| Property | Thermal Neutrons | Fast Neutrons |
|---|---|---|
| Definition | Neutrons in thermal equilibrium with surroundings (low kinetic energy). | High kinetic energy neutrons emitted from nuclear reactions. |
| Typical Energy Range | ≈ 0.025 eV (at ~300 K) | ≈ 1 MeV – 10 MeV (or higher) |
| Speed | ≈ 2,200 m/s | ≈ 10⁷ m/s |
| Source | Produced when fast neutrons are slowed by a moderator (water, graphite, heavy water). | Produced directly from fission or other nuclear reactions. |
| Interaction with Nuclei | High probability of absorption and inducing fission in certain isotopes (e.g., U-235). | Lower absorption probability; more likely to cause scattering (elastic/inelastic). |
| Use in Reactors | Used in thermal reactors (with moderators) to sustain chain reactions. | Used in fast reactors/breeder reactors (no moderator) for breeding and fast-spectrum reactions. |
| Cross-section (Interaction Probability) | Generally higher absorption and fission cross-sections for many fissile nuclei. | Generally lower absorption cross-sections; scattering dominates. |
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