### cross section

("area" representation of probability of an interaction of particles)

A use of the term **cross section** in the physics of particles
moving close to each other is a representation of the probability
of interaction between the particles as if it were an area around one of the
particles.
Among the areas of physics where the concept is used
is nuclear physics (fission and fusion, such as the power generation
within stars), in radiative transfer, and in the physics of
gases as interacting particles (**kinetic theory of gases**). The
term **collision cross section** is sometimes used when the interactions
are thought of as collisions between particles.

In the mechanics of small particles (quantum mechanics), particles
passing at a certain distance of each other may or may not interact,
an interaction having some probability whose calculation is the
business of quantum mechanics. If a specific region were
selected around one particle that includes nearly all points at
which another passing particle might interact (so the probability
of interacting with a particle passing outside this region is
virtually zero), then the cross section is a smaller region sized
so the ratio of region sizes matches the probability of an interaction
of a particle passing through the larger region. It is a convenient
and simplifying method of modeling how many interactions will take
place.

Given such a cross section, the path of a moving particle can be
viewed as a straight, round, tube-shaped volume, whose calculated
volume along with the density of the material allows calculation
of properties of interest such as how long the particle is likely
to move before an interaction (the mean free path).

Cross sections are used in quantifying scattering effects such
as Compton scattering. For scattering,
the **differential cross section** is the ratio between the
area-size of the cross section and the solid-angle size
of the possible directions to which something can be scattered.
It is a function of the scattering angle.
In some kinds of experiments it can be worked out through
scattering statistics and serves as a step toward calculating
the cross section.

(*physics*)
**Further reading:**

http://en.wikipedia.org/wiki/Cross_section_(physics)

**Referenced by pages:**

absorption coefficient

dark matter annihilation

extinction

Klein-Nishina formula

oscillator strength

pebble accretion

radiation pressure

solar neutrino unit (SNU)

Thomson optical depth (τ_{T})

ZEPLIN

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