World Library  
Flag as Inappropriate
Email this Article

Millibarn

Article Id: WHEBN0011502882
Reproduction Date:

Title: Millibarn  
Author: World Heritage Encyclopedia
Language: English
Subject: MB
Collection:
Publisher: World Heritage Encyclopedia
Publication
Date:
 

Millibarn

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.[1] It is one of the very few units that is accepted for use 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]

Etymology

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. 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]

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 μb 10−34 10−30
nanobarn nb 10−37 10−33
picobarn pb 10−40 10−36
femtobarn fb 10−43 10−39
attobarn ab 10−46 10−42
zeptobarn zb 10−49 10−45
yoctobarn yb

[5][6]

10−52 10−48

Two related units are the outhouse (1 μb, or 10−34 m2) and the shed (10−24 b (1 yb), or 10−52 m2),[7] although these are rarely used in practice.

Conversions

Calculated cross sections are often written in units of ħ2c2/GeV2 (approximately 0.3894 mb).

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

Inverse femtobarn

The "inverse femtobarn" (fb−1) is a measurement of particle collision events per femtobarn of target cross-section, and is the conventional unit for time-integrated luminosity.

In a particle accelerator 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 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 productivity.[8][9]

Fermilab has produced 10 fb−1 in the first decade of the 21st century.[10] Fermilab's Tevatron took about 4 years to reach 1 fb−1 in 2005, while two of CERN's LHC experiments, ATLAS and CMS, reached over 5 fb−1 of proton-proton data in 2011 alone.[11][12][13][14][15][16] A record of over 23 fb−1 was achieved during 2012.[17]

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 = 0.008 64 fb−1 during this period. If this is multiplied by the cross-section, then a dimensionless number is obtained which would be simply the number of expected scattering events.

See also

References

External links

  • citation for this usage of "barn"
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 


Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.