# 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 (100 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]

Two related units are the outhouse (10−34 m2, or 1 μb) and the shed (10−52 m2, or 1 yb), although these are never used in practice.

## Contents

### Conversions

Calculated cross sections are often written in units of ħ2c2/GeV2 (approximately 0.3894 mb).

### Origin

The etymology of the unit barn is whimsical and jocular. During wartime research on the atomic bomb, American physicists at Purdue University who were deflecting neutrons off uranium nuclei, (similar to Rutherford scattering) described the uranium nucleus as "big as a barn". Physicists working on the project adopted the name "barn" for a unit equal to 10−24 square centimetres, about the size of a uranium nucleus. Initially they hoped the American slang name would obscure any reference to the study of nuclear structure; eventually, the word became a standard unit in particle physics.[6][7]