What type of Photon use in Compton Scattering Radiology?

Compton scattering is known as an interaction between a free electron at rest and a high-energy photon (which is typically an x-ray or gamma-ray). Compton (1923) presented the first semi-quantum-mechanical treatment of such an interaction in 1923. In contrast, inverse Compton scattering (ICS) is known as an interaction between a high-energy electron and a low-energy photon. In such scattering, part of the energy of the electron is transferred to the scattering photon. In 1948, Feenberg and Primakoff (1948) first suggested the kinematic formulas for ICS and revealed that gamma-rays emitted from the vicinity of the Sun are produced by the interaction between the cosmic ray electrons and the Sun's light.

The collision between a photon and an atomic electron
The Compton effect occurs for most of the atomic electrons. A gamma photon plays the role of a projectile that collides with an electron in an atom that serves as a target. Gamma was represented as a punctual particle because of its very short wavelength at the atomic scale. As the vast majority of electrons possess smaller energy than the one of gamma, physicists are accustomed to neglecting it and considering the electron as a target at rest. In the collision, the electron is put in motion at a certain angle, while the gamma scattered with another angle loses its energy.
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Compton collisions can be viewed as elastic collisions between a photon and an electron. These elastic collisions become predominant when the photon energy becomes large compared to the energy that holds the electron in an atom, its binding energy. For a light atom such as carbon, the Compton effect prevails on the photoelectric effect above 20 keV. For copper, it is above 130 keV, and for lead 600 keV.

In this gamma energy range, which is rather extended, the phenomenon concerns all the electrons of the atom, whereas in the photoelectric effect these are the two electrons from the innermost K shell, which play a role. For an absorber, it is the density of electrons that is crucial in the range where the Compton effect dominates. Lead has thus also an advantage over lighter materials, even if less important than for the photoelectric effect, which came at the fourth power of the high electrical charge of its nucleus.

Unequal energy sharing
The energy sharing between the gamma photon and the electron that has served as a target depends on the gamma scattering angle. The most likely case is the one in which the photon is not scattered and do not lose power. The rarest case is the one where the gamma bounces backwards propelling the electron forward. On average, the photon transfers only a small portion of its energy to the electron and undergoes a significant shift in direction (the values in the figure have been calculated for the energy of 500 keV).
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The gamma is not destroyed in the collision. The photon that emerges with the electron, called "scattered" photon shares the initial energy with the electron set in motion. The electron then loses its energy by ionization as a beta electron. The scattered gamma propagates through the material without depositing energy until it interacts again.

The energy sharing is unequal. It depends on the angle between the scattered photon and the initial gamma (the gamma probability distribution at a given angle is given by a formula called the "Klein-Nishima formula"). Despite its extremely light mass, the electron is indeed a heavy target for a photon that is massless. The laws of physics governing the Compton effect are such as the scattered photon carries most of the initial energy: 96% on average at 50 keV, 83% at 500 keV.

The scattered photon emerges usually in a different direction than the incident photon one. It can even go backward ( backscattering). On average it scatters with an angle of 30 to 45 degrees. Gamma of hundreds of keV can undergo multiple Compton scattering before being absorbed by the photoelectric effect.

When the gamma energy exceeds 1 MeV, which is rarely the case for the gamma rays emitted by nuclei, Compton scattering begins to be challenged by a new phenomenon: the transformation of a gamma into an electron and its antiparticle, a positron. This phenomenon becomes prominent with the high-energy gamma produced by example with particle accelerators.