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 m
2 (100 fm) 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.
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).
Two related units are the outhouse (10−34 m, or 1 μb) and the shed (10−52 m, or 1 yb), although these are rarely used in practice.