Examination of the structure or internal soundness of an object using radiography consists in submitting it to electromagnetic radiation at a very short wavelength (X-ray or gamma ray) and collecting modulations of beam intensity in the form of an image on an appropriate receiver, usually film.
Radiation of the same type, but of different origins, X-rays and gamma rays are electromagnetic waves with very short wavelengths (between about 0.1 pm and 1000 pm).
X-rays or gamma rays propagate in a straight line, at the speed of light in a vacuum. Their very short wavelength makes it so they are susceptible to the laws of geometrical optics.
The interaction between X-rays or gamma rays and the material, involves the following phenomena:
- a) a photoelectric effect corresponding to a complete transfer of energy between the incident photon and an electron, leading to its total absorption and finally re-emission of X-ray fluorescence. This effect increases as the energy of photons decreases.
- b) the Compton effect, corresponding to a partial transfer of the photon’s energy to an electron. Photons are deviated whereas the activated electrons cause a secondary emission. This Compton diffusion becomes dominant around 1 MeV with regard to the photoelectric effect, and constitutes the predominant phenomenon in industrial radiography.