Compton Scattering Application 1

Radiation Therapy

An application of Compton scattering can be seen from the theme of radiobiology where it is applied in radiation therapy. This happens due to it being the most probable interaction of high energy X-rays with atomic nuclei in living beings. It also produces high-energy photon beams, peaked at the high energy and having small divergence which is preferable in radiotherapy. 

Compton backscattering, in this case, is an alternate method of photon production for cancer treatment. It is evident that the photons scattered in the direction opposite to the direction of the initial photon (backscattered) will have the energy desired for photon beam therapy. The output of Compton backscattering is a high-energy photon beam (gamma-ray beam), which is well collimated and has minimal low-energy components. Such beams may be used for conventional high-energy photon treatments, production of radionuclides, and generation of positrons and neutrons.

This process is basically Compton scattering. Here, we specifically consider the scattering of a low-energy photon by a high-energy (ultrarelativistic) electron. A relativistic description of the process shows that a high-energy photon beam wherein the most probable photons have high energy is the result of this process. Furthermore, the beam will be spatially highly focused. The interaction process that one considers is simply electron-photon scattering. In the laboratory frame of reference, this concept mainly revolves around how an electron of energy collides “head-to-head” with a photon of energy where either one of them is at rest while the other is moving or both are moving. This is attainable by using free electron lasers. The photon momentum might be directed towards the negative z-direction. It will also then have its energy multiplied. 

The collision itself is then understood as Compton scattering. The special case of completely backscattered photons is considered first as the energy will be reduced due to electron recoil. Said recoil plays an important role only for low-energy electron beams and/or high-energy photons (~0.5 MeV in the rest frame of the electron). Since practical production applications could involve the use of polarized photons for the collision process, head-to-head (p and k antiparallel) collisions of photons and electrons would be preferred.

Compared to bremsstrahlung, the beam spectrum of Compton is completely different. The backscattered Compton beam energy distribution is weighted to the high end of the spectrum, whereas the bremsstrahlung beam is weighted to the low-energy end. The completely backscattered photon energy rises linearly with an increase in incident energy as compared to bremsstrahlung which relative number of protons increase abruptly as a small amount of energy is applied then gradually decrease as more energy is supplied. It is also evident that the usage of Compton scattering concept will lead to intrinsic narrowness that is accentuated by the increase in electron energy. 

In actual practice, one must produce a therapeutically useful output. In other words, the flux of photons must be sufficiently high enough to treat patients in minutes. These beams will allow that as it is contained and is peaked on the central axis, making it not only release sufficient useful output yet also released in pencil-like beams regardless of the treatment distance which helps in targeting one specific location. This eases the process as field shaping materials would then be unnecessary. Treatment of larger volumes can be achieved either by electronically scanning the pencil beam by varying the interaction point and/or by mechanical scanning of the immobilized patient in the beam. Also to be mentioned, the dose profile of Compton scattering at the depth of maximum dose reflects sharp intensity as the dose distribution spreading with depth is minimal. 

It is important to take note of Compton scattering as it is responsible for a majority of occupational worker exposure to radiation. The advantages Compton backscattering have over regular use of bremsstrahlung includes the continuously tunable output energy and the less divergence from the point of interaction is produced compared to bremsstrahlung. It is also to be highlighted that large energy photons are used, thus highly penetrating photon beams may be utilized without the complication of air and tissue activation and neutron contamination.