### cross section

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

One 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 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.

If two particles can interact, one particle passing sufficiently
close to the other has a chance of interacting with it. When the
point of interest is the frequencies of the interactions within a
considerable amount of some substance, this "chance of interaction"
between two particles is the same as it would be if the particle
had some specific size such that passing close enough to touch
results in an interaction. If such a size can be determined
through experiment or analysis, it provides an intuitive way of
thinking of the interactions that can be useful in thinking about
what happens and can be used in some useful calculation as well.
Given the probabilities inherent in quantum mechanics, such a cross
section might be merely theoretical (e.g., considered quantum-mechanically,
there could well be a non-zero probability that one particle passes
right through another) but one can still be devised that represents
the chances of an interaction.

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).

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

**Referenced by:**

absorption coefficient

dark matter annihilation

extinction

oscillator strength

solar neutrino unit (SNU)

index