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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.

Contents

Definition

Commonly used prefixed versions

Conversions

Origin

References

External links

Definition

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 picobarn ('pb') = 10⁻⁴⁰ m².

★ 1 femtobarn ('fb') = 10⁻⁴³ m².

Conversions

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

Origin

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. During wartime research on the atomic bomb, American physicists who were bouncing neutrons off uranium nuclei 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 centimeters, 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. ^[1]
The origin of the ''barn'' is described in the February 2006 issue of ''symmetry'' magazine.

References

1. http://ed.fnal.gov/painless/pdfs/cross.pdf

External links

★ 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''