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A barn (symbol b) is a unit of area
Area

Area is a quantity expressing the two-dimensional size of a defined part of a surface, typically a region bounded by a closed curve. The term surface area refers to the total area of the exposed surface of a 3-dimensional solid, such as the sum of the areas of the exposed sides of a polyhedron....
. While the barn is not an SI
Si

Si, si, or SI may refer to :...
 unit, it is accepted for use with the SI. Originally used in nuclear physics
Nuclear physics

Nuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei.The most commonly known applications of nuclear physics are nuclear power and nuclear weapons, but the research field is also the basis for a far wider range of applications, including in the medical sector , in materials engineering...
 for expressing the cross sectional
Cross section (physics)

In nuclear physics and particle physics, the concept of a cross section is used to express the likelihood of interaction between particles.When particles are thrown against a foil made of a certain substance, the cross section is a hypothetical area measure around the target particles that represents a surface....
 area of nuclei and nuclear reactions, today it is used in all fields of high energy physics
Particle physics

Particle physics is a branch of physics that studies the elementary particle constituents of matter and radiation, and the interactions between them....
 to express the cross sections of any scattering process. A barn is defined as being 10-28 m2, and corresponds to approximately the cross sectional area of a uranium
Uranium

Uranium is a silvery-gray metallic chemical element in the actinide series of the periodic table that has the chemical symbol U and atomic number 92....
 nucleus.






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A barn (symbol b) is a unit of area
Area

Area is a quantity expressing the two-dimensional size of a defined part of a surface, typically a region bounded by a closed curve. The term surface area refers to the total area of the exposed surface of a 3-dimensional solid, such as the sum of the areas of the exposed sides of a polyhedron....
. While the barn is not an SI
Si

Si, si, or SI may refer to :...
 unit, it is accepted for use with the SI. Originally used in nuclear physics
Nuclear physics

Nuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei.The most commonly known applications of nuclear physics are nuclear power and nuclear weapons, but the research field is also the basis for a far wider range of applications, including in the medical sector , in materials engineering...
 for expressing the cross sectional
Cross section (physics)

In nuclear physics and particle physics, the concept of a cross section is used to express the likelihood of interaction between particles.When particles are thrown against a foil made of a certain substance, the cross section is a hypothetical area measure around the target particles that represents a surface....
 area of nuclei and nuclear reactions, today it is used in all fields of high energy physics
Particle physics

Particle physics is a branch of physics that studies the elementary particle constituents of matter and radiation, and the interactions between them....
 to express the cross sections of any scattering process. A barn is defined as being 10-28 m2, and corresponds to approximately the cross sectional area of a uranium
Uranium

Uranium is a silvery-gray metallic chemical element in the actinide series of the periodic table that has the chemical symbol U and atomic number 92....
 nucleus. To phrase it in terms of the closest SI unit, a barn is 100 square femtometers (fm2). The barn is also the unit of area used in nuclear quadrupole resonance
Nuclear Quadrupole Resonance

Nuclear quadrupole resonance or NQR is a technique related to nuclear magnetic resonance . In NMR, nuclei with a magnetic dipole moment have their energies split by a magnetic field, allowing resonance absorption of energy related to the difference between the ground state energy and the excited state....
 and nuclear magnetic resonance
Nuclear magnetic resonance

Nuclear magnetic resonance is the name given to a physical resonance phenomenon involving the observation of specific quantum mechanics magnetism properties of an atomic atomic nucleus in the presence of an applied, external magnetic field....
 to quantify the interaction of a nucleus with an electric field gradient
Electric field gradient

Mathematically, the electric field gradient is the hessian matrix of the electrical potential V:It is an important structural property of a crystalline solid, where it is defined at the location of a atomic nucleus....
.

Commonly used prefixed versions

Conversion to SI units
Unit Symbol(although discouraged) m2 cm2
barn b 10-28 10-24
millibarn mb 10-31 10-27(although discouraged)
microbarn(although discouraged) µb 10-34 10-30
nanobarn nb 10-37 10-33
picobarn pb 10-40 10-36
femtobarn fb 10-43 10-39


Conversions

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

Origin

The etymology is clearly whimsical and jocular—the unit is said to be "as big as a barn" compared to the typical cross sections for nuclear reaction
Nuclear reaction

In nuclear physics, a nuclear reaction is the process in which two atomic nucleus or subatomic particles collide to produce products different from the initial particles....
s. During wartime
World War II

World War II, or the Second World War , was a global military conflict which involved a Participants in World War II, including all of the great powers, organised into two opposing military alliances: the Allies of World War II and the Axis powers....
 research on the atomic bomb, American physicists who were deflecting neutrons off uranium nuclei, (similar to Rutherford scattering
Rutherford scattering

In physics, Rutherford scattering is a phenomenon that was explained by Ernest Rutherford in 1909, and led to the development of the Rutherford model of the atom, and eventually to the Bohr model....
) 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.

The origin of the barn is described in the .

Shed

The shed
Shed (physics)

A shed is a non-SI unit of area or cross-section used in nuclear physics. One shed is defined as being equal to 10−24 barn , an extraordinarily small unit of area....
 was devised to describe incredibly small areas. One shed is 10-52m2, or 10-24b. The shed is to the barn what the barn is to the square centimeter.

See Also

  • Orders of magnitude (area)
    Orders of magnitude (area)

    This page is a progressive and labeled list of the SI area orders of magnitude, with certain examples appended to some list objects....


External links


  • IUPAC