm²), representing the effective “target area” a nucleus presents to an incoming neutron—a concept that determines whether nuclear reactors generate power or nuclear weapons explode.
Types of Interactions
Absorption Cross Section
- Radiative capture: Neutron absorbed, gamma ray emitted
- Fission: Neutron absorbed, nucleus splits
- Charged particle emission: Neutron absorbed, proton or alpha particle emitted
Scattering Cross Section
- Elastic scattering: Neutron bounces off, no energy loss
- Inelastic scattering: Neutron bounces off, some energy lost
Energy Dependence
Cross sections vary dramatically with neutron energy:
Thermal Neutrons (~0.025 eV)
- U-235 fission: ~580 barns
- Pu-239 fission: ~750 barns
- U-238 absorption: ~2.7 barns
Fast Neutrons (~1 MeV)
- U-235 fission: ~1.2 barns
- Pu-239 fission: ~1.8 barns
- U-238 fission: ~0.5 barns
Resonance Peaks
Many isotopes show resonance peaks where cross sections increase dramatically at specific energies:
- U-238: Major resonance at 6.7 eV
- Pu-240: Strong resonance at 1.0 eV
- Xe-135: Enormous thermal absorption (~2.6 million barns)
Practical Applications
Reactor Design
- Fuel enrichment: Optimizing fissile isotope concentration
- Control rods: Materials with high absorption cross sections
- Moderator choice: Balancing absorption and scattering
Nuclear Weapons
- Criticality calculations: Determining critical mass
- Neutron reflectors: Materials that scatter neutrons back
- Tamper materials: High-density materials to compress core
Key Materials
High Fission Cross Section
- U-235: Excellent thermal fission cross section
- Pu-239: High fission cross section at all energies
- U-233: Superior thermal fission properties
High Absorption Cross Section
- Boron-10: 3,840 barns (thermal)
- Cadmium: 2,520 barns (thermal)
- Gadolinium: 49,000 barns (thermal)
Good Moderators (High Scattering, Low Absorption)
- Water: Good scattering, moderate absorption
- Heavy water: Excellent scattering, very low absorption
- Graphite: Good scattering, low absorption
Measurement and Calculation
Cross sections are determined through:
- Experimental measurement: Neutron beam experiments
- Theoretical calculation: Nuclear physics models
- Evaluated data libraries: ENDF/B, JEFF, JENDL
Relevance to Nuclear Weapons
Understanding neutron cross sections is crucial for:
- Critical mass calculations: Determining weapon requirements
- Neutron multiplication: Optimizing chain reactions
- Material selection: Choosing appropriate isotopes and reflectors
- Weapon efficiency: Maximizing energy release
Sources
Authoritative Sources:
- National Nuclear Data Center (NNDC) - Nuclear data and cross sections
- International Atomic Energy Agency (IAEA) - Nuclear physics standards
- Los Alamos National Laboratory - Nuclear weapons physics research
- Oak Ridge National Laboratory - Nuclear data evaluation
- Nuclear Regulatory Commission - Reactor physics and safety