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barn, abbr. b, in physics, unit of nuclear cross section, i.e., the effective target presented by a nucleus for collisions leading to nuclear reactions; it is equal to 10-24 square centimeters. The barn is approximately the size of the geometric cross section of an atomic nucleus; the term was coined because an effective cross section that large would present a target "as big as a barn," i.e., an easy target for nuclear bombardment. In practice, effective cross sections of nuclei for many reactions are measured in millibarns (10-3 barn) because, for most interactions, only a small fraction of collisions cause reactions.

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barn. Building to store agricultural produce, especially grain, to protect it from the weather. Barns of the Middle Ages often had architectural pretensions. Examples include the C13 barn at Great Coxwell, Berks., and the (probably) C12 barley-barn at Cressing Temple, Essex, both with timber nave-and-aisle interiors.


Endersby et al . (1992);
Kirk (1994)