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INTERACTION OF X RAYS WITH MATTER

INTERACTION OF X RAYS WITH MATTER. INTERACTION BETWEEN X-RAYS AND MATTER. Coherent Scattering Photoelectric effect Compton Scattering. COHERENT SCATTERING. 8 %. Classic scattering. Incident X-ray photon. Scattered X-ray photon. Low energy photons

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INTERACTION OF X RAYS WITH MATTER

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  1. INTERACTION OF X RAYS WITH MATTER

  2. INTERACTION BETWEEN X-RAYS AND MATTER Coherent Scattering Photoelectric effect Compton Scattering

  3. COHERENT SCATTERING 8 % Classic scattering Incident X-ray photon Scattered X-ray photon Low energy photons only scattering, no Ionization and only little energy loss

  4. COHERENT SCATTERING – Rayleigh (coherent) scatter interaction with entire atom . Photon changes direction (though usually by small angle) . “Elastic” scattering: Photon energy unchanged . Most important for “soft” (low energy) . and x rays (< 10 keV) . Adds very little to film fog

  5. COMPTON SCATTERING Compton effect or Compton scattering (C) incoherent scattering: Directly proportional to electron density Can occur with very low atomic weight targets even at relatively low X-ray energies 62 %

  6. COMPTON SCATTERING • An x-ray photon of lower energy is scattered from the • atom • Proportional to electron density of absorber • Interaction with outer electron • Changes direction • Adds to film fog 62 %

  7. COMPTON SCATTERING • Incident X-ray changes direction and loses energy, imparting that energy to the electron (Compton electron) • Compton electron interacts with other atoms producing secondary ionizations • Possess relatively low energy, produce low energy x- rays • Energies of 100 keV -- 10 MeV – absorption of radiation mainly due to the Compton effect

  8. PHOTOELECTRIC EFFECT Interaction with bound atomic electron . Incident photon disappears . Photon energy absorbed by electron . Momentum absorbed by atom . Probability of PE absorption increases for: » Low incident photon energy » High electron density in medium (mass density x Z) . Following absorption, photoelectrons, and “characteristic” x rays are emitted

  9. PHOTOELECTRIC EFFECT 30 % • Critical indiagnostic X rays • Ionization occurs • Binding energy is small for biologic tissues • Usually with - K shell electron • Recoil electron travels only short distance

  10. DIFFERENTIAL ABSORPTION Different tissues - different amount of absorption Makes radiographs possible Hard tissue - mostly absorbed Soft tissue - heterogeneous beam Mainly by photoelectric effect - varies with third power of Z Of absorber 6.5 times more in bone than water

  11. SECONDARY ELECTRONS • Electrons ejected by photoelectric effect or • Compton effect • Energy given up by: • Collisional ionization or excitation • Radiative Bremsstrahlung radiation low energy

  12. BEAM ATTENUATION Reduction in intensity while passing through mattter Either by photo electric effect or Compton scattering Half Value Layer - thickness of an absorber needed to reduce the number of x ray photon by half

  13. RADIATION DOSE The International System (SI) unit • Radioactivity • Exposure • Absorbed Dose • Dose Equivalent • Effective dose

  14. RADIATION DOSE Activity: rate at which the isotope decays. Radioactivity may be thought of as the volume of radiation produced in a given amount of time. International System (SI) unit - Becquerel (Bq) (curie (Ci) Exposure: measure of the strength of a radiation field at some point in air. measure made by a survey meter.  Gray Gy (Roentgen (R) Effective dose: Sum of product of equivqlent dose to each organ or tissue Sivert Si

  15. RADIATION DOSE Absorbed Dose:amount of energy that ionizing radiation imparts to a given mass of matter. SI unit gray (Gy), “rad” (Radiation Absorbed Dose)   1 rad - 0.01 Gy  Different materials that receive the same exposure may absorb different energy  Dose Equivalent:  relates the absorbed dose to the biological effect of that dose absorbed dose of specific radiation X "quality factor" = dose equivalent (SI unit - sievert (SV), (rem)" One rem = 0.01 SV  X- or Gamma radiation - the quality factor is 1

  16. Radiation quantities and units

  17. DOSIMETRY Quantity of radiation exposure Radiation dosimeters are devices which are capable of measuring an accumulated absorbed dose of ionizing radiation • issued four times a year • - rate measuring instruments and    - personal dose measuring instruments • Direct • Indirect. 

  18. DOSIMETERS Film Badges Ring Dosimeters

  19. DOSIMETERS Pocket Dosimeter - Direct Read Pocket Dosimeter Digital Electronic Dosimeter Ionization Counters Scintillation Counters Particle Track Devices The Cloud Chamber Geiger Muller Counter

  20. THERMOLUMINESCENT DOSIMETERS (TLDS) 15% for low doses 3% for high doses TLD - a phosphor, such as lithium fluoride (LiF) or calcium fluoride (CaF), in a solid crystal structure exposed to ionizing radiationradiation interaction ionization trapping of free electrons Heating the crystal Release of energy as light 1 millirem

  21. Thermoluminescence dosimeter T L material T L D reader EXPOSURE Equipment used to measure T L heated Emitted light proportonal to absorbed dose

  22. Parts of TLD reader • HEATER • PHOTOMULTIPLIER TUBE • HEAT FILTER ALOW THE LIGHT TO PASS • MEASURE THE EMMITED LIGHT • ELECTRONIC SYSTEM • (POWER SUPPLY)

  23. AVAILABILITY • TLD AVAILABLE IN • SOLID • POWDER • APPLICATION OF TLD • RADIOTHERAPY • RADIODIAGNOSIS • PERSONAL MONITORING

  24. P M TUBE FILTER TLD READER PROCEDURE TRAY HEATER

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