Transport theory is preferred over diffusion theory in which scenario?

• A. Deep inside a large homogeneous core
• B. Near boundaries with strong angular anisotropy
• C. Very high scattering and isotropic flux
• D. Steady-state diffusion

Answer: (B)

When the angular variation of the flux is strong, you need to keep track of directionality rather than assume the flux is almost the same in all directions. Diffusion theory treats the flux as nearly isotropic and uses Fick’s law to relate the current to the spatial gradient, which works well deep inside a large, scattering-dominated medium where many collisions erase direction information. But near boundaries, particles preferentially stream out in certain directions, creating a highly anisotropic angular distribution and boundary layers. Transport theory handles this by solving for the angular flux directly, capturing how particles approach and leave the boundary, how leakage occurs, and how the boundary conditions affect the angular distribution. So near boundaries with strong angular anisotropy, transport theory provides a much more accurate description than diffusion.