design of phantom and metallic implants for 3d - ePrints@USM

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2.1.3 PAIR PRODUCTION Pair production is a direct conversion of radiant energy to matter. For pair production to occur, the electromagnetic energy, in a discrete quantity called a photon, must have energy greater than 1.02 MeV strikes a material of high Z, it is found that the photon is completely absorbed and a pair of electron and positron is produced (Figure 2.3). Figure 2.3 Kinematics of pair production process. (Taken from Sprawls Educational Foundation) The threshold energy of the photon is 1.02 MeV. The conversion of energy yields as shown in Eq. 2.7. The presence of the nucleus is essential for the conversion of linear momentum. hν = 2m0c 2 + E+ + E- + Enuc where hν = The incident photon energy 2m0c 2 = The rest mass of the electron-positron pair E+ = The kinetic energy of positron E+ = The kinetic energy of electron Enuc = The kinetic energy of nucleus 14 2.7
2.1.4 RAYLEIGH (COHERENT) SCATTERING Rayleigh scattering is called coherent because the photon is scattered by the combined action of the whole atom. The event is elastic in the sense that the photon loses essentially none of its energy; the atom moves just enough to conserve momentum. The photon is usually redirected through only a small angle. Therefore the effect on a photon beam can only be detected in narrow-beam geometry. Rayleigh scattering has more practical importance at low energies, partly because the scattering angle is greater. The relative importance of this process is seen to be fairly small, as it contributes only a few percent or less of the narrow-beam attenuation coefficient. 2.1.5 PHOTONUCLEAR INTERACTIONS Photonuclear reactions are one of the methods for generating radionuclide. These reactions require minimum photon energy of about 2 MeV and they are not significant in most elements until photon energies exceed about 10 MeV. Also, even at these energies, the probability of photonuclear reactions is much smaller than that of Compton scattering or pair production. Furthermore, the effective total sum of attenuation coefficients is the total sum of attenuation coefficient of photoelectric absorption and Compton scattering. Therefore, photonuclear reactions are of no practical importance in terms of photon beam attenuation or the transfer of photon energy to matter. 15
Page 1 and 2: DESIGN AND FABRICATION OF MULTIPURP
Page 3 and 4: ACKNOWLEDGEMENTS With the name of A
Page 5 and 6: CONTENTS iv PAGE Acknowledgements i
Page 7 and 8: CHAPTER 4: METHODOLOGY OF PREPARATI
Page 9 and 10: CHAPTER 5: RESULTS AND DISCUSSIONS
Page 11 and 12: Table 5.9 Linear attenuation coeffi
Page 13 and 14: Figure 4.3 Cylindrical shaped of PT
Page 15 and 16: Figure 4.37 A cylindrical shaped al
Page 17 and 18: Figure 5.18 Experimental values of
Page 19 and 20: Figure 5.54 Two-dimensional PET sin
Page 21 and 22: eFOV Extended Field of View CTAC Co
Page 23 and 24: Name Definition Z Atomic number LIS
Page 25 and 26: Ci Curie mCi Milicurie γ Gamma Gy
Page 27 and 28: LIST OF APPENDICES Figure A.1 Conne
Page 29 and 30: DESIGN AND FABRICATION OF MULTIPURP
Page 31 and 32: 1.1 BACKGROUND CHAPTER 1: INTRODUCT
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Page 39 and 40: 1.5 THESIS ORGANISATION In this sec
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Page 43: 2.1.2 COMPTON SCATTERING Compton sc
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Page 49 and 50: 2.3.3 EXPONENTIAL DECAY With the pa
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