Editing Cross section (physics) (section)

== Units ==
Although the [[SI unit]] of total cross sections is [[square metre|m<sup>2</sup>]], a smaller unit is usually used in practice.

In nuclear and particle physics, the conventional unit is the barn [[Barn (unit)|b]], where 1&nbsp;b = 10<sup>−28</sup>&nbsp;m<sup>2</sup> = 100&nbsp;[[Femtometre|fm]]<sup>2</sup>.<ref>{{SIbrochure8th|pages=127–28}}</ref> Smaller [[Metric prefix|prefixed]] units such as [[Milli-|mb]] and [[Micro-|μb]] are also widely used. Correspondingly, the differential cross section can be measured in units such as mb/sr.

When the scattered radiation is visible light, it is conventional to measure the path length in [[centimetre]]s. To avoid the need for conversion factors, the scattering cross section is expressed in cm<sup>2</sup>, and the number concentration in cm<sup>−3</sup>. The measurement of the scattering of visible light is known as [[nephelometry]], and is effective for particles of 2–50&nbsp;[[micrometre|μm]] in diameter: as such, it is widely used in [[meteorology]] and in the measurement of [[atmospheric pollution]].

The scattering of [[X-ray]]s can also be described in terms of scattering cross sections, in which case the square [[ångström]] is a convenient unit: 1&nbsp;Å<sup>2</sup> = 10<sup>−20</sup>&nbsp;m<sup>2</sup> = {{val|10000|u=[[picometre|pm]]<sup>2</sup>}} = 10<sup>8</sup>&nbsp;b. The sum of the scattering, photoelectric, and pair-production cross-sections (in barns) is charted as the "atomic attenuation coefficient" (narrow-beam), in barns.<ref>Nondestructive Testing Handbook Volume 4 Radiographic Testing, ASNT, 2002, chapter 22.</ref>