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. It is used in nuclear physics for expressing the cross sectional area of nuclei and nuclear reactions. A barn is approximately equal to the area of a uranium nucleus.

Definition

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1 barn (b) = 10⁻²⁸ square metres (m²) = 100 square femtometres (fm²)

Commonly used prefixed versions

• 1 millibarn (mb) = 10⁻³¹ m².
• 1 picobarn (pb) = 10⁻⁴⁰ m².
• 1 femtobarn (fb) = 10⁻⁴³ m².

Origin

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The etymology is clearly whimsical—the unit is said to be "as big as a barn" compared to the typical cross sections for nuclear reactions. It may have been thought as beneficial to use the term to obscure discussions of weapons research during World War II.

External links

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• Official SI website: Table 8. Other non-SI units currently accepted for use with the International System Their use is not encouraged.
• IUPAC citation for this usage of "barn"
• Article on cross-section

Units of area

Barn | Barn | Barn | Barn | バーン (単位) | Barn | Барн | Barn (enhet)