number of particles crossing a sphere of unit cross-sectional area (i.e. of radius 1/ ) per unit time
NOTE 1 For arbitrary angular distributions, it is normally known as omnidirectional flux.
NOTE 2 Flux is often expressed in 'integral form' as particles per unit time (e.g. electrons cm-2 s-1) above a certain energy threshold.
NOTE 3 The directional flux is the differential with respect to solid angle (e.g. particles-cm-2steradian-1s-1) while the 'differential' flux is is differential with respect to energy (e.g. particles-cm-2MeV-1s-1). In some cases fluxes are treated as a differential with respect to linear energy transfer rather than energy.