Fast Neutron
Definition and meaning of Fast Neutron in chemistry.
A fast neutron is a free neutron with kinetic energy above roughly 1 megaelectronvolt (MeV), such as those emitted directly during nuclear fission before any slowing down occurs.
In more detail
Fast neutrons are produced directly by nuclear fission and carry far more kinetic energy than thermal neutrons, which have equilibrated with their surroundings at roughly 0.025 eV. Because fast neutrons have a much lower probability of inducing further fission in uranium-235, most power reactors use a moderator (water, heavy water, or graphite) to slow them via elastic collisions. Fast neutrons are also central to fast breeder reactors, where they efficiently convert non-fissile uranium-238 into fissile plutonium-239, and they are used industrially for neutron radiography and materials testing.
Key facts
| Field | Physical Chemistry |
|---|---|
| Typical energy | > 1 MeV (fission neutrons average ~2 MeV) |
| Contrast | Thermal neutron: ~0.025 eV |
| Key application | Fast breeder reactors convert U-238 to Pu-239 |
Fission of uranium-235 releases neutrons with an average energy of about 2 MeV; in a light-water reactor these fast neutrons collide repeatedly with hydrogen nuclei in the water moderator, losing energy until they become thermal neutrons capable of sustaining the chain reaction.
Frequently asked questions
How does a fast neutron differ from a thermal neutron?
A fast neutron has kinetic energy above about 1 MeV, whereas a thermal neutron has been slowed by repeated collisions with a moderator until its energy matches the thermal motion of the surrounding medium, roughly 0.025 eV at room temperature.
Why do most nuclear reactors need to slow fast neutrons?
Uranium-235 is far more likely to undergo fission when struck by a slow (thermal) neutron than a fast one, so moderators like water or graphite are used to reduce fast fission neutrons to thermal energies and sustain an efficient chain reaction.