# Cross section

$\sigma = \frac{N}{\Phi}$
where $N$ is the mean number of such interactions per target entity subjected to the particle fluence, $Φ$. The SI unit of cross section is m2. A special unit often used for the cross section is the barn, where 1 barn (b) = 10-28 m2. A full description of an interaction process requires, inter alia, knowledge of the distributions of cross sections in terms of energy and direction of all emergent particles from the interaction. Such distributions, sometimes called ‘differential cross sections’, are obtained by differentiations of $\sigma$ with respect to energy and solid angle.