T.O. 33B-1-16-38Figure 6-15. Illustration of Various Radiation Absorption Interactions.6.5.1.1 PhotoelectricAbsorption.When the photons have energies of 100 keV or less, they are readily absorbed by the electrons in the orbital shells ofthe atoms of the absorber. The energy of the photon is transferred to the electron; often dislodging it from its orbit andthe remainder of the energy in the photon is used to give the electron kinetic energy or velocity. These ejected electronsare called photoelectrons and the process is known as photoelectric absorption. The moving electrons then lose theirenergy, producing ion pairs. It should be noted that the radiation photon has given up all of its energy and no longerexists. This mechanism of absorption has a very high probability for very low energy radiation and accounts for themajor absorption of radiation when photon energies are 100 keV and less.6.5.1.2 ComptonEffect.When the photon energies are in the 100 keV to 10 MeV range, all of the energy is not required to dislodge an orbitalelectron and accelerate it by induction of kinetic energy. In this case the photoabsorption can occur, but the photoncontinues at some different path and at a reduced energy level, due to the loss of energy to the electron. By thismechanism of absorption, the path of the photon is altered and its energy decreased. This mechanism of absorption isreferred to as Compton effect. Compton effect accounts for the major absorption of radiation in the energy rangebetween 100 keV and 10 MeV.6.5.1.3 PairProduction.When photon energies exceed 1.02 MeV, their energy can cause pair production. In this event the high-energy photonis disintegrated by the nuclear field surrounding the nucleus of the atom. The energy of the photon converts into anelectron-positron pair. The positron has the same mass as an electron and is of equal, but opposite charge. It may benoted that in this absorption mode, the energy of the massless photon is converted to mass. Einstein's equation statesthat energy equals mass times the square of the velocity of light (E = mc2). If this equation is used it can be found that
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business
