fast neutron
C2 / Very specializedTechnical / Scientific
Definition
Meaning
A neutron emitted during nuclear fission or other nuclear reactions that has a high kinetic energy, typically above 0.1 MeV (mega-electron volts), and is not easily captured by atomic nuclei.
In nuclear physics and engineering, a neutron that has not been slowed down (or 'thermalized') by a moderator, allowing it to sustain certain types of nuclear chain reactions (e.g., in fast breeder reactors). Its high speed makes it less likely to be absorbed by fissile material like Uranium-235 compared to slow (thermal) neutrons, but it can cause fission in other isotopes like Plutonium-239 or Uranium-238.
Linguistics
Semantic Notes
The term is almost exclusively used within nuclear physics, reactor engineering, and related scientific fields. It is a compound noun where 'fast' denotes high kinetic energy/speed, not temporal quickness. The opposite is a 'slow neutron' or 'thermal neutron'.
Dialectal Variation
British vs American Usage
Differences
No significant lexical differences. Potential minor spelling preferences in related compound terms (e.g., 'fast-neutron reactor' may be hyphenated slightly more often in UK English).
Connotations
Identical technical connotations in both varieties.
Frequency
Used with identical frequency in relevant scientific communities in both the UK and US.
Vocabulary
Collocations
Grammar
Valency Patterns
The [reactor] produces fast neutrons.Fast neutrons induce fission in [Uranium-238].[Material] is bombarded with fast neutrons.Vocabulary
Synonyms
Neutral
Weak
Vocabulary
Antonyms
Usage
Context Usage
Business
Rare, except in the context of the nuclear energy industry, e.g., 'The company specializes in fast neutron reactor technology.'
Academic
Core term in nuclear physics papers, textbooks, and lectures.
Everyday
Extremely rare; would only appear in news coverage of nuclear technology or science documentaries.
Technical
The primary context of use. Essential vocabulary for reactor design, nuclear safety analysis, and particle physics.
Examples
By Part of Speech
adjective
British English
- The fast-neutron cross-section was measured.
- They conducted a fast-neutron irradiation experiment.
American English
- The fast neutron cross section was calculated.
- Fast neutron spectroscopy requires specialized detectors.
Examples
By CEFR Level
- A fast neutron has much more energy than a thermal neutron.
- The design relies on fast neutrons to breed more fissile material from uranium-238.
- Shielding against fast neutrons requires materials with high hydrogen content.
Learning
Memory Aids
Mnemonic
Imagine a neutron in a hurry ('fast') to smash into a nucleus before being caught, unlike its 'slow' cousin who is easily captured.
Conceptual Metaphor
SPEED IS (KINETIC) ENERGY. The adjective 'fast' maps the concept of high speed onto the specific physical property of high kinetic energy.
Watch out
Common Pitfalls
Translation Traps (for Russian speakers)
- Прямой перевод 'быстрый нейтрон' is correct. Trap: Confusing 'fast' with 'rapid' in a temporal sense. The term is purely about particle physics, not speed of process.
Common Mistakes
- Using 'fast neutron' to refer to any rapidly moving particle.
- Confusing it with 'free neutron'.
- Misspelling as 'fast neutron' without the hyphen in compound adjectives (e.g., 'fast-neutron reaction' is often hyphenated).
Practice
Quiz
What is the primary characteristic of a fast neutron?
FAQ
Frequently Asked Questions
Typically above 0.1 MeV (mega-electron volts), though the exact threshold can vary slightly by context.
No, most commercial light-water reactors (LWRs) use thermal (slow) neutrons. Fast neutrons are used in specialized designs like fast breeder reactors.
Fast neutrons can efficiently cause fission in fertile materials like Uranium-238, transforming them into fissile Plutonium-239, thus 'breeding' new fuel.
No. A free neutron is any neutron not bound within an atomic nucleus. It can be fast or slow. 'Fast' describes its energy state, not its bound/unbound status.