amount of radiation crossing a surface per unit of time
NOTE 1 The directional flux is the differential with respect to solid angle (e.g. particles cm-2 steradian-1s-1) while the 'differential' flux is differential with respect to energy (e.g. particles cm-2 MeV-1s-1). In some cases fluxes are also treated as a differential with respect to Linear Energy Transfer (see 3.2.28).
NOTE 2 Flux is often expressed in 'integral form' as particles per unit area per unit time (e.g. electrons cm-2s-1) above a certain threshold energy.