- Barn (unit)
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, and is best understood as a measure of the probability of interaction between small particles. A barn is defined as 10−28 m2 (100 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. 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).
Two related units are the outhouse (10−34 m2, or 1 μb) and the shed (10−52 m2, or 1 yb), although these are rarely used in practice.
The etymology of the unit barn is whimsical. During wartime research on the atomic bomb, American physicists at Purdue University 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.
Commonly used prefixed versions
Conversion to SI units Unit Symbol m2 cm2 megabarn Mb 10−22 10−18 kilobarn kb 10−25 10−21 barn b 10−28 10−24 millibarn mb 10−31 10−27 microbarn (or "outhouse") μb 10−34 10−30 nanobarn nb 10−37 10−33 picobarn (or "silo") pb 10−40 10−36 femtobarn fb 10−43 10−39 attobarn ab 10−46 10−42 zeptobarn zb 10−49 10−45 yoctobarn (or "shed") yb 10−52 10−48
SI units with prefix
In SI, one can use units such as square femtometers (fm²).
Conversion from squared prefixed SI units 1 pm² = 10 kb 1 fm² = 10 mb 1 am² = 10 nb 1 zm² = 10 fb 1 ym² = 10 zb
The "inverse femtobarn" (fb−1) is a measurement of particle collision events per femtobarn. One inverse femtobarn is equal to around 70 million million (70 x 1012) collisions. 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 nine months). Inverse femtobarns are often quoted as an indication of particle collider effectiveness. Fermilab has produced 10 fb−1 in the last decade. Fermilab's Tevatron took about 4 years to reach 1 fb−1 in 2005, while the Large Hadron Collider experiments ATLAS and CMS reached over 5 inverse femtobarns of proton-proton data in 2011 alone.
- 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. "By next year, collisions will be occurring – if all continues to go well – at a rate producing what physicists call one "inverse femtobarn," best described as a colossal amount of information for analysts to ponder."
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- ^ "CDF, DZero reach 5 inverse femtobarns of luminosity". Fermilab Today. September 26, 2008. http://www.fnal.gov/pub/today/archive_2008/today08-09-26.html. Retrieved 2011-07-23.
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- ^ "Thanks to recent fine tuning, the LHC has delivered 2 inverse femtobarns of data already this year; peak luminosity is now over 2x10^33.". CERN. August 5, 2011. http://twitter.com/#!/CERN/status/99405126694993920. Retrieved 2011-08-05.
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