particle flux density

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:
  1. Particle flux density is identical with the product of the particle density and the average speed of the particles.
  2. The SI unit of flux density is \(\pu{m-2 s-1}\).
  3. 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.
Source:
PAC, 2021, 93, 69. 'Vocabulary of radioanalytical methods (IUPAC Recommendations 2020)' on page 91 (https://doi.org/10.1515/pac-2019-0302)