ABSTRACT
The interaction of photons and particles with matter is fundamental to radiation therapy physics, since photons and particles are used to treat and image patients. The energy of the incoming photon largely determines which process will dominate. The energy of the photoelectron is related to the energy of the incident photon. The Compton scattering probability depends somewhat on the energy of the incident photon and, more importantly, depends on the number of electrons per unit gram in the material. The interaction of most importance for therapy physics is a high-energy photon interacting with a nucleus to eject one of the neutrons. This can happen in high-Z material in and around the head of a linear accelerator. This creates an extra neutron component which may need to be considered in shielding for high-energy linear accelerators.
