Lec5 X-Ray Interaction With Matter PDF

Summary

These lecture notes cover the interactions of X-rays with matter, focusing on the photoelectric effect, Compton effect, and pair production. The notes explain how these interactions differ based on photon energy and atomic number. They discuss applications of the photoelectric effect in medical imaging.

Full Transcript

Interaction of X-ray with matter v The interactions are classified on the basis of: A- Photon scattering B- Photon disappearance Compton scattering Photoelectric effect Pair production (1) Photoelectric effect Ø An incident photon encounter a K...

Interaction of X-ray with matter v The interactions are classified on the basis of: A- Photon scattering B- Photon disappearance Compton scattering Photoelectric effect Pair production (1) Photoelectric effect Ø An incident photon encounter a K-shell electron and ejects it from the orbital. Ø The photon disappear, giving up all of its energy to the electron. Ø The electron flies off into space as a photoelectron carrying the excess Characteristic X-ray energy as kinetic energy. K-edge vThe binding energy of a K electron in iodine is 33 keV, while that in lead is 88 keV. vWhen the energy of the x-ray is just slightly greater than the binding energy, the probability that the photoelectric effect will occur increases greatly, and this accounts for the sharp rises in the curve for iodine at 33 keV and in the curve for lead at 88 keV. These rises are called K-edges. vbone, muscle and fat have K-edge, but they are at such low energies (~ 6 keV for calcium). Probability of occurrence Directly proportional to atomic number Z3 Inversely proportional to the photon energy 1/ E3 N ot e v Photoelectric reactions are more likely to occur with low energy photons and elements with high atomic numbers (2) Compton effect Ø When an x-ray photon can collide with a loosely bound electron. This interaction take place at the outer shell electrons. Ø At the collision, the electron receives part of the energy and the remainder is given to a Compton (scattered) photon, which then travels in a direction different from that of the original x-ray. (3) Pair production Ø very energetic photon interact with the nucleus, the photon disappears and its energy is converted into matter in the form of two particles (electron and positron). v Positron: particle with the same mass of electron, but with positive charge. Note v This interaction cannot take place with photon energy less than 1.022 MeV. m mass of electron 9.11x 10-31 Kg Rest mass energy C velocity of light 3x 108 m/ sec v Rest mass energy of electron = 0.511 MeV v Rest mass energy of two electron = 2 x 0.511 = 1.022 MeV vPositron annihilation: positron combines with free electron and produce two photons (each with 0.511 MeV ejected in opposite direction). v 2 mass units are converted giving a total energy of 1.022 MeV. electron v To make further use of the photoelectric effect radiologists often inject high Z materials, or contrast media into different parts of the body v Compounds containing iodine are often injected into the bloodstream to show the arteries. vOily mist containing iodine is sometimes sprayed into the lungs to make the airways visible. vBarium compounds orally to see parts of the gastrointestinal tract (upper G1). vBarium enemas to view the other end of the digestive system (lower G1).

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