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2 Radiation Physics Relevant to Advanced Imaging Technology
Pages 27-32

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From page 27...
... This chapter summarizes the physics principles and the associated transfer of energy to the passenger that enable image formation. Additional information, including details of these processes, can be found in radiological physics and health physics texts.1 THE PHYSICS OF X-RAY ABSORPTION X-ray backscatter AIT uses a narrow beam of X-ray photons with energies, hv, less than 100 keV.2 There are five basic interactions that can occur as X rays penetrate material.
From page 28...
... Because no scattered photon is generated, the photoelectric effect does not contribute photons to a backscatter image, but its contribution to energy absorbed in the material is significant. Furthermore, because the probability of a photoelec tric interaction increases rapidly with the atomic number, Z, of the target atom, photons are more likely to be absorbed than scattered by high Z materials such as metals.
From page 29...
... The energy of the electron is dissipated in the material, whereas the scattered photon either escapes or interacts in the material. While it is unlikely that an inci dent photon will backscatter at exactly 180°, several Compton scattering events can occur, resulting in the photon emerging in a backward direction or terminating in a photoelectric event.
From page 30...
... Hence, ­ either n Thompson nor Rayleigh scattering make important contributions either to back scatter image formation or to the deposition of energy in the scanned object. BEAM ATTENUATION AND DEPOSITED ENERGY If it were possible to use monochromatic 50 keV photons for backscatter screening, it would be relatively easy to estimate the intensity of backscattered ra diation reaching the detector and the energy deposited in the scanned person.
From page 31...
... The photons at very low energies are preferentially removed by absorption either in the tungsten anode or in the glass or metal body of the tube and any purposely inserted exter nal filtration. The typical output spectrum peaks at around 30 keV, as shown in Figure 2.2.6 However, the details of the spectrum depend on the atomic number of the anode material, the amount of anode material that the photons must traverse before exiting the tube (e.g., as a function of the exit angle)
From page 32...
... But electron ranges are typically a small fraction of the mean penetration depths of photons, so the location where the incoming energy is deposited is determined by the photons. For example, the mean free path of a 30 keV photon in water is 2.7 cm, three orders of magnitude larger than the 18 µm range of a 30 keV electron in the same material.


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