Barn (unit)
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It has been suggested that Shed (physics) be merged into this article or section. (Discuss) |
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.[1][2] 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 defined as being 10−28 m2, and corresponds to approximately the cross sectional area of a uranium nucleus. To phrase it in terms of the closest SI unit, a barn is 100 square femtometers (fm2). The barn is also the unit of area used in nuclear quadrupole resonance and nuclear magnetic resonance to quantify the interaction of a nucleus with an electric field gradient.
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[edit] Commonly used prefixed versions
| Unit | Symbol | m2 | cm2 |
|---|---|---|---|
| barn | b | 10−28 | 10−24 |
| millibarn | mb | 10−31 | 10−27 |
| microbarn | μb | 10−34 | 10−30 |
| nanobarn | nb | 10−37 | 10−33 |
| picobarn | pb | 10−40 | 10−36 |
| femtobarn | fb | 10−43 | 10−39 |
[edit] Conversions
Calculated cross sections are often written in units of ℏ2 c2/GeV2 (approximately 0.3894 mb).
[edit] Origin
The etymology is clearly whimsical and jocular. 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.[3][4]
[edit] Related units
The "inverse femtobarn" (fb−1) is a measurement of particle collision events per unit of time. This unusual method of expressing a ratio comes from the means by which particle collisions are detected. Over a period of time, two streams of particles with a cross-sectional area, measured in femtobarns, are directed to collide. The total number of collisions is directly proportional to the luminosity of the collisions measured over this time. Therefore, the collision count can be calculated by dividing the integrated luminosity by the sum of the cross-sectional area of all the particles. This count is then expressed as inverse femtobarns for the time period (e.g., 100 fb−1 in 9 months). Inverse femtobarns are often quoted as an indication of particle collider effectiveness.[5][6]
The "shed" was devised to describe incredibly small areas, far tinier than a barn. One shed is 10−52 m2, or 10−24 b.[7]
[edit] See also
[edit] References
- ^ "Chapter 4.1: Non-SI units accepted for use with the SI, and units based on fundamental constants". SI brochure (8th edition). International Bureau of Weights and Measures (BIPM). May 2006. http://www.bipm.org/en/si/si_brochure/chapter4/4-1.html. Retrieved on 2009-03-13.
- ^ "Table 8. Other non-SI units". SI brochure (8th edition). BIPM. May 2006. http://www.bipm.org/en/si/si_brochure/chapter4/table8.html. Retrieved on 2009-03-13.
- ^ Perricone, Mike (February 2006). "Signal to Background". Symmetry 03 (01). ISSN 1931-8367. http://www.symmetrymagazine.org/cms/?pid=1000258. Retrieved on 2009-03-13.
- ^ Wackeroth, Doreen; Leila Belkora. "Cross Section" (PDF). High Energy Physics Made Painless. Fermilab Science Education Office. http://ed.fnal.gov/painless/pdfs/cross.pdf. Retrieved on 2009-03-13.
- ^ Metropolis, Kate (July 21, 2004). "Understanding luminosity through 'barn', a unit that helps physicists count particle events". Stanford News Service. http://news-service.stanford.edu/news/2004/july21/femtobarn-721.html. Retrieved on 2009-03-13.
- ^ Inman, Mason; Emily Ball (April 16, 2004). "PEP-II's luminous life". SLAC National Accelerator Laboratory. http://www2.slac.stanford.edu/tip/special/PEP-II-04-16-04.htm. Retrieved on 2009-03-13.
- ^ Rowlett, Russ. "Units: S". How Many? A Dictionary of Units of Measurement. University of North Carolina at Chapel Hill. http://www.unc.edu/~rowlett/units/dictS.html. Retrieved on 2009-03-13.
