Barn (unit)

A barn (symbol: b) is a metric unit of area equal to e−28m2 (100 fm2). Originally used in nuclear physics for expressing the cross sectional area of nuclei and nuclear reactions, today it is also 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 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 While the barn never was an SI unit, the SI standards body acknowledged it in the 8th SI Brochure (superseded in 2019) due to its use in particle physics. During Manhattan Project research on the atomic bomb during World War II, American physicists at Purdue University needed a secretive name for a unit with which to quantify the cross-sectional area presented by the typical nucleus (~10−28 m2) and decided on "barn". 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". 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. The unit symbol for the barn (b) is also the IEEE standard symbol for bit. In other words, 1 Mb can mean one megabarn or one megabit. Calculated cross sections are often given in terms of inverse squared gigaelectronvolts (GeV−2), via the conversion ħ2c2/GeV2 = 0.3894mb = 38940am2. In natural units (where ħ = c = 1), this simplifies to GeV−2 = 0.3894mb = 38940am2. In SI, one can use units such as square femtometers (fm2).