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Compton effect and pair production

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  • 1. DR. VIKAS JAGTAP JR I DR. LEE SR I
  • 2.  
  • 3.
    • Ionizing Radiation : Any electromagnetic or particulate radiation capable of producing ion pairs by interaction with matter. Scope limited to X and gamma rays, alpha particles, beta particles (electrons), neutrons , and charged nuclei.
    • Particulate radiations e.g. beta, alpha particle
    • Electromagnetic radiations
  • 4.  
  • 5.
    • Electricity and magnetism are different facets of electromagnetism
      • a moving electric charge produces magnetic fields
      • changing magnetic fields move electric charges
    • This connection first elucidated by Faraday, Maxwell
    • Einstein saw electricity and magnetism as frame-dependent facets of unified electromagnetic force
  • 6.
    • Rapidaly fluctuating electric and magnetic field oscillates at right angles to each other and also to the direction of propagation and are in phase with each other, so also called transverse wave
    • Travel through space in straight line with same velocity= ‘speed of light’ c = 3  10 8 m/s
    • Composed of packets of energy ‘ photons’
    • Follows wave and particle model
  • 7. All electromagnetic waves travel at the same speed The speed of light: 300,000 Km/s trough crest
  • 8. Wavelength (length/cycle) Wavelength (  ): the length of one complete cycle trough crest
  • 9. Wavelength (length/cycle) Amplitude : 1/2 height between trough and crest Amplitude trough crest
  • 10. Wavelength (length/cycle) Frequency (  ): the number of cycles/second Amplitude trough crest
  • 11.
    • Relationship between frequency, speed and wavelength
    • v·  = c
    • v is frequency,  is wavelength, c is speed of light
    • Different frequencies of electromagnetic radiation are better suited to different purposes
    • The frequency of a radio wave determines its propagation characteristics through various media
  • 12.
    • A beam of EM radiation delivers energy in series of discrete packets of energy called quanta but in EM radiations it is called as photon
    • E=hv where h=Planck’s constant
    • so
  • 13.  
  • 14.
    • AM and FM A beam of EM radiation delivers energy series of discrete packets of energy radio waves (including TV signals)
    • Cell phone communication links
    • Microwaves
    • Infrared radiation
    • Light
    • X-rays
    • Gamma rays
  • 15.
    • X-rays are photons (Electromagnetic or EM radiations) emitted from electron orbits , such as when an excited orbital electron "falls" back to a lower energy orbit called ‘extranuclear’
    • Gamma rays are photons emitted from the nucleus , often as part of radioactive decay called ‘intranuclear’
  • 16.
    • Photons are indirectly ionising radiation
    • They interact with matter via 5 processes:
      • Elastic scattering
      • Photoelectric effect
      • Compton Effect
      • Pair production
      • Photonuclear interactions
  • 17.
    • Photon scattering photon disappearing
    • Compton Photoelectric
    • Elastic scattering Pair production
    • Photonuclear reaction
  • 18.
        • Elastic scattering
        • Compton effect
        • Photo-electric effect
        • Pair production
        • Photonuclear interactions
        • Auger effect
        • Scattered radiation
        • Secondary electrons
        • Linear energy transfer
        • Range versus energy
  • 19.
    • Ionization vs. Excitation :
    • Excitation: transfers enough energy to an orbital electron to displace it further away from the nucleus
  • 20.
    • Ionization vs. Excitation :
    • Ionization: the electron is removed, resulting in an ion pair (the newly freed electron(-) and the rest of the atom(+)
  • 21. Compton effect
    • Arthur Holly Compton Compton won the Nobel in 1927 for the discovery of the Compton Effect, the increase in the wavelengths of X rays and gamma rays when they collide with and are scattered from loosely bound electrons in matter
  • 22.
    • Photon interacts with atomic electron as though it were a ‘free electron’
    • The energy of incident photon must be large compared with the electron binding energy
    • Electron receives some energy from photon and is emitted at an angle  . The photon with reduced energy is scattered at an angle 
    • The angle through which the photon is scattered, the energy handed on to the electron, and the energy lost by the photon (and hence the wavelength change) are all interconnected.
  • 23.  
  • 24.
    • by law of conservation of energy and momentum
    • E = hv 0  (1- cos  )
    • 1+  (1-cos   )
    • hv’ = hv 0 1
    • 1+  (1- cos  )
    • hv 0= energy of incident photon
    • hv 1 = scattered photon
    • E= electron
    •  = hv 0 = hv 0
    • m0c2 0.511MeV
  • 25.
    • Energy transfer
    • to recoil electron  initial photon energy
    • Change of wavelength
    •       cos 
    • Special cases of compton effect
    • A] direct hit
    •   deg  deg
    • Electron will travel forward and the scattered photon will travel backward
  • 26.
    • E max = hv 0 2 
    • 1+2 
    • hv’ min = hv 0 1
    • 1+2 
    • B] Grazing hit
    •  = 90 
    •   hv’ = hv 0
  • 27.
    • C] 90 degree photon scatter
    • 
    • Angle of electron emission
    • cos  = (1+  ) tan   
  • 28.
    • As it involves essentially free electron in the absorbing material, it is independent of the atomic No. Z and depends only on No. of atoms per gram of element
    • Most material except hydrogen can be considered as having the same no of electron per gram thus  /  is nearly the same for all material
    •  /  = mass scattering coefficient
  • 29. Material Density(g/cm3) Atomic no no,. Of electrons per gram hydrogen 0.0000899 1 6 × 10 carbon 2.25 6 3.01 oxygen 0.001429 8 3.01 copper 8.9 29 2.75 Effective atomic no. fat 0.916 5.92 3.48 muscle 1.00 7.42 3.36 water 1.00 7.42. 3.34 bone 1.85 13.8 3.00 air 0.001293 7.64 3.01
  • 30. Bone Hydrogen Water Muscle
  • 31.
    • Thus the attenuation per g/cm2 for bone is nearly the same as that of soft tissue.
    • But 1 cm of bone will attenuate more than 1 cm of soft tissue, because bone has a higher electron density i.e. no. of electron per cubic cm
    • So compton effect results in both attenuation of beam
    • and also absorption of beam i.e. photon energy
  • 32.
    • Energy of initial photon is shared between the scattered photon and recoil electron
    •  =  s +  a
    •  s = fraction of energy removed by scattered photon
    •  a = fraction of energy removed by recoil electron
    • So energy distribution depends only upon the angle and is independent of incident photon energy and material
  • 33. Increasing share of total energy is taken up recoil electron
  • 34.  
  • 35.
    • So high energy radiations are less likely to be scattered because the energy removed by the scatter photon is smaller fraction of the total
    • In other words they also yield smaller scatter radiation intensity
    • So, High energy  most goes to electron
    • Low energy  most is scattered
  • 36.
    • Direction of scattering and of the recoil electron
    • Low energy photon  equal chance to be scattered in any direction
    • High energy photon  more likely to travel in forward direction
  • 37.  
  • 38. Applications This implies that as the photon energy increases there is a corresponding increase in the forward scatter of the beam. This results in better dose distribution. Direction of the scatter depends on the energy of the incident photon beam This means that higher beam energies allow greater absorption of the dose in the body with less scattering of energy. Thus with increasing photon energy greater absorption occurs relative to attenuation. The fraction of the energy imparted to the recoil electron increases as the beam energy increases
    • Thus concrete is as good as lead in shielding of megavoltage equipment!
    • The absorption in bones doesn't exceed that produced in the soft tissues – unlike in PE effect seen in orthovoltage radiation era.
    • There is no Bone shielding phenomenon unlike that seen in orthovoltage radiation.
    • Port films produced in megavoltage equipment have very little detail.
    Attenuation doesn't depend on the atomic number
  • 39. This has several important implications in designing radiation protection. The maximum energy of photons with 90° scatter is 0.511 MeV while that for 180° scatter ( i.e.. Back scatter) is 0.255 MeV . The energy of the photons scattered at angles <90 ° will be more than .511 MeV and will gradually approach the incident photon energy Energy of the scattered radiation is independent of the incident beam energy
  • 40. Applications
    • Radiotherapy
    • Same for all body tissue
    • So most of our LA’s are 6/10 MV (further increase in energy will also increase exit dose)
    • by production of electrons and subsequent biochemical effects
    • IGRT- kvCT vs MVCBCT
    • Metallic objects can produce artifacts with kvCT & MRI images which can impair image fusion & delineation of anatomy thereby limiting further electron density correction & dose calculation
    • So MVCBCT minimizes this distorsion
  • 41.    0 0 0 0 0 7 50 95 84 16 100.00 50 50 24.00 23 77 10.00 6 94 4.00 0 100 0.150 0 93 0.060 0 50 0.026 0 5 0.01 Relative no. of interaction Photon energy (MeV)
  • 42.  
  • 43.  
  • 44.
    • Gamma spectroscopy
    • Wave function of electron in momentum preservation
  • 45.
    • Megavoltage Imaging
    • Only 1% efficient. Semiconductors are 20% efficient but costly.
    • Tuned down to 3.5Mev
    • Dose: 0.5-3cGy
    • Can detect 3% contrast object with3cm dimension. I.e Fat & muscle can be identified.
    • Furthermore, higher contrast objects of 1.2cm can be resolved.
    • No high-Z artifacts (hip prostheses, fillings etc.)
    • Can be used for Treatment planning.
  • 46. Pair production
    • Creation of an elementary particle and its antiparticle, usually from a photon (or another neutral boson).
    • Provided there is enough energy available to create the pair – at least the total rest mass energy of the two particles – and that the situation allows both energy and momentum to be conserved
  • 47.  
  • 48.
    • Is the chief method by which energy from gamma rays is observed in condensed matter.
    • Photon need only have a total energy of twice the rest mass (me) of an electron (1.022 MeV) for this to occur
    • If it is much more energetic, heavier particles may also be produced.
    • These interactions were first observed in Patrick Blackett 's counter-controlled bubble chamber , leading to the 1948 Nobel Prize
  • 49.
    • The photon interacts strongly with the EM field of an atomic nucleus and gives up all its energy in the process of creating a pair of negative electron (e-) and a positive electron (e+)
    • Threshold energy 1.02MeV (rest mass energy of electron = 0.511 MeV)
    • The photon energy in excess of this shared between the particles as kinetic energy
    • =( hv -1.02)MeV
    • Energy is converted into masses E=mc2
    • Energy distribution is equal, although any energy distribution is possible
  • 50.
    • Also increases as the logarithm of the incident photon energy above threshold energy
    • Since the momentum of the initial photon must be absorbed by something, pair production cannot occur in empty space out of a single photon; the nucleus is needed to conserve both momentum and energy
    • All the energy removed from the beam (attenuation) is absorbed
  • 51.
    •   Z [  = pair production coefficient]
    • As it is mainly due interaction of photon with magnetic field of nucleus its likelihood goes on increasing with nuclear charge i.e. with atomic no. Z
    • Also in contrast to other scattering the pair production increases with energy
  • 52. Annihilation
    • Electron-positron annihilation occurs when an electron and a positron (the electron's anti-particle) collide.
    • The result of the collision is the conversion of the electron and positron and the creation of gamma ray photons or, less often, other particles.
    • The process must satisfy a number of conservation laws, including:
    • 1. Conservation of charge. The net charge before and after is zero
    • 2. Conservation of linear momentum and total energy
    • This forbids the creation of a single gamma ray.
    • 3. Conservation of angular momentum.
  • 53.  
  • 54. Applications
    • Radiotherapy –
    • 1. we cant increase the dose energy of beam as beyond 10MV pair production is dominant
    • 2. it can directly cause photonuclear reaction by ejecting a nucleon(p or n)
    • 3. PET scan( 2 FDG emits positrons as it is a radioisotope)
  • 55. Applications This leads to dosimetric inaccuracies when using air containing ion chambers. Polarization in heavier atomic weight elements. This leads to a “smudging” of the Bragg's peak which is not seen in electrons. The low mass of the electron leads to greater scattering. This is of practical importance as radioactive isotopes which are produce high energy beta radiation are better stored in low atomic number materials e.g. plastics as they will lead to lesser bremsstrahlung radiation. Also higher atomic number elements are better for x ray production. The amount of radiative loss is proportional to the square of the atomic number of the material This leads to the phenomenon of greater ionization in soft tissues relative to bones. Ionization and excitation are more for low atomic materials
  • 56. Photoelectric effect
    • Photon interacts with an atom and ejects one of the orbital electron
    • Entire energy of is absorbed by atom and then transferred to atomic electron
    • Photoelectron ejected
    • Kinetic energy of ejected electron
    • hv- E B
    • vacancy is created in shell
    • Emission of characteristic x-rays
  • 57.  
  • 58.
    • Auger electrons: internal photoelectric effect
    • Because the kinetic energy of k shell electron is only 0.5 kev in case of soft tissue the characteristic photon produced has very little energy and can be considered to be locally absorbed
    • Mass photoelectric coefficient
    • τ / ρ  1/E3
    • τ / ρ  Z3
    • Angular distribution of electrons produced in photoelectric process depend on the photon energy
  • 59.
    • Useful in diagnostic radiology
    • Difference in Z of various tissues such as bone, muscle, fat amplifies the difference in energy absorption
    • Also forms the basis of use of contrast material
  • 60. Coherent scattering
    • Also Thomson and Rayleigh scattering
    • EM wave passing near an electron and sets it into oscillation
    • The electron reradiates the energy at the same frequency as incident EM wave
    • Energy is taken from beam & is scattered in all direction
    • Same wavelength as incident beam no energy change
  • 61.
    • No energy is permanently taken up by irradiated material
    • Thus it is attenuation without absorption
    • Involves bound electrons so more with higher atomic no. materials and more with low photon energy
    • Important in x- ray crystallography
  • 62.  
  • 63. Correlation
    •  /  =  /  +  coh /  +  /  +  / 
    • total = photoelectric coherent compton pair
  • 64.
    • The total mass attenuation coefficient is the sum of three individual coefficients; photoelectric coefficient, mass scattering coefficient and pair production coefficient:
    • ( μ / ρ ) = ( τ / ρ )+( σ / ρ )+( π / ρ )
    • When we plot the total coefficient versus the photon energy, in different media, the following effects are seen:
    • At low energies the mass attenuation coefficient is larger, especially in high atomic number media, because of the predominance of photoelectric interactions in these circumstances.
    • That attenuation coefficient then decreases rapidly with the energy till the photon energy far exceeds the electron binding energy and Compton effect becomes the predominant mode of interaction. In between the ranges of 200 KeV- 4 MeV, Compton scattering is the predominant mode of interaction.
    • At this energy range, the mass attenuation coefficients also become independent of the atomic number and actually become more for soft tissues, which have more hydrogen content.
    • Beyond 4 MeV pair production results in increasing mass attenuation coefficients specially for high atomic number elements.
    • Thus very high-energy radiations (> 20 MeV) are less-penetrating than some lower energy radiations and are not used in radiotherapy!!
  • 65.  
  • 66.
      • Up to 50KeV – PE effect is important.
      • 60 KeV - 90 KeV – Both PE and Compton effects are important.
      • 200 KeV – 4 MeV – Compton effect is increasingly important.
      • Beyond 20 MeV – Pair production becomes important.
  • 67.  
  • 68.  
  • 69.    0 0 0 0 0 7 50 95 84 16 100.00 50 50 24.00 23 77 10.00 6 94 4.00 0 100 0.150 0 93 0.060 0 50 0.026 0 5 0.01 Relative no. of interaction Photon energy (MeV)
  • 70. Conclusion
    • The three major forms of interaction of radiation with matter, which are of clinical importance in radiotherapy are:
      • Compton effect.
      • Photoelectric effect.
      • Pair production.
    • Out of these, the Compton effect is the most important in modern-day megavoltage radiation therapy.
    • The reduced scattering suffered by high-energy radiation as well as the almost homogeneous tissue dosage is primarily due to the Compton effect.
    • The photoelectric effect is of primary importance in diagnostic radiology and has only historical importance in present day radiotherapy.
    • Despite several decades of research, photon-beam still constitute the main therapeutic modality in radiotherapy, because of several unresolved technical problems with the use of particulate radiation.
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