A graphite target was bombarded with monochromatic x-rays and the wavelength of the scattered radiation was measured with a rotating crystal spectrometer. Compton's experiment convinced physicists that light can behave as a stream of particles whose energy is proportional to the frequency.Ī schematic diagram of the apparatus used by Compton is shown in the Figure below.

Light must behave as if it consists of particles in order to explain the low-intensity Compton scattering. Thomson scattering, the classical theory of an electromagnetic wave scattered by charged particles, cannot explain low intensity shift in wavelength (Classically, light of sufficient intensity for the electric field to accelerate a charged particle to a relativistic speed will cause radiation-pressure recoil and an associated Doppler shift of the scattered light, but the effect would become arbitrarily small at sufficiently low light intensities regardless of wavelength.). The effect was first demonstrated in 1923 by Arthur Holly Compton (for which he received a 1927 Nobel Prize).The effect is important because it demonstrates that light cannot be explained purely as a wave phenomenon. The scattered radiation experiences a wavelength shift that cannot be explained in terms of classical wave theory, thus lending support to Einstein's photon theory. Philadelphia, PA: Wolters Kluwer Health 2012.The Compton effect (also called Compton scattering) is the result of a high-energy photon colliding with a target, which releases loosely bound electrons from the outer shell of the atom or molecule. Reprinted with permission from Fosbinder RA, Orth D. As the atomic number of the absorber decreases, the probability of characteristic X-ray emission decreases. The difference in binding energy is released as either characteristic X-rays or Auger electrons. This will, in turn, create another vacancy, which is filled from an even lower binding energy shell, creating an electron cascade from outer to inner shells. An electron from a more outer shell, with a lower binding energy, will fill this vacancy. As a result of a photoelectric interaction, atoms are ionized with a vacancy of an inner shell electron. For example, photons whose energies exceed the K-shell binding energies will most likely result in photoelectric interactions with the K-shell electrons.

For a photoelectric effect to occur, the incident photon energy must be equal to or greater than the binding energy of the ejected electron. Illustration of the photoelectric effect.For example, an iodine atom which has a K-edge of 33.2 keV is six times more likely to have a photoelectric interaction with a 33.2 keV X-ray photon than with a 33.1 keV photon ( Figure 2.4). Every element has one or a few sharp “absorption edges” in which the probability of a photoelectric interaction dramatically increases for photons of energies just above the absorption edge relative to energies just below the edge. As the incident photon energy increases to just above the binding energy of the K shell, the absorption of the photon markedly increases. However, there are sharp discontinuities to be observed at certain photon energies. Also, the probability of photoelectric absorption generally decreases with the increase in photon energy. This is why barium ( Z = 56) and iodine ( Z = 53), which have high atomic numbers, are used as contrast agents in X-ray imaging. Therefore, as the atomic number increases, the photoelectric absorption effect becomes more pronounced. The probability of photoelectric absorption, by approximation, is considered proportional to Z 3/E 3, where Z is the atomic number and E is the energy of the incident photon.

Graph compton effect