AskDefine | Define barns

User Contributed Dictionary



  1. Plural of barn

Extensive Definition

A barn (symbol b) is a unit of area. While the barn is not an SI unit, it is accepted (although discouraged) for use with the SI. Originally used in nuclear physics for expressing the cross sectional area of nuclei and nuclear reactions, today it is used in all fields of high energy physics to express the cross sections of any scattering process. A barn is approximately equal to the cross sectional area of a uranium nucleus.


1 barn (b) = 10−28 square metres (m²) = 100 square femtometres (fm²) = 10−24 square centimetres (cm²) (Most commonly used)

Commonly used prefixed versions

  • 1 millibarn (mb) = 10−31 m².
  • 1 microbarn (μb) = 10−34 m².
  • 1 nanobarn (nb) = 10−37 m².
  • 1 picobarn (pb) = 10−40 m².
  • 1 femtobarn (fb) = 10−43 m².


Calculated cross sections are often found in units of (GeV)−2(c)2 = 0.3894 mb.


The etymology is clearly whimsical and jocular—the unit is said to be "as big as a barn" compared to the typical cross sections for nuclear reactions. During wartime research on the atomic bomb, American physicists who were deflecting neutrons off uranium nuclei, (similar to Rutherford scattering) described the uranium nucleus as “big as a barn.” Physicists working on the project adopted the name barn for a unit equal to 10-24 square centimetres, about the size of a uranium nucleus. Initially they hoped the American slang name would obscure any reference to the study of nuclear structure; eventually, the word became a standard unit in particle physics.
The origin of the barn is described in the February 2006 issue of symmetry magazine.


The shed was devised to described incredibly small areas. One shed is 10m, or 10b. A shed is to the barn what a barn is to the square meter.


External links

barns in Bosnian: Barn
barns in Bulgarian: Барн
barns in Danish: Barn (enhed)
barns in German: Barn
barns in Spanish: Barn
barns in French: Barn
barns in Korean: 바안
barns in Croatian: Barn
barns in Italian: Barn
barns in Japanese: バーン (単位)
barns in Norwegian: Barn (måleenhet)
barns in Polish: Barn
barns in Portuguese: Barn
barns in Russian: Барн
barns in Slovenian: Barn
barns in Serbian: Барн
barns in Swedish: Barn (enhet)
barns in Ukrainian: Барн
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