symbol: $j$, units: $\pu{m-2 s-1}$, $\pu{cm-2 s-1}$
synonyms: fluence rate (obsolete), photon flux density
https://doi.org/10.1351/goldbook.08747
Number of particles (or photons) incident on a plane perpendicular to the incident radiation per unit time per unit area.
Notes:
- Particle flux density is identical with the product of the particle density and the average speed of the particles.
- The SI unit of flux density is \(\pu{m-2 s-1}\).
- Often an indication of the type of incident particles is wrongly added to the unit of flux density, e.g., the neutron flux density is sometimes indicated as n \(\pu{cm-2 s-1}\). As 'n' is not a unit, it is metrologically unacceptable and therefore not recommended.