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A barn (symbol b) is a unit of area. 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 10⁻²⁸ m² or 100 square femtometers (fm²) and is approximately the cross sectional area of a uranium nucleus. 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. While the barn is not an SI unit, it is accepted for use with the SI due to its continued use in particle physics.^[1] It is one of the very few units which are accepted to be used with SI units, and one of the most recent units to have been established (cf. the knot and the bar, other non-SI units acceptable in limited circumstances).^[2]

Contents 1 Commonly used prefixed versions 2 Conversions 3 Origin 4 Usage example 5 Related units 6 See also 7 References 8 External links

[edit] Commonly used prefixed versions

Conversion to SI units

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 ħ²c²/GeV² (approximately 0.3894 mb).

[edit] Origin

The etymology of the unit barn 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] Usage example

As a simplified example, if a beamline runs for 8 hours (28,800 seconds) at an instantaneous luminosity of 300 × 10³⁰ cm⁻²s⁻¹ = 300 μb⁻¹s⁻¹, then it will gather data totaling an integrated luminosity of 8,640,000 μb⁻¹ = 8.64 pb⁻¹ during this period.

[edit] Related units

The "inverse femtobarn" (fb⁻¹) is a measurement of particle collision events per femtobarn. 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 multiplying the integrated luminosity by the sum of the cross-section for those collision processes. 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

• Orders of magnitude (area)

[edit] References

[edit] External links

• IUPAC citation for this usage of "barn"