According to the theory of special relativity, any particle in nature obeys the relativistic energy equation For example, how can we find the linear momentum or kinetic energy of a body whose mass is zero? This apparent paradox vanishes if we describe a photon as a relativistic particle. From the point of view of Newtonian classical mechanics, these two characteristics imply that a photon should not exist at all. In a vacuum, unlike a particle of matter that may vary its speed but cannot reach the speed of light, a photon travels at only one speed, which is exactly the speed of light. Unlike a particle of matter that is characterized by its rest mass m 0, m 0, a photon is massless. This idea proved useful for explaining the interactions of light with particles of matter. ![]() A beam of monochromatic light of wavelength λ λ (or equivalently, of frequency f) can be seen either as a classical wave or as a collection of photons that travel in a vacuum with one speed, c (the speed of light), and all carrying the same energy, E f = h f. Beyond 1905, Einstein went further to suggest that freely propagating electromagnetic waves consisted of photons that are particles of light in the same sense that electrons or other massive particles are particles of matter. Two of Einstein’s influential ideas introduced in 1905 were the theory of special relativity and the concept of a light quantum, which we now call a photon. Describe how experiments with X-rays confirm the particle nature of radiation.At beam energies above this, the Compton effect predominates.By the end of this section, you will be able to: 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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