Final answer:
The principal source of scatter radiation in radiography is Compton scattering, where X-rays interact with electrons resulting in a change in X-ray wavelength and ejected electrons.
Step-by-step explanation:
The principal source of scatter radiation in radiography is Compton scattering. In Compton scattering, X-rays are used and the interaction involves the scattering of these X-rays off the material's electrons, resulting in a change of wavelength of the X-rays. This kind of scattering is inelastic, meaning that the scattered radiation has a longer wavelength than the incident radiation. The photons in X-rays collide with free, weakly bound electrons, causing the electron to be ejected and the X-ray to lose energy. This interaction obeys the conservation laws of total energy and momentum but is more dependent on the density of the material and its thickness rather than its chemical composition. The particles that interact in Compton scattering are the photon and the electron.