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LECTURE 6: INTERACTION OF RADIATION WITH MATTER All radiation is detected through its interaction with matter! INTRODUCTION: What happens when radiation passes through matter? Interlude – The concept of cross-section For a thin target: ) )( )( ( t I N N = # of interactions in the target per unit time I = # of incident particles per unit time = cross-section or interaction probability. It is usually expressed in cm 2 . t = target thickness expressed in # of target nuclei/cm 2 1 barn = 1 b = 1x10 -24 cm 2 Exercise A target of 197 Au 52 nm thick is bombarded by a 20 MeV proton beam with an intensity of 1 x 10 8 p/s. If the total reaction cross-section is 1.5 b, what is the number of interactions occurring in the target per second? The density of Au is 19.3 g/cm 3 . How does the total cross-section compare with the geometric cross-section of the 197 Au nucleus? I

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III. Electromagnetic Radiation -- Photons A. Sources: Electromagnetic Spectrum 1. Rearrangement of nuclear orbitals: -rays 2. Rearrangement of atomic and molecular orbitals: x-rays, uv … 3. Annihilation radiation ; e.g., e + e two 0.511 MeV s 4. Bremsstrahlung: electron deceleration 5. Cosmic ray showers B. Interactions 1. Photon: Carriers of Electromagnetic force must interact with electric charge M e d i u m : a. electrons b. protons in nucleus 2. Mechanisms a. Photoelectric Effect: E b. Compton Scattering: E c. Pair Production: E C. Photoelectric Effect 1. Mechanism: Photon is completely absorbed by a charged particle; all e n e r g y E is transferred to an atomic electron, which is ejected from the atom 2. ONE COLLISION STOPS PHOTON e (photoelectron); monoenergetic - e most probable – size argument again photon disappears e e E
E e = E E B ( n ) ; i.e., electron is monoenergetic where E B (n ) is electron binding energy

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