Barn (unit)
A barn is a unit of area
Area is a quantity that expresses the extent of a two-dimensional surface or shape in the plane. Area can be understood as the amount of material with a given thickness that would be necessary to fashion a model of the shape, or the amount of paint necessary to cover the surface with a single coat...

. 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 generation and nuclear weapons technology, but the research has provided application in many fields, including those...

 for expressing the cross sectional
Cross section (physics)
A cross section is the effective area which governs the probability of some scattering or absorption event. Together with particle density and path length, it can be used to predict the total scattering probability via the Beer-Lambert law....

 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 existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...

 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
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...

 nucleus. The barn is also the unit of area used in nuclear quadrupole resonance
Nuclear Quadrupole Resonance
Nuclear quadrupole resonance spectroscopy or NQR is a chemical analysis technique related to nuclear magnetic resonance .-Principle:In NMR, nuclei with spin ≥ 1/2 have a magnetic dipole moment so that their energies are split by a magnetic field, allowing resonance absorption of energy related to...

 and nuclear magnetic resonance
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...

 to quantify the interaction of a nucleus with an electric field gradient
Electric field gradient
In atomic, molecular, and solid-state physics, the electric field gradient measures the rate of change of the electric field at an atomic nucleus generated by the electronic charge distribution and the other nuclei...

. While the barn is not an SI
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...

 unit, it is accepted for use with the SI due to its continued use in particle physics
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...

. 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
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...

, 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
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

 research on the atomic bomb, American physicists at Purdue University 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 1911, and led to the development of the Rutherford model of the atom, and eventually to the Bohr model. It is now exploited by the materials analytical technique Rutherford backscattering...

) 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


Calculated cross sections are often written in units of ħ2c
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...

In physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...

2 (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. 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.

Fermi National Accelerator Laboratory , located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics...

 has produced 10 fb−1 in the last decade. Fermilab's Tevatron
The Tevatron is a circular particle accelerator in the United States, at the Fermi National Accelerator Laboratory , just east of Batavia, Illinois, and is the second highest energy particle collider in the world after the Large Hadron Collider...

 took about 4 years to reach 1 fb−1 in 2005, while the Large Hadron Collider
Large Hadron Collider
The Large Hadron Collider is the world's largest and highest-energy particle accelerator. It is expected to address some of the most fundamental questions of physics, advancing the understanding of the deepest laws of nature....

 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
In particle physics, a beamline is the line in a linear accelerator along which a beam of particles travels. It may also refer to the line of travel within a bending section such as a storage ring or cyclotron, or an external beam extracted from a cyclic accelerator.In materials science, physics,...

 runs for 8 hours (28,800 seconds) at an instantaneous luminosity of , 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."

External links

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