A unit of effective nuclear cross section, = 10−24 square centimeters per nucleus. Symbol, b.

The unit was named by M. G. Holloway and C. P. Baker in December 1942, while having after dinner coffee in the Union Building of Purdue University. A value of 10−24 square centimeters was already being used as a unit for nuclear cross sections in their work for the Manhattan Project, the secret American effort to construct the first atomic bomb, but it had no name. Holloway and Baker considered and rejected the names “bethe,” “oppenheimer,” and “manley” (for John Manley, head of their group), and finally arrived at “barn” because “for nuclear processes [10-24 square centimeters] was really as big as barn.”1

The first use of the barn in a publication seems to have been in Los Alamos Research Report LAMS-2 (June 28, 1943). The value 10−24 centimeters approximates the actual cross section (in the geometric sense) of a typical nucleus, and for effective nuclear cross section, places 1 barn about in the middle of the range of observed nuclear reactions. Nuclear reactions typically have cross sections in the range 0.1 to 10 barns, though cross sections from 10−8 to 106 barns are known. The fission product xenon-135, which poisons fuel rods in reactors by absorbing slow neutrons, has an extraordinary cross section of 3,500,000 barns.4

Though the barn has the dimension of area, it is not a measure of area, but of the probability of a reaction.

The International Union for Pure and Applied Physics recommended the use of the barn at its 10th General Meeting in 1960.2

According to the current national standard in the United States3, the barn is not to be used. Square femtometers should be used instead.

1. M. G. Holloway and C. P. Baker.
Note on the origin of the term “barn”.
Los Alamos Research Report, LAMS 523.
Report submitted: 13 September 1944. Report issued: 5 March 1947.  [back]

2. International Union of Pure and Applied Physics.
Report of the 10th General Assembly, Ottawa, 1960.

Pages 7 and 24.  [back]

3. IEEE/ASTM SI 10™-2002.
American National Standard for Use of the International System of Units (SI): The Modern Metric System.
New York: IEEE, 30 December 2002.

See Section 3.3.3.


The barn and its symbol "b" appear in

4. National Research Council.
A Glossary of Terms in Nuclear Science and Technology.
New York: American Society of Mechanical Engineers, 1955.

Page 16. Xe-135, page 65.

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