Monte Carlo Photon Transport in a Tissue Slab

A Monte Carlo photon-transport playground. Photons normally incident on a water slab are followed history by history: a free path is sampled as $-\ln(U)/\mu$, the interaction type is drawn from the photoelectric, Compton and Rayleigh cross sections, Compton scattering is sampled from the Klein-Nishina distribution by Kahn's method, and the released electron energy is deposited over a forward CSDA range, which produces the depth-dose build-up before the exponential falloff. Panel A shows the photon histories coloured by interaction type, Panel B the depth dose and the 2D dose map, Panel C the interaction fractions versus energy and the energy balance. The sampled mean free path follows $1/\mu$, the photoelectric-to-Compton dominance crosses over with energy, energy is conserved history by history, and the depth dose shows the build-up before the Beer-Lambert falloff.