the interaction probability is greatest when the electron is very tightly bound, i.e.once the incoming X-ray photon has an energy greater than the binding energy, then the interaction probability is at a maximum and reduces approximately with the cubed root of the photon energy, E,.the incoming X-ray photon must have an energy greater than the binding energy of the inner shell so that it can eject a tightly-bound electron,.The probability of a Photoelectric Effect occurring is found to be dependent on three major considerations: 3.1: The Photoelectric Effect - (a) photon absorption and electron ejection and (b) fluorescent X-ray emission. The resultant vacancy in the K-shell can be filled by an electron from another shell and a K-fluorescent X-ray is generated from the transition - panel (b). The ejected electron is called a photoelectron. An incoming X-ray collides with, is totally absorbed by and ejects a K-shell electron, for instance, as in panel (a). Firstly, there’s the Photoelectric Effect - which is illustrated in Figure 3.1. Photoelectric Effect There’s basically three major processes we need consider. In addition, excitations can occur where electrons in the material are raised to higher energy levels. We will see that the energy loss by X-ray photons in different materials is characterised by ionizations. Instead of considering gross electron behavior as in the Energy Band Theory, we will return in this chapter to considering what happens at the level of an individual atom. The attenuation of X-rays by materials used for radiation detection has been discussed in an earlier chapter. At beam energies above this, the Compton effect predominates.Interaction Processes The dependence of photoelectric absorption on Z and E means that it is the major contributor to beam attenuation up to approximately 30 keV when human tissues (Z = 7.4) are irradiated. Photoelectric absorption is also utilized in mammography and when using contrast agents to improve image contrast. This has practical implications in the field of radiation protection and is the reason why materials with a high Z such as lead (Z = 82) are useful shielding materials 3. Small changes in Z and E can therefore significantly affect photoelectric absorption. Therefore if Z doubles, photoelectric absorption will increase by a factor of 8 (2³ = 8), and if E doubles photoelectric absorption will reduce by a factor of 8. Thus the overall the probability of photoelectric absorption can be summarized as follows: The probability of photoelectric effect rapidly approaches zeo at incident photon beam energy of 140keV 4. Proportional to the physical density of the attenuating medium (p) Inversely proportional to the cube of the energy of the incident photon (E), and Proportional to the cube of atomic number of the attenuating medium (Z), and The probability of photoelectric absorption occurring is The energy which is lost by this electron as it drops to the inner shell is emitted as characteristic radiation (an x-ray photon) or as an Auger electron. To stabilize the atom an outer shell electron fills the vacancy in the inner shell. Hence, the photoelectric effect contributes to the attenuation of the x-ray beam as it passes through matter. The electron that is removed is then called a photoelectron and the incident photon is completely absorbed in the process. The electron is tightly bound (as in K shell) 4 The energy of the incident photon is equal to or just greater than the binding energy of the electron in its shell ( K-absorption edge) and The probability of this effect is maximum when: photoelectric absorption, is one of the principal forms of interaction of x-ray and gamma photons with matter. A photon interacts with the inner shell electron of the atom and removes it from its shell.
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