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 In nuclear physics it is conventional to consider that the impinging particles have negligible diameter. Cross sections can be computed for any sort of process, such as capture scattering, production of neutrons, etc. In many cases, the number of particles emitted or scattered in nuclear processes is not measured directly; one merely measures the attenuation produced in a parallel beam of incident particles by the interposition of a known thickness of a particular material. The cross section obtained in this way is called the total cross section and is usually denoted by a &sigma; or &sigma;<sub>T</sub>. 
 
The typical nuclear diameter is of the order of 10<sup>&minus;12</sup> cm. We might therefore expect the cross sections for nuclear reactions to be of the order of &pi;''r'',<sup>2</sup> or roughly 10<sup>&minus;24</sup> cm<sup>2</sup> and this unit is given its own name, the [[barn (unit)|barn]], and is the unit in which cross sections are usually expressed. Actually the observed cross sections vary enormously. Thus for slow neutrons absorbed by the (n, gamma) reaction the cross section in some cases is as much as 1,000 barns, while the cross sections for transmutations by gamma-ray absorption are in the neighborhood of 0.001 barns.
 
 == [[Radar]] ==
 The (monostatic) [[radar cross section]] is defined as 4 π times the [[radio]] differential cross section at 180 degrees.
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 In nuclear physics it is conventional to consider that the impinging particles have negligible diameter. Cross sections can be computed for any sort of process, such as capture scattering, production of neutrons, etc. In many cases, the number of particles emitted or scattered in nuclear processes is not measured directly; one merely measures the attenuation produced in a parallel beam of incident particles by the interposition of a known thickness of a particular material. The cross section obtained in this way is called the total cross section and is usually denoted by a &sigma; or &sigma;<sub>T</sub>. 
 
The typical nuclear diameter is of the order of 10<sup>&minus;12</sup> cm. We might therefore expect the cross sections for nuclear reactions to be of the order of <math>\pi r^2</math> or roughly 10<sup>&minus;24</sup> cm<sup>2</sup> and this unit is given its own name, the [[barn (unit)|barn]], and is the unit in which cross sections are usually expressed. Actually the observed cross sections vary enormously. Thus for slow neutrons absorbed by the (n, gamma) reaction the cross section in some cases is as much as 1,000 barns, while the cross sections for transmutations by gamma-ray absorption are in the neighborhood of 0.001 barns.
 
 == [[Radar]] ==
 The (monostatic) [[radar cross section]] is defined as 4 π times the [[radio]] differential cross section at 180 degrees.
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