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Physical unit
Unit name Barn
Unit symbol
Physical quantity (s) Cross section
Formula symbol
In SI units
Named after English barn , "barn"
Derived from square meters

The barn b ( English for 'barn') is a unit of measurement of the area that is used to indicate cross- sections in atomic , nuclear and particle physics .

The Barn is not an SI unit. In the EU and Switzerland, it is a legal unit for specifying effective cross-sections, so it is not a generally applicable unit of area.

A barn is in the order of magnitude of the geometric cross-section of heavy atomic nuclei such as uranium . The definition is:

1 b = 10 −24  cm 2 = 10 −28  m 2 .

Common decimal parts

Small cross-sections are given in millibarn (mb), microbarn (µb), nanobarn (nb) or picobarn (pb). For example is

1 pb = 10 −12  b = 10 −40  m 2 .


The unit shed was intended to describe very small cross- sections, especially of neutrino reactions, but it never really caught on. It applies

1 shed = 10 −24  b = 10 −52  m 2 .

Word origin

The designation barn for the already common cross section unit 10 −24  cm 2 was introduced in December 1942 by two scientists from Purdue University who were involved in the US nuclear weapons development ( Manhattan Project ). It played a role that an effective cross-section of this size for nuclear reactionsas big as a barn ” (German: “big as a barn door”) appeared.

Individual evidence

  1. The International System of Units (SI) . German translation of the BIPM brochure "Le Système international d'unités / The International System of Units (8e edition, 2006)". In: PTB-Mitteilungen . tape 117 , no. 2 , 2007, p. 156 ( [PDF; 1.4 MB ]). Online version (PDF; 1.4 MB) ( Memento from March 4, 2016 in the Internet Archive )
  2. Directive 80/181 / EEC (PDF)
  3. Unit Ordinance
  4. ^ Nuclear Glossary
  5. ^ MG Holloway, CP Baker: How the barn was born. In: Physics Today , July 1972