| barn | |
|---|---|
| Unit of | area |
| Symbol | b |
| Named after | the broad side of abarn |
| Conversions | |
| 1 bin ... | ... is equal to ... |
| SI base units | 10−28 m2 |
| square fm | 100 fm2 |
Abarn (symbol:b) is a non-SImetric unit ofarea equal to10−28 m2 (100 fm2). This is equivalent to a square that is10−14 m (10 fm) each side, or a circle of diameter approximately1.128×10−14 m (11.28 fm).
Originally used innuclear physics for expressing thecross sectional area ofnuclei andnuclear reactions, today it is also used in all fields ofhigh-energy physics to express the cross sections of anyscattering process, and is best understood as a measure of the probability of interaction between small particles. A barn is approximately the cross-sectional area of auranium nucleus. The barn is also the unit of area used innuclear quadrupole resonance andnuclear magnetic resonance to quantify the interaction of a nucleus with anelectric field gradient. While the barn never was anSI unit, theSI standards body acknowledged it in the 8th SI Brochure (superseded in 2019) due to its use inparticle physics.[1]
DuringManhattan Project research on theatomic bomb duringWorld War II, American physicistsMarshall Holloway andCharles P. Baker were working atPurdue University on a project using a particle accelerator to measure thecross sections of certain nuclear reactions. According to an account of theirs from a couple years later, they were dining in a cafeteria in December 1942 and discussing their work. They "lamented" that there was no name for the unit of cross section and challenged themselves to develop one. They initially triedeponyms, names of "some great men closely associated with the field" that they could name the unit after, but struggled to find one that was appropriate. They considered "Oppenheimer" too long (in retrospect, they considered an "Oppy" to perhaps have been allowable), and considered "Bethe" to be too easily confused with the commonly used Greek letterbeta. They then considered naming it afterJohn Manley, another scientist associated with their work, but considered "Manley" too long and "John" too closely associated withtoilets. But this latter association, combined with the "rural background" of one of the scientists, suggested to them the term "barn", which also worked because the unit was "really as big as a barn". According to the authors, the first published use of the term was in a (secret) Los Alamos report from late June 1943, on which the two originators were co-authors.[2]
The unit symbol for the barn (b) is also the IEEE standard symbol forbit, and both are commonly used with SI prefixes, which may give rise to ambiguity.
| Submultiples | Multiples | ||||
|---|---|---|---|---|---|
| Value | SI symbol | Name | Value | SI symbol | Name |
| 10−1 b | db | decibarn | 101 b | dab | decabarn |
| 10−2 b | cb | centibarn | 102 b | hb | hectobarn |
| 10−3 b | mb | millibarn | 103 b | kb | kilobarn |
| 10−6 b | μb | microbarn | 106 b | Mb | megabarn |
| 10−9 b | nb | nanobarn | 109 b | Gb | gigabarn |
| 10−12 b | pb | picobarn | 1012 b | Tb | terabarn |
| 10−15 b | fb | femtobarn | 1015 b | Pb | petabarn |
| 10−18 b | ab | attobarn | 1018 b | Eb | exabarn |
| 10−21 b | zb | zeptobarn | 1021 b | Zb | zettabarn |
| 10−24 b | yb | yoctobarn | 1024 b | Yb | yottabarn |
| 10−27 b | rb | rontobarn | 1027 b | Rb | ronnabarn |
| 10−30 b | qb | quectobarn | 1030 b | Qb | quettabarn |
Calculated cross sections may be given in terms of inverse squared gigaelectronvolts (GeV−2), via the conversionħ2c2/GeV2 =0.3894 mb =38940 am2.
Innatural units (whereħ =c = 1), this simplifies toGeV−2 =0.3894 mb =38940 am2.
| barn | GeV−2 |
|---|---|
| 1 mb | 2.56819 GeV−2 |
| 1 pb | 2.56819×10−9 GeV−2 |
| 0.389379 mb | 1 GeV−2 |
| 0.389379 pb | 10−9 GeV−2 |
In the SI, one can use a unit such as the square femtometer (fm2). The most common prefixed form of the barn is the femtobarn, which is equal to a tenth of a square zeptometer. Many scientific papers discussing high-energy physics mention quantities of that are a fraction of a femtobarn.
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The inverse femtobarn (fb−1) is the unit typically used to measure the number ofparticle collision events per femtobarn oftarget cross-section, and is the conventional unit for time-integratedluminosity. Thus, if a detector has accumulated100 fb−1 of integrated luminosity, one expects to find 100 events per femtobarn of cross-section within these data.
Consider aparticle accelerator where two streams of particles, with cross-sectional areas measured in femtobarns, are directed to collide over a period of time. The total number of collisions will be 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 nine months). Inverse femtobarns are often quoted as an indication ofparticle collider productivity.[4][5]
Fermilab produced10 fb−1 in the first decade of the 21st century.[6] Fermilab'sTevatron took about 4 years to reach1 fb−1 in 2005, while two ofCERN'sLHC experiments,ATLAS andCMS, reached over5 fb−1 of proton–proton data in 2011 alone.[7][8][9][10][11][12] In April 2012, the LHC achieved the collision energy of8 TeV with a luminosity peak of 6760 inverse microbarns per second; by May 2012, the LHC delivered 1 inverse femtobarn of data per week to each detector collaboration. A record of over 23 fb−1 was achieved during 2012.[13] As of November 2016, the LHC had achieved40 fb−1 over that year, significantly exceeding the stated goal of25 fb−1.[14] In total, the second run of the LHC has delivered around150 fb−1 to both ATLAS and CMS in 2015–2018.[15]
As a simplified example, if abeamline runs for 8 hours (28 800 seconds) at an instantaneous luminosity of300×1030 cm−2⋅s−1 =300 μb−1⋅s−1, then it will gather data totaling an integrated luminosity of8640000 μb−1 =8.64 pb−1 =0.00864 fb−1 during this period. If this is multiplied by the cross-section, then a dimensionless number is obtained equal to the number of expected scattering events.
