Fluence, probability, cross-section, and the barn

[dj_strom@ccmail.pnl.gov]

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Fluence is the number of particles traversing a sphere of unit area and 
therefore has dimension m^-2 (per square meter, also expressible as per 
barn or per acre; ICRU Report 33, 1980).  In atomic, nuclear, and 
particle physics, cross sections can be viewed as "interaction 
probability per unit fluence" (ICRU Report 33, 1980).  

Thinking of an interaction cross section as a probability per unit 
fluence avoids the conceptual difficulty of imagining a "target" much 
larger than an interaction diameter for billiard ball-like collisions.