Consider a steady uniform stream of particles approaching the body at point F.
What is the probability that a particle is scattered to the solid angle d between θ
and θ + dθ? In order that particles scatter into this interval, their impact parameters
must lie within a corresponding interval b, b + db (see Fig. 3.10). To find out how
many particles scatter into this solid angle we define the cross-section σ(θ) d for
scattering to the solid angle