Ionizing Radiation : Any electromagnetic or particulate radiation capable of producing ion pairs by interaction with matter. Scope limited to X and gamma rays, alpha particles, beta particles (electrons), neutrons , and charged nuclei.
Arthur Holly Compton Compton won the Nobel in 1927 for the discovery of the Compton Effect, the increase in the wavelengths of X rays and gamma rays when they collide with and are scattered from loosely bound electrons in matter
Applications This implies that as the photon energy increases there is a corresponding increase in the forward scatter of the beam. This results in better dose distribution. Direction of the scatter depends on the energy of the incident photon beam This means that higher beam energies allow greater absorption of the dose in the body with less scattering of energy. Thus with increasing photon energy greater absorption occurs relative to attenuation. The fraction of the energy imparted to the recoil electron increases as the beam energy increases
Thus concrete is as good as lead in shielding of megavoltage equipment!
The absorption in bones doesn't exceed that produced in the soft tissues – unlike in PE effect seen in orthovoltage radiation era.
There is no Bone shielding phenomenon unlike that seen in orthovoltage radiation.
Port films produced in megavoltage equipment have very little detail.
Attenuation doesn't depend on the atomic number
This has several important implications in designing radiation protection. The maximum energy of photons with 90° scatter is 0.511 MeV while that for 180° scatter ( i.e.. Back scatter) is 0.255 MeV . The energy of the photons scattered at angles <90 ° will be more than .511 MeV and will gradually approach the incident photon energy Energy of the scattered radiation is independent of the incident beam energy
1. we cant increase the dose energy of beam as beyond 10MV pair production is dominant
2. it can directly cause photonuclear reaction by ejecting a nucleon(p or n)
3. PET scan( 2 FDG emits positrons as it is a radioisotope)
Applications This leads to dosimetric inaccuracies when using air containing ion chambers. Polarization in heavier atomic weight elements. This leads to a “smudging” of the Bragg's peak which is not seen in electrons. The low mass of the electron leads to greater scattering. This is of practical importance as radioactive isotopes which are produce high energy beta radiation are better stored in low atomic number materials e.g. plastics as they will lead to lesser bremsstrahlung radiation. Also higher atomic number elements are better for x ray production. The amount of radiative loss is proportional to the square of the atomic number of the material This leads to the phenomenon of greater ionization in soft tissues relative to bones. Ionization and excitation are more for low atomic materials
The total mass attenuation coefficient is the sum of three individual coefficients; photoelectric coefficient, mass scattering coefficient and pair production coefficient:
( μ / ρ ) = ( τ / ρ )+( σ / ρ )+( π / ρ )
When we plot the total coefficient versus the photon energy, in different media, the following effects are seen:
At low energies the mass attenuation coefficient is larger, especially in high atomic number media, because of the predominance of photoelectric interactions in these circumstances.
That attenuation coefficient then decreases rapidly with the energy till the photon energy far exceeds the electron binding energy and Compton effect becomes the predominant mode of interaction. In between the ranges of 200 KeV- 4 MeV, Compton scattering is the predominant mode of interaction.
At this energy range, the mass attenuation coefficients also become independent of the atomic number and actually become more for soft tissues, which have more hydrogen content.
Beyond 4 MeV pair production results in increasing mass attenuation coefficients specially for high atomic number elements.
Thus very high-energy radiations (> 20 MeV) are less-penetrating than some lower energy radiations and are not used in radiotherapy!!