A barn (symbol: b) is a
metric unit
The metric system is a system of measurement that succeeded the decimalised system based on the metre that had been introduced in France in the 1790s. The historical development of these systems culminated in the definition of the Internati ...
of
area
Area is the quantity that expresses the extent of a region on the plane or on a curved surface. The area of a plane region or ''plane area'' refers to the area of a shape or planar lamina, while '' surface area'' refers to the area of an ope ...
equal to (100
fm2). Originally used in
nuclear physics
Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions, in addition to the study of other forms of nuclear matter.
Nuclear physics should not be confused with atomic physics, which studies the ...
for expressing the
cross sectional area of
nuclei and
nuclear reaction
In nuclear physics and nuclear chemistry, a nuclear reaction is a process in which two nuclei, or a nucleus and an external subatomic particle, collide to produce one or more new nuclides. Thus, a nuclear reaction must cause a transformatio ...
s, today it is also used in all fields of
high-energy physics
Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) a ...
to express the cross sections of any
scattering process
Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...
, and is best understood as a measure of the probability of interaction between small particles. A barn is approximately the cross-sectional area of a
uranium
Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...
nucleus. The barn is also the unit of area used in
nuclear quadrupole resonance and
nuclear magnetic resonance
Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with a ...
to quantify the interaction of a nucleus with an
electric field gradient In atomic, molecular, and solid-state physics, the electric field gradient (EFG) measures the rate of change of the electric field at an atomic nucleus generated by the electronic charge distribution and the other nuclei. The EFG couples with t ...
. While the barn never was an
SI unit, the
SI standards body acknowledged it in the 8th SI Brochure (superseded in 2019) due to its use in
particle physics
Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) an ...
.
[
]
Etymology
During
Manhattan Project
The Manhattan Project was a research and development undertaking during World War II that produced the first nuclear weapons. It was led by the United States with the support of the United Kingdom and Canada. From 1942 to 1946, the project w ...
research on the
atomic bomb during
World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing ...
, American physicists at
Purdue University
Purdue University is a public land-grant research university in West Lafayette, Indiana, and the flagship campus of the Purdue University system. The university was founded in 1869 after Lafayette businessman John Purdue donated land and mone ...
needed a secretive name for a unit with which to quantify the cross-sectional area presented by the typical nucleus (~10
−28 m
2) and decided on "
barn
A barn is an agricultural building usually on farms and used for various purposes. In North America, a barn refers to structures that house livestock, including cattle and horses, as well as equipment and fodder, and often grain.Alle ...
". They considered this a large target for particle accelerators that needed to have direct strikes on nuclei, and the proposers, physicists Marshall Holloway and Richard Baker, said that the constant "for nuclear purposes was really as big as a barn". In the American idiom "
couldn't hit the broad side of a barn" refers to someone whose aim is very bad. Initially they hoped the name would obscure any reference to the study of nuclear structure; eventually, the word became a standard unit in nuclear and particle physics.
[
][
]
Commonly used prefixed versions
Conversions
Calculated cross sections are often given in terms of gigaelectronvolts (
GeV GEV may refer to:
* ''G.E.V.'' (board game), a tabletop game by Steve Jackson Games
* Ashe County Airport, in North Carolina, United States
* Gällivare Lapland Airport, in Sweden
* Generalized extreme value distribution
* Gev Sella, Israeli-Sou ...
), via the conversion
''ħ''2 ''c''2/
GeV GEV may refer to:
* ''G.E.V.'' (board game), a tabletop game by Steve Jackson Games
* Ashe County Airport, in North Carolina, United States
* Gällivare Lapland Airport, in Sweden
* Generalized extreme value distribution
* Gev Sella, Israeli-Sou ...
2 = = .
In
natural units
In physics, natural units are physical units of measurement in which only universal physical constants are used as defining constants, such that each of these constants acts as a coherent unit of a quantity. For example, the elementary charge ma ...
(where
''ħ'' =
''c'' = 1), this simplifies to
GeV GEV may refer to:
* ''G.E.V.'' (board game), a tabletop game by Steve Jackson Games
* Ashe County Airport, in North Carolina, United States
* Gällivare Lapland Airport, in Sweden
* Generalized extreme value distribution
* Gev Sella, Israeli-Sou ...
−2 = = .
SI units with prefix
In SI, one can use units such as square femtometers (fm
2). The most common SI prefixed unit for 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 fractions of femtobarn level.
Inverse femtobarn
The inverse femtobarn (fb
−1) is the unit typically used to measure the number of
particle collision events per femtobarn of
target cross-section, and is the conventional unit for time-integrated
luminosity. Thus if a detector has accumulated of integrated luminosity, one expects to find 100 events per femtobarn of cross-section within these data.
Consider a
particle accelerator
A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies, and to contain them in well-defined beams.
Large accelerators are used for fundamental research in particle ...
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 of
particle collider
A collider is a type of particle accelerator which brings two opposing particle beams together such that the particles collide. Colliders may either be ring accelerators or linear accelerators.
Colliders are used as a research tool in particle ...
productivity.
[
]
Fermilab
Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a United States Department of Energy national laboratory specializing in high-energy particle physics. Since 2007, Fermilab has been opera ...
produced in the first decade of the 21st century. Fermilab's
Tevatron
The Tevatron was a circular particle accelerator (active until 2011) in the United States, at the Fermi National Accelerator Laboratory (also known as ''Fermilab''), east of Batavia, Illinois, and is the second highest energy particle collider ...
took about 4 years to reach in 2005, while two of
CERN's
LHC experiments,
ATLAS
An atlas is a collection of maps; it is typically a bundle of maps of Earth or of a region of Earth.
Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geograp ...
and
CMS
CMS may refer to:
Computing
* Call management system
* CMS-2 (programming language), used by the United States Navy
* Code Morphing Software, a technology used by Transmeta
* Collection management system for a museum collection
* Color manag ...
, reached over of proton–proton data in 2011 alone. In April 2012 the LHC achieved the collision energy of 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. As of November 2016, the LHC had achieved over that year, significantly exceeding the stated goal of . In total, the second run of the LHC has delivered around to both ATLAS and CMS in 2015–2018.
Usage example
As a simplified example, if a
beamline
In accelerator physics, a beamline refers to the trajectory of the beam of particles, including the overall construction of the path segment (guide tubes, diagnostic devices) along a specific path of an accelerator facility. This part is either ...
runs for 8 hours (28 800 seconds) at an instantaneous luminosity of , then it will gather data totaling an integrated luminosity of = = 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.
See also
* "
Shake", a unit of time created by the same people at the same time as the barn
*
Orders of magnitude (area)
This page is a progressive and labelled list of the SI area orders of magnitude, with certain examples appended to some list objects.
to square metres
10−8 to 10−1 square metres
100 to 107 square metres
108 to 1014 square metres
101 ...
*
List of unusual units of measurement
An unusual unit of measurement is a unit of measurement that does not form part of a coherent system of measurement, especially because its exact quantity may not be well known or because it may be an inconvenient multiple or fraction of a base ...
*
List of humorous units of measurement
Many people have made use of, or invented, units of measurement intended primarily for their humor
Humour (Commonwealth English) or humor (American English) is the tendency of experiences to provoke laughter and provide amusement. The ...
References
External links
*
IUPAC
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
br>
citation for this usage of "barn"
{{DEFAULTSORT:Barn (Unit)
Units of area
Non-SI metric units
Particle physics