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# Fluence

In physics, fluence is defined as the number of particles that intersect a unit area . Its units are m-2 (Number of particles per meter squared). In particular, it is used to describe the strength of a radiation field, in which case the unit used is J/m2. It is considered one of the fundamental units in dosimetry.

It has two equivalent definitions:

1) Imagine that an infinitesimal sphere of cross sectional area da is impinged upon by dN particles of a certain type. Then, the fluence is:

$\Phi = \frac{{\rm d} N}{{\rm d} a}$.

2) Using the same image of a small sphere as above:

$\Phi = \frac{\sum {\rm d \ell} }{{\rm d} V}$,

where dV is the infintesimal volume and $\sum {\rm d \ell}$ is the sum of all the path lengths of the particles that transverse the volume.

## References

ICRU 33: Radiation Quantities and Units, April, 1980. (www.icru.org)