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fast neutron
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Diagram illustrating the moderated, controlled fission of uranium-235, showing uranium nuclei splitting after absorbing slow neutrons, releasing fast neutrons, and how moderators slow neutrons to sustain a controlled chain reaction.
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fast neutron

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Diagram illustrating the moderated, controlled fission of uranium-235, showing uranium nuclei splitting after absorbing slow neutrons, releasing fast neutrons, and how moderators slow neutrons to sustain a controlled chain reaction.
chain reaction in a nuclear reactor at a critical state Slow neutrons strike nuclei of uranium-235, causing the nuclei to fission, or split, and release fast neutrons. The fast neutrons are absorbed or slowed by the nuclei of a graphite moderator, which allows just enough slow neutrons to continue the fission chain reaction at a constant rate.

Fast neutrons are energetic neutrons with kinetic energy above approximately 1 keV. These neutrons are produced during nuclear fission and have an average energy of about two million electron volts (MeV), significantly higher than atoms at room temperature.12

Unlike charged particles, fast neutrons do not interact continuously with matter and can travel considerable distances before interacting with atomic nuclei. When they do interact, it's primarily through elastic scattering, where they can transfer a significant portion of their energy to the struck nucleus, creating a recoil nucleus. Hydrogen is often used as a target nucleus because a fast neutron can transfer its entire energy to a hydrogen nucleus in a single collision.1

Fast neutrons are crucial in nuclear reactors, particularly in fast reactors, which operate without moderators. These reactors require higher concentrations of fissile material but are more efficient at converting fertile material into fissile material, potentially acting as breeder reactors.2

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6 Britannica Sources
  1. nuclear reactor - Thermal , intermediate, and fast reactors
  2. thermal neutron
  3. slow neutron
  4. nuclear fission - Uses of fission reactors and fission products
  5. radiation measurement - Fast neutrons
  6. radiation - Neutrons

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