barn

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

Farm building used for sheltering animals, their feed and other supplies, farm machinery, and farm products. Barns are named according to their purpose (e.g., hog barns, dairy barns, tobacco barns, and tractor barns). The principal type in the U.S. is the general-purpose barn, used for housing livestock and for storing hay and grain. Most North American and European farms have one or more barns. They usually consist of two stories, though one-story barns gained in popularity in the late 20th century.

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barn¹

US and Canadian a large shed for sheltering railroad cars, trucks, etc.

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barn²

a unit of nuclear cross section equal to 10^--28 square metre.

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barn [′bärn]

(agriculture)

A farm building used for storage of agricultural products and equipment or for housing farm animals.

(nuclear physics)

A unit of area equal to 10^-24square centimeter; used in specifying nuclear cross sections. Symbolized b.