text: Nuclear cross sections are used in determining the nuclear reaction rate, and are governed by the reaction rate equation for a particular set of particles (usually viewed as a "beam and target" thought experiment where one particle or nucleus is the "target", which is typically at rest, and the other is treated as a "beam", which is a projectile with a given energy).
  For neutron interactions incident upon a thin sheet of material (ideally made of a single isotope), the nuclear reaction rate equation is written as:
  where:
  *  : number of reactions of type x, units: [1/time&sdot;volume]
  *  : beam flux, units: [1/area&sdot;time]
  *  : microscopic cross section for reaction , units: [area] (usually barns or cm).
  *  : density of atoms in the target in units of [1/volume]
  * : macroscopic cross-section [1/length]
  Types of reactions frequently encountered are s: scattering, : radiative capture, a: absorption (radiative capture belongs to this type), f: fission, the corresponding notation for cross-sections being: , , , etc. A special case is the total cross-section , which gives the probability of a neutron to undergo any sort of reaction ().
  Formally, the equation above defines the macroscopic neutron cross-section (for reaction x) as the proportionality constant between a neutron flux incident on a (thin) piece of material and the number of reactions that occur (per unit volume) in that material. The distinction between macroscopic and microscopic cross-section is that the former is a property of a specific lump of material (with its density), while the latter is an intrinsic property of a type of nuclei.