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Barn (unit)

A barn is a unit of area, equal to 10-28 m2. Although not an official SI unit, it is widely used by nuclear physicists, since it is convenient for expressing the cross sectional area of nuclei and nuclear reactions. A barn is approximately equal to the area of a uranium nucleus. The etymology is clearly whimsical -- the unit is said to be "as big as a barn" compared to the typical cross sections for nuclear reactions.

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