The second way that photons can interact is through the Compton interaction (Figure 7.2). This interaction occurs over a broad range of photon energies and is the most important interaction between 200 keV and 2 Mev. It is not dependent on the atomic number of the target atom, so it does not cause preferential absorp­tion in bone. The photon with energy hf interacts with a very loosely bound outer

shell electron (essentially a free electron) and gives up part of its energy in the col­lision. After the collision, the photon keeps on going with less energy hf’ (hence a longer wavelength), and the electron moves away with the rest of the energy, much like what would happen in a collision of pool balls. The less energetic photon can then interact again by either a Compton or photoelectric interaction until finally it loses all of its energy or passes through the target. The electron does damage on its own in a way to be discussed shortly. The Compton interaction is the primary interaction that occurs when cancers are treated with high energy у or X-rays and is also the principal interaction with у rays from natural or artificial radioactivity. Both the photoelectric and Compton interactions provide excellent confirmation that light can indeed act like a particle (a photon) since these interactions can be described mathematically as collisions of particles.