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Fluence, probability, cross-section, and the barn -Reply



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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.

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Although 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, it requires one to
view the target as a uniform surface, i.e. no nuclei (or atoms).  If one
prefers viewing that way, that is fine.  In many if not most low energy
(~<100 MeV) nuclear physics experiments the beam spot is considered a
point (an idealization) and fluence is not a very useful concept.  The
billiard ball analogy is also the one to which Fermi referred when he coined
the term "barn."

My response was to a previous mail message, which, concerning the
sievert being equal to J/kg, stated, "In the more disciplined areas of
physical sciences, a new unit would be/have-been defined to reflect the
physical reality . . . ."  I am not certain which of the physical sciences are
"more disciplined," but certainly in physics there are a number of similar
situations, the barn as square centimeters being an example that sprung to
mind.

Keith Brown
kdb1@nrc.gov