Assume for simplicity that the foil is sufficiently thin and the
reflectors sufficiently small compared to their average spacing that that
the reflectors do not occult one another and that the vast majority of
particles pass through the foil colliding with at most one reflector.
What fraction of the particles can be expected to pass through the
foil without a single collision? Ignoring multiple scattering events,
at what rate (particles/unit time) will particles emerge from the foil
with deflection angles less than 
away from the 
direction?
