Barn (unit)

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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 1 Definition 1.1 Commonly used prefixed versions 2 Conversions 3 Origin 4 External links

[edit] Definition

1 barn (b) = 10⁻²⁸ square metres (m²) = 100 square femtometres (fm²) = 10⁻²⁴ square centimeters (cm²) (Most commonly used)

[edit] Commonly used prefixed versions

• 1 millibarn (mb) = 10⁻³¹ m².
• 1 picobarn (pb) = 10⁻⁴⁰ m².
• 1 femtobarn (fb) = 10⁻⁴³ m².

[edit] Conversions

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

[edit] 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. It may have been thought as beneficial to use the term to obscure discussions of weapons research during World War II.

The origin of the barn is described in the February 2006 issue of symmetry magazine.

[edit] 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

SI derived unit square metre, from base unit metre						Orders of magnitude for area						Conversion of units for area
1 fm²	1 pm²	1 nm²	1 µm²	1 mm²			1 m²			1 km²	1 Mm²	1 Gm²	1 Tm²	1 E+30 m²	1 E+36 m²
10 fm²	10 pm²	10 nm²	10 µm²	10 mm²			10 m²			10 km²	10 Mm²	10 Gm²	10 Tm²
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
1,000 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
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
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+41 m²