6720 Lecture 3_interaction

6720 Lecture 3_interaction - Lecture III 1. 2. 3. 4. 5....

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1 PHYS 6720 - Lecture III 1 Lecture III Radiation-matter interaction 1. Cross sections 2. Air ionization and exposure 3. Absorbed dose 4. Radiation attenuation in matters 5. Energy absorption and transfer PHYS 6720 - Lecture III 2 III-1 Cross sections General features of radiation-matter interaction Æ Radiation-matter interaction is based on the interaction between the electromagnetic field and the charged particles of electrons (higher probability for their smaller masses) and protons. Æ This interaction can only be understood through quantum mechanics. Æ For optical radiation with photons of ~1eV energy, the electrons in atoms and molecules can be forced to make transitions among different quantum states, but all remains bound to the atoms and molecules. PHYS 6720 - Lecture III 3 III-1 Cross sections General features of radiation-matter interaction Æ For high energy photons (E>10keV) of medical x-ray and γ -ray, ionization of bound electrons from atoms is inevitable through either absorption or inelastic scattering. Æ Because of their high energy, photons interact with atoms in a form of cascading events: photon+atoms Æ primary ions+electrons Æ secondary ions+electrons … Æ As a result, the x-ray/ γ -ray photon interaction with matter consists of long tracks along which photon energy is transferred in small amount to electrons and ions for many times. PHYS 6720 - Lecture III 4 III-1 Cross sections General features of radiation-matter interaction For high energy photons (10keV<E<10MeV), radiation-matter interaction can be classified in four major types, each is described by a cross section : Æ (1) photoelectric absorption: photons are absorbed to ionize and energize electrons from atoms Æ (2) incoherent scattering: photon “changes” direction with reduced energy Æ (3) coherent scattering: photon “changes” direction with unchanged energy Æ (4) pair-production: generation of electron-positron pair as the photon enters the field of an atomic nucleus PHYS 6720 - Lecture III 5 III-1 Cross sections Cross sections - definitions Æ differential scattering cross-section σ s ( θ,φ ): σ s ( θ,φ )= scattered power at ( , ) direction total incident power per area θφ θ incident radiation scattered radiation φ z x y PHYS 6720 - Lecture III 6 III-1 Cross sections Cross sections - definitions Æ differential scattering cross-section σ s ( θ,φ ): If the wave-scatterer interaction posses axial symmetry, then σ s ( θ,φ )= σ s ( θ ) Example: for coherent (Rayleigh) Scattering, we have σ s ( θ )= σ soh (E, θ )= where r e =2.8x10 -15 (m) is the classical radius of the electron, x=sin( θ /2)/ λ is the momentum transfer variable, E and λ are photon energy and wavelength, Z is the atomic number (or number of elementary charges in the nucleus). F is called the atomic form factor and depends on Z as Z n with 1.0 n 1.5.
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This note was uploaded on 04/25/2010 for the course PHYS 6720 taught by Professor Hu during the Spring '10 term at East Carolina University .

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6720 Lecture 3_interaction - Lecture III 1. 2. 3. 4. 5....

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