Lesson10a_1 - Lesson 10a Interaction of Photons with Matter...

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Lesson 10a Interaction of Photons with Matter MP200 Radiation Physics - 2010 Duke Medical Physics Graduate Program 1
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Introduction Photon interaction with matter is either with the nuclei or the orbital electrons of atoms in the medium. There are five types of photon interactions with matter, which must be important in radiological physics.. Compton Effect Pair production Rayleigh scattering Photo-nuclear interaction Photoelectric effect If the photon interacts directly with the nuclei, it is called photo- disintegration (photonuclear). The interaction of the photon and the electrostatic field of the nucleus is called the pair production. Photon interactions with the orbital electrons of the atoms are charac- terized as follows. Interaction with loosely bounded electrons ( hν >>> E b ). This interaction is considered to be an interaction between a photon and a free electron: Thomson scattering; Compton effect; triplet production; Interaction with tightly bound electron ( E b ). This interac- tion is considered to be an interaction between a photon and the atom as a whole. 2
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photoelectric effect; Photon’s fate after the interaction Photon disappears (or absorbed completely); part of its energy is transferred to charged particles ( electrons or positrons) Photon scatters ( two situations ); The resulting photon has the same energy as the incident pho- ton (Rayleigh scattering); The resulting photon has a lower energy than the incident photon and the energy excess is transferred to an electron; The electrons produced in the medium through photon interactions will; deposit their energy to the medium through Coulomb interactions with orbital electrons in the medium ( ionization loss ); radiate their kinetic energy through Coulomb interaction with nu- clei in the medium ( radiative loss or bremsstrahlung); In Photoelectric, Compton and pair production interactions, photons transfer energy to the electrons which then impart energy to the matter. Rayleigh scattering is elastic (no loss of energy). Photonuclear interactions are significant at very high energy. The relative importance of these interactions depends on the both the photon energy and the atomic number Z , of the medium. 3
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Figure 1: Relative importance of photon interactions. The photoelectric effect is dominant at lower photon energies, the Compton effect takes over at medium energies, and pair production at the higher energies. For low Z media ( carbon, air, water, tissue etc.) the region of Compton effect dominance is very broad, extending from 20 keV to 30 MeV . This will narrow with increasing Z. Compton Effect The scattering of a photon with a loosely bound (free) orbital electron and the appearance of a scattered photon at a longer wave length is called the Compton effect.
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