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Barn (unit)
From Wikipedia, the free encyclopedia
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 |
1 barn (b) = 10−28 square metres (m²) = 100 square femtometres (fm²) = 10−24 square centimetres (cm²) (Most commonly used)
- 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.
Calculated cross sections are often found in units of (GeV)−2(ℏc)2 = 0.3894 mb.
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 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.
- 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
| SI derived unit square metre, from base unit metre | Orders of magnitude for area | Conversion of units for area | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 E-30 m² = 1 fm² | 1 E-24 m² = 1 pm² | 1 E-18 m² = 1 nm² | 1 E-12 m² = 1 µm² | 1 E-6 m² = 1 mm² | 1 E0 m² = 1 m² | 1 E+6 m² = 1 km² | 1 E+12 m² = 1 Mm² | 1 E+18 m² = 1 Gm² | 1 E+24 m² = 1 Tm² | 1 E+30 m² = 1 Pm² | ||||
| 10 fm² | 10 pm² | 10 nm² | 10 µm² | 10 mm² | 10 m² | 10 km² | 10 Mm² | 10 Gm² | 10 Tm² | 1 E+36 m² = 1 Em² | ||||
| 100 fm² | 100 pm² | 100 nm² | 100 µm² | 100 mm² | = | 1 cm² | 100 m² | = | 1 dam² aka a | 100 km² | 100 Mm² | 100 Gm² | 100 Tm² | 1 E+40 m² |
| 1,000 fm² | 1,000 pm² | 1,000 nm² | 1,000 µm² | 1,000 mm² | = | 10 cm² | 1,000 m² | = | 10 dam² aka a | 1,000 km² | 1,000 Mm² | 1,000 Gm² | 1,000 Tm² | 1 E+41 m² |
| 10,000 fm² | 10,000 pm² | 10,000 nm² | 10,000 µm² | 10,000 mm² | = | 1 dm² | 10,000 m² | = | 1 hm² aka ha | 10,000 km² | 10,000 Mm² | 10,000 Gm² | 10,000 Tm² | 1 E+42 m² = 1 Zm² |
| 100,000 fm² | 100,000 pm² | 100,000 nm² | 100,000 µm² | 100,000 mm² | = | 10 dm² | 100,000 m² | = | 10 hm² aka ha | 100,000 km² | 100,000 Mm² | 100,000 Gm² | 100,000 Tm² | 1 E+48 m² = 1 Ym² |