COMPTON SCATTERING

The Compton Scatter, also known as the Compton Effect is one of the most common types of photon interaction. In material, it is the primary source of scattered radiation. It happens when a photon (x-ray or gamma) interacts with free electrons (atoms that aren't attached to them) or loosely bound valence shell (outer shell) electrons. The incident photon scatters (changes direction) and provides energy to the electromagnet.  The scattered photon would have a different wavelength and as a result, different energy (E=hc/). This method conserves both energy and momentum. The Compton effect is a partial absorption process that results in a Compton shift (i.e. a wavelength/frequency shift) as the original photon loses energy. 0.024 (1- cosine) can be used to calculate the wavelength shift of the scattered photon. Thus, the energy of the scattered photon decreases with increasing scattered photon angle. 

   On December 12, 2005, APS President-elect John Hopfield presented a plaque in honor of Arthur H. Compton at Washington University in St. Louis. Compton was a professor at Washington University, studying the scattering of X-rays when he discovered the effect that is named after him in 1922. A graduate of the University of Wooster and Princeton University, Compton developed a theory of the intensity of X-ray reflection from crystals as a means of studying the arrangement of electrons and atoms. In 1918 he started a study of X-ray scattering. In 1919 Compton was awarded one of the first National Research Council fellowships. He took his fellowship to the Cavendish Laboratory in Cambridge, England, and then to Washington University, St. Louis when the equipment in England turned out to be inadequate for his needs. Working with X-rays, he perfected his apparatus to measure the shift of wavelength with a scattering angle that is now known as the Compton effect. Compton observed t...

In Compton Scattering, the x-ray photon interacts with an electron in the outer shell, and hence the likelihood of Crompton Scattering doesn’t depend on Z. From the above figure, we can see that the electron was removed from the shell by the  X-Ray photon . I n order to conserve momentum, t he photon then goes out in an opposing direction from the knocked-out electron. Differ from the photoelectric effect, the energy is not all deposited locally. The scattered photon may still have a significant fraction of the energy of the incoming photon. It can still travel through the patient and potentially could have a secondary scatter effect or could get measured on the detector.

Probability of Compton effect 1. directly proportional to number of outer shell electrons (i.e., the electron density) physical density of the material 2. inversely proportional to photon energy 3. does not depend on atomic number (unlike photoelectric and pair production) In other words, the Compton effect's probability is determined by the number of electrons per gram in the absorbing material, which is roughly the same for most elements (approximately 3 x 1023). With the exception of hydrogen, which has no neutrons in its nucleus and thus has an electron density twice that of all other elements (approximately 6 x 1023 ), therefore the Compton effect is independent of the absorber's atomic number (Z). When human tissues are irradiated in the 30 keV to 30 MeV energy range, which is the diagnostic and therapeutic radiation range, the Compton effect becomes the dominant process.

Compton scattering occurs when a photon interacts with an outer orbital electron. About 57% of interactions in a dental x-ray beam exposure involve Compton scattering. In this interaction, the incident photon collides with an outer electron, which receives kinetic energy and recoils from the point of impact.  The path of the incident photon is deflected by this interaction and is scattered in a new direction from the site of the collision. The energy of the scattered photon equals the energy of the incident photon minus the sum of the kinetic energy gained by the recoil electron and its binding energy. The probability of a Compton interaction is directly proportional to the electron density of the absorber. The density of electrons in bone (5.55 × 1023/cm3) is greater than in soft tissue (3.34 × 1023/cm3); therefore the probability of Compton scattering is correspondingly greater in bone than in tissue. As a result, Compton interactions also contribute to the formation of an image....

The significance of Compton's effect is that it shows that light cannot be explained solely as a wave phenomenon. As demonstrated in experiments, the classical theory of an electromagnetic wave cannot explain low-intensity wavelength shifts because that radiation must behave as particles to explain low-intensity Compton scattering. As shown in the figure above, because energy and momentum are conserved in this process, the electron cannot simply move in the direction of the incident photon. When electrons interact with high-energy photons, a portion of the energy is transferred to the electrons, allowing them to change their direction. The process can be repeated if the scattered photon still has enough energy, but as previously stated, the penetrating energy of this same photon is much lower, and the process can be repeated until the photon loses its penetrating energy.