If the finite size of the nucleus is taken into account in the ``single-scattering'' tail of the multiple-scattering distribution, there is a critical thickness xc beyond which the single-scattering tail is absent.
(a) Define xc in a reasonable way and calculate its value (in cm) for aluminum and lead, assuming that the incident particle is relativistic.
(b) For these thicknesses calculate the number of collisions that occur and determine whether the Gaussian approximation is valid.