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 defined as being 10−28 m2, and corresponds to approximately to the cross sectional area of a uranium nucleus. To phrase it in terms of the closest SI unit, a barn is 100 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.
Contents |
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 |
Conversions
Calculated cross sections are often written in units of ℏ2·c2⁄GeV2 (approximately 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. 1
The origin of the barn is described in the February 2006 issue of symmetry magazine.
Shed
The shed was devised to describe incredibly small areas. One shed is 10−52m2, or 10−24b. 2 The shed is to the barn what the barn is to the square centimeter.
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
Wikipedia content modification information:
- This page was last modified on 29 December 2008, at 14:57.
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