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A “barn” is a unit used in nuclear physics to measure the effective cross-sectional area of atomic nuclei. Here’s a detailed explanation of its origin and usage:

Definition and Origin:

  1. Unit of Area:
  • A barn (symbol: b) is defined as \( 10^{-28} \) square meters \(( 10^{-28} \, \text{m}^2 )\).
  • It is used to describe the likelihood of a nuclear reaction occurring when atomic particles or radiation interact with atomic nuclei.
  1. Origin of the Term:
  • The term “barn” originated during the development of nuclear physics in the 1940s.
  • Physicists humorously coined the term based on the phrase “as big as a barn door,” emphasizing that to a subatomic particle (like a neutron or proton), a target with a cross-sectional area of one barn is so large that it’s virtually impossible to miss.
  1. Relative Size:
  • In nuclear physics, interactions between particles and nuclei are probabilistic. A larger cross-sectional area (measured in barns) indicates a higher likelihood of interaction or collision between the particle and the nucleus.

Usage and Applications:

  1. Nuclear Reactions:
  • Barns are used extensively in nuclear reactions and particle interactions.
  • They quantify the probability of a particle being absorbed, scattered, or otherwise interacting with a target nucleus.
  1. Cross-Sectional Calculations:
  • Cross-sections (measured in barns) are calculated based on experimental data and theoretical models to predict and analyze nuclear reactions and particle behavior.
  1. Subdivisions:
  • For smaller interactions, fractions of a barn (such as millibarns, microbarns) are commonly used to describe smaller cross-sectional areas.

Practical Examples:

  1. Neutron Capture:
  • In neutron physics, the probability of a neutron being captured by a nucleus is often expressed in barns.
  1. Particle Collisions:
  • High-energy physics experiments and particle accelerators use barns to describe the likelihood of particle collisions with target materials.


A barn is a unit of measurement in nuclear physics that quantifies the effective cross-sectional area of atomic nuclei for particle interactions. Its humorous origin highlights its utility in describing very small areas relative to the size of atomic particles. Barns play a crucial role in understanding and predicting nuclear reactions, particle interactions, and the behavior of atomic nuclei in various experimental and theoretical contexts within the field of nuclear physics.


  • Cross-section
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