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1 interaction of radiation with matter


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1 interaction of radiation with matter

  2. 2. Introduction Lecture 5  Manner of interactions  Nature of Materials  Nature of Incident particle  Interactions are probabilities  Noble Prize
  3. 3. Types and sources of ionizing radiation Lecture 5 The important types of ionizing radiation  -rays  X-rays  Fast Electrons  Heavy Charged Particles  Neutrons
  4. 4. -rays Lecture 04  Electromagnetic radiation emitted from a nucleus or in annihilation reactions between matter and antimatter.  Practical range of photon energies emitted by radioactive atoms extends from 2.6 keV to 7.1 MeV
  5. 5. X-rays Lecture 04  Electromagnetic radiation emitted by charged particles (usually electrons) in changing atomic energy levels (called characteristic or fluorescence x-rays)  or in slowing down in a Coulomb field (continuous or bremsstrahlung x-rays).
  6. 6. X-rays Lecture 04 20 – 120 kV Diagnostic-range x-rays 120 – 300 kV Orthovoltage x-rays 300 kV – 1 MV Intermediate-energy x-rays 1 MV upward Megavoltage x-rays
  7. 7. X-rays Lecture 04 Do you know the name of x-rays of range 0.1 – 20 kV. Soft x-rays or Grenz rays
  8. 8. Fast Electrons Lecture 04 Do you know the name of electrons if they result from a charged- particle collision. Delta Rays “-rays”
  9. 9. Fast Electrons Lecture 04  Positrons if positive in charge  If emitted from a nucleus they are usually referred to as -rays (positive or negative)  Intense continuous beams of electrons up to 12 MeV available from Van de Graff generators.
  10. 10. Heavy Charged Particles Lecture 04  Acceleration by a Coulomb force field.  Alpha particles also emitted by some radioactive nuclei.  Proton – the hydrogen nucleus  Deuteron – the deuterium nucleus  Triton – a proton and two neutrons bound by nuclear force  Alpha particle – the helium nucleus
  11. 11. Heavy Charged Particles Lecture 04 Do you know the name of some other heavy charged particles Pions – negative -mesons
  12. 12. Neutrons Lecture 04  Neutral particles obtained from nuclear reactions [e.g., (p, n) or fission].  They cannot themselves be accelerated electrostatically
  13. 13. ICRU Terminology Lecture 04 Directly ionizing radiation Fast charged particles Deliver their energy to matter directly, through many small Coulomb- force interactions along the particle’s track
  14. 14. ICRU Terminology Lecture 04 Indirectly ionizing radiation  X- or -ray photons or neutrons.  First transfer their energy to charged particles.  Resulting fast charged particles then in turn deliver the energy to the matter as above
  15. 15. Specific Ionization Lecture 04  Number of primary and secondary ion pairs produced per unit length of charged particle’s path is called specific ionization.  Expressed in ion pairs (IP)/mm.
  16. 16. Specific Ionization Lecture 04  Increases with electrical charge of particle.  Decreases with incident particle velocity.  ~ 7000 IP/mm in air by Alpha particle.
  17. 17. Specific Ionization Lecture 04 Do you know the medical usage of specific ionization? Radiotherapy
  18. 18. Specific Ionization Lecture 04 7.69 MeV Alpha Particle from Polonium 214.
  19. 19. Charged Particle Tracks Lecture 03 Do you know the difference between “Range” and “Path Length” for ionizing particles? Path length is actual distance particle travels whereas range is actual depth of penetration in matter
  20. 20. Charged Particle Tracks Lecture 03  Electrons follow tortuous paths in matter as the result of multiple scattering events.  Ionization track is thin and non-uniform
  21. 21. Charged Particle Tracks Lecture 03  Larger mass of heavy charged particle results in dense and usually linear ionization track.
  22. 22. Linear Energy Transfer Lecture 04  Amount of energy deposited per unit path length is called the linear energy transfer (LET).  Expressed in units of eV/cm.  LET is the product of specific ionization (IP/cm) and the average energy deposited per ion pair (ev/IP).
  23. 23. Linear Energy Transfer Lecture 04 Radiation LET (keV/ um) 1 MeV Gamma Rays 0.5 100 kVp X-rays 6 20 keV β-particles 10 5 MeV neutrons 20 5 MeV α-particles 50 1 MeV Electron 0.2 100 keV Electron 0.3
  24. 24. Linear Energy Transfer Lecture 04 Can you establish a relation between LET and biological damage? High LET Radiation (alpha particles, protons etc)are more dangerous to tissue than low LET radiation (gamma, X-rays, electrons)
  25. 25. Linear Energy Transfer Lecture 04 What is the probability of charged particle passing through a medium without interaction. ZERO
  26. 26. Linear Energy Transfer Lecture 04 What is the probability of charged particle passing through a medium without interaction. ZERO
  27. 27. Scattering Lecture 04  Interaction resulting in the deflection of a particle or photon from its original trajectory.  Elastic : Billiard Ball  In-elastic
  28. 28. Scattering Lecture 04 Is there any effect of scattering on image quality. bone air soft tissue bone primary diaphragm film, fluorescent screen or image intensifier primary radiological image intensity at detector scattered radiation grid Image taken from: Johns & Cunningham, The Physics of Radiology, 4th Edition
  29. 29. Scattering Lecture 04 Veil over image. Add patient dose.
  30. 30. Particle interactions Lecture 04  Energetic charged particles interact with matter by electrical forces and lose kinetic energy via:  Excitation  Radiative losses  Ionization  ~ 70% of charged particle energy deposition leads to nonionizing excitation
  31. 31. Particle interactions Lecture 04 A: Excitation (left) and de-excitation on (right) B: Ionization and the production of delta rays
  32. 32. INTERACTIONS Lecture 04 All Interactions are Probabilities. Different passengers in a car; faces different level of injuries or enjoy different grades of safety in the situation of an accident.
  33. 33. PHOTON INTERACTIONS Lecture 04  Chance of event happening  Relative predictions can be made,  energy of photons  type of matter with which photons are gong to interact
  34. 34. Radiative Interactions-Bremsstrahlung Lecture 04 Path of the electron is deflected / de- accelerated by the positively charged nucleus.
  35. 35. Radiative Interactions-Bremsstrahlung Lecture 04  Angle of emission changes with the incident electron energy.  Probability of production is ~ Z2 of the absorber.  Energy emission varies inversely with the square of the mass.  Protons and alpha particles produce less than one-millionth the amount of bremsstrahlung radiation as electrons of the same energy.
  36. 36. Radiative Interactions-Bremsstrahlung Lecture 04 Disadvantages:  two edged sword  It is not especially useful for therapeutic  Bremsstrahlung produced by a beta electron is more harmful to the technologist than the beta particle that produces it, because of the penetrability of an electromagnetic ray.
  37. 37. Radiative Interactions-Bremsstrahlung Lecture 04 Ratio of electron energy loss by bremsstrahlung production to that lost by excitation and ionization, kinetic energy of incident electron * atomic number 820
  38. 38. Radiative Interactions-Characteristics Lecture 04  If a high speed beta particle approaches an electron in an inner orbital.  Ejected from the atom.
  39. 39. Neutron interactions Lecture 04  Don’t interact with electrons.  Don’t create direct ionization.  They do interact with atomic nuclei, sometimes liberating charged particle.  Neutrons may also be captured by atomic nuclei- Retention of the neutron converts the atom to a different nuclide (stable or radioactive)
  40. 40. Rayleigh Scattering Lecture 04  Incident photon interacts with and excites the total atom as opposed to individual electrons.
  41. 41. Rayleigh Scattering Lecture 04 Electric field of the incident photon’s electromagnetic wave expands energy. Causing all of the electrons in the scattering atom to oscillate in phase. Atom’s electron cloud immediately radiates this energy. Emitting photon of same energy but slightly different direction.
  42. 42. Rayleigh Scattering Lecture 04  Occurs mainly with very low energy diagnostic x-rays, as used in mammography (15 to 30 keV)  Less than 5% of interactions in soft tissue above 70 keV; at most only 12% at ~30 keV.
  43. 43. Rayleigh Scattering Lecture 04
  44. 44. Compton Scattering Lecture 04  Predominant interaction in the diagnostic energy range with soft tissue.  Predominate Energy Region: 26 keV to 30 MeV.  Most likely to occur between photons and outer (“valence”) shell electrons.
  45. 45. Compton Scattering Lecture 04  Electron ejected from the atom.  Binding energy comparatively small and can be ignored.  Photon scattered with reduction in energy.
  46. 46. Compton Scattering Lecture 04  Energy of scattered photon can be calculated by, )cos1(1 2 0 0 0 0     cm E E E EEE sc esc
  47. 47. Compton Scattering Lecture 04  Ionization of the atom.  Ejected electron lose K.E. by excitation and ionization of atoms in the surrounding material.
  48. 48. Compton Scattering Lecture 04  As incident photon energy increases, scattered photons and electrons are scattered more toward the forward direction.
  49. 49. Compton Scattering Lecture 04  In diagnostic imaging (18 to 150 keV), the majority of the incident photon energy is transferred to the scattered photon.  In x-ray transmission imaging, these photons are much more likely to be detected by the image receptor.  Thus reducing image contrast.
  50. 50. Compton Scattering Lecture 04  Probability of interaction increases as incident photon energy increases.  probability also per atom of the absorber depends on the number of electrons available as scattering targets and therefore increases linearly with Z.
  51. 51. Compton Scattering Lecture 04  Laws of conservation of energy and momentum place limits on both scattering angle and energy transfer.  Energy of the scattered electron is usually absorbed near the scattering site
  52. 52. Compton Scattering Lecture 04 Do you know the angles of maximum energy transfer and scattering of Compton Electron. Maximal energy transfer 180-degree- photon backscatter Maximal Scattering angle is 90 degrees
  53. 53. PHOTOELECTRIC EFFECT / ABSORPTION Lecture 04  All of the incident photon energy is transferred to an electron, which is ejected from the atom.  Kinetic energy of ejected photoelectron (Ec) is equal to incident photon energy (E0) minus the binding energy of the orbital electron (Eb). Ec = Eo - Eb
  54. 54. Photoelectric absorption Lecture 04 Incident photon energy must be greater than or equal to the binding energy of the ejected photon.
  55. 55. The Photoelectric Effect / Absorption Lecture 04  Atom is ionized, with an inner shell vacancy.  Electron cascade from outer to inner shells- Characteristic x-rays or Auger electrons
  56. 56. The Photoelectric Effect / Absorption Lecture 04  Probability of characteristic x-ray emission decreases as Z decreases  Does not occur frequently for diagnostic energy photon interactions in soft tissue
  57. 57. The Photoelectric Effect / Absorption Lecture 04 Most likely to occur • With inner-shell electrons • With tightly bound electrons. • When the x-ray energy is greater than the electron-binding energy.
  58. 58. The Photoelectric Effect / Absorption Lecture 04 As the x-ray energy increases • Increased penetration through tissue without interaction. • Less photoelectric effect relative to Compton effect. • Reduced absolute absorption.
  59. 59. The Photoelectric Effect / Absorption Lecture 04  As the atomic number of the absorber increases  As mass density of the absorber increases • Increases ~ Z3 . • Proportional increase in photoelectric effect.
  60. 60. The Photoelectric Effect / Absorption Lecture 04  Low atomic number target atoms such as soft tissue have low binding energies.  Therefore the photoelectric electron is released with kinetic energy nearly equal to the incident x-ray.  Higher atomic number target atoms will have higher binding energies.
  61. 61. The Photoelectric Effect / Absorption Lecture 04  Probability of photoelectric absorption per unit mass is approximately proportional to,  No additional non-primary photons to degrade the image. 33 / EZ
  62. 62. The Photoelectric Effect / Absorption Lecture 04  1 / E 3 explains why image contrast decreases when higher x-ray energies are used in imaging process.  For 1 / E 3 there is an exceptions.  Absorption Edges (Discontinuities).
  63. 63. The Photoelectric Effect / Absorption Lecture 04  Photon energy corresponding to an absorption edge is the binding energy of electrons in a particular shell or subshell
  64. 64. The Photoelectric Effect / Absorption Lecture 04 • The photoelectric effect predominates when lower energy photons interact with high Z materials like Lead and Iodine. • Compton scattering will predominates at most diagnostic photon energies in materials of lower Z such as tissue and air.
  65. 65. Principle of radiological image formation Lecture 04 Attenuation of an X Ray beam  Air: negligible  Bone: significant due to relatively high density (atom mass number of Ca)  Soft tissue (e.g. muscle ): similar to water  fat tissue: less important than water
  66. 66. Principle of radiological image formation Lecture 04 Attenuation of an X Ray beam  lungs: weak due to density.  bones can allow to visualize lung structures with higher kVp (reducing photoelectric effect)  body cavities are made visible by means of contrast products (Iodine, Barium).
  67. 67. Contribution of photoelectric and Compton interactions to attenuation of X Rays in water (muscle) and Bone Lecture 04 Water (Muscle) Bone
  68. 68. Lecture 04 X Ray penetration in human tissues 60 kV - 50 mAs 70 kV - 50 mAs 80 kV - 50 mAs
  69. 69. Lecture 04 X Ray penetration in human tissues  Higher kVp reduces photoelectric effect  The image contrast is lowered  Bones and lungs structures can simultaneously be visualized  Note: body cavities can be made visible by means of contrast media: iodine, barium