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The cross section is an effective area that quantifies the intrinsic likelihood of a scattering event when an incident beam strikes a target object, made of discrete particles. The cross section of a particle is the same as the cross section of a hard object, if the probabilities of hitting them with a ray are the same. It is typically denoted and measured in units of area. In scattering experiments, one is often interested in knowing how likely a given event occurs. However, the rate depends strongly on experimental variables such as the density of the target material, the intensity of the beam, or the area of overlap between the beam and the target material. To control for these mundane differences, one can factor out these variables, resulting in an area-like quantity known as the cross section. == Definition == Cross section is associated with a particular event (e.g. elastic collision, a specific chemical reaction, a specific nuclear reaction) involving a certain combination of beam (e.g. light, elementary particles, nuclei) and target material (e.g. colloids, gases, atoms, nuclei). Often there are additional factors that can affect the cross section in complicated ways, such as the energy of the beam. For a given event, the cross section is given by : where * is the cross section of this event (SI units: ), * is the attenuation coefficient ''due to the occurrence of this event'' (SI units: ), and * is the number density of the target particles (SI units: ). Equivalently, if the target material is a thin slab placed perpendicular to the beam, one may express the cross section in terms of flux: : where * is the amount of flux lost ''due to the occurrence of this event'', * is the thickness of the target material, and * is the flux of the incident beam. For discrete events involving a beam of particles, the cross section is given by: : where * is the rate at which the event occurs (SI units: ), * is the particle flux (or intensity) of the incident beam (SI units: ), and * is the area of overlap between the beam and the target (SI units: ). right Schematically, an event is said to have a cross-section of if its rate is equal to that of collisions in an idealized classical experiment where: * the beam is replaced by a stream of inert point-like particles, and * the target particles are replaced by inert and impenetrable disks of area (and hence the name “cross-section”), with all other experimental variables kept the same as the original experiment. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cross section (physics)」の詳細全文を読む スポンサード リンク
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