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.

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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:

.

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

,

where dV is the infintesimal volume and 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)