English

Noun

barns

1. Plural of barn

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.

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 microbarn (μb) = 10−34 m².
• 1 nanobarn (nb) = 10−37 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 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.

Shed

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.

References

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