Barn (unit)
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A barn (symbol b) is a unit of area. Originally used in nuclear
physics for expressing the cross sectional area of nuclei and nuclear
reactions, today it is used in all fields of high energy physics to
express the cross sections of any scattering process. A barn is
defined as 10?28 m2 or 100 square femtometers (fm2) and is
approximately the cross sectional area of a uranium nucleus. The barn
is also the unit of area used in nuclear quadrupole resonance and
nuclear magnetic resonance to quantify the interaction of a nucleus
with an electric field gradient. While the barn is not an SI unit, it
is accepted for use with the SI due to its continued use in particle
physics.[1] It is one of the very few units which are accepted to be
used with SI units, and one of the most recent units to have been
established (cf. the knot and the bar, other non-SI units acceptable
in limited circumstances).[2]
