Fermi at the blackboard

Enrico Fermi

Courtesy Argonne National Laboratory



A unit of length used to measure nuclear distances, = 10⁻¹⁵ meter. Named for the Italian physicist Enrico Fermi (1901–1954). Now obsolete, replaced by the SI term femtometer (symbol, fm); one fermi = one femtometer. According to the current national standard in the United States¹, the fermi is not to be used.

1. 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.


A unit of nuclear cross-section = 10⁻²⁴ square centimeters, the same as the barn. Symbol, F.

Alvin M. Weinberg and Eugene P. Wigner.
The Physical Theory of Neutron Chain Reactors.
Chicago: University of Chicago Press, 1958.

Page 22-23: “Cross-sections are often given in barns instead of square centimeters; a barn is 10⁻²⁴ cm². However, we shall use the word ‘fermi,’ in honor of Enrico Fermi, to denote 10⁻²⁴ cm²; the letter ‘F’ will be used for its abbreviation.”

2. W. W. Havens.
Modern physics has its unit problems.
in Systems of Units. National and International Aspects.
Carl F. Kayan, editor.
Publication No. 57 of the AAAS.
Washington, D. C.: American Association for the Advancement of Science, 1959.

Haven's remark that Fermi invented the barn is incorrect, at least as far as the naming of the unit goes.

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