INTERACTION OF RADIATION WITH MATTER 


  INTRODUCTION 

  • Radiation: emission or propagation of energy through space or medium
  • Particulate radiation: consisting of atomic or subatomic particles (electrons, protons, etc.) that carry energy in the form of kinetic energy of mass in motion
  • Electromagnetic radiation: energy is carried by oscillating electrical and magnetic fields travelling through space at the speed of light.
  •  EM radiation most of the time behave like a wave but certain situation specially in the case of interaction with atoms it behaves like photons 





  IONIZING RADIATION  

  • Directly ionizing Radiation: charge particles such as electrons, protons  and alpha particles
  • Indirectly ionizing Radiation: uncharged particles such  X and Gamma rays, neutrons , because they liberate directly ionizing particles while interact through medium.




  • X rays and gamma rays-major forms used in radiotherapy.


  • X rays-origin    - Extra nuclear 

                               - produced electrically     

    

  • Gamma rays-origin   - Intra nuclear 

                                          -due to instability of nucleus  


 ATTENUATION OF A BEAM 

  • Absorption- Photon disappears
  • Scattering -  deflection from its path 
  • Transmission- passing through the medium 
  • Attenuation-removal of radiation from the beam by the matter

Absorption + scattering= Attenuation


 INTERACTIONS 

  • Interaction between Photons and matter can take place with the result that energy is transferred to the medium
  • The initial step in the energy transfer involves the ejection of electrons from the atom of the absorbing medium
  • Electrons transfer their energy by producing ionization and excitation of the atom along their paths.
  • Excitation: transfers enough energy to an orbital electron to raise the electron to higher energy levels
  • Ionization: removes electron from the atom resulting in an ion pair the newly freed electron(-) and the rest of the atom(+)

  • If medium consists of body tissues, sufficient energy may be deposited with in the cells 
  • Most of the absorbed energy is converted into heat
  • Attenuation of photon beam by an absorbing medium is caused by five major types of interactions

  1. Coherent scattering
  2. Photoelectric effect
  3. Compton effect
  4. Pair production
  5. Photodisintegration

COHERENT OR RAYLEIGH SCATTERING

  • Bound electrons momentarily vibrate at frequency equal to the incident radiation
  • Vibrating electron in turn emit X- rays of same frequency in all directions
  • No ionization
  • Occurs mainly with  very low diagnostic X-ray(15-30KeV) and high Z

PHOTOELECTRIC EFFECT

  • Emission of electrons ( called photoelectrons) from a surface when light is shined on it.
  • If sufficient photon energy available, the photoelectron is most likely to be ejected from the innermost possible shell
  • Kinetic energy of a photoelectron is given by
        K.E. = hv – B.E.



•          Vacancy thus created is filled by an outer shell electron with emission

      of characteristic X rays.

•          There is also the possibility of emission of Augor electrons,

•          which occurs when the energy released as a result of the outer electron filling the vacany is given to another electron in higher shell which is subsequently ejected

•          The angular distribution of emitted photo electrons depend on the photon energy:

•          For low energy: 90 degrees

•           For higher energies : progressively  forward direction.

 

IMPLICATIONS 

  • It is the primary mode of interaction in diagnostic radiology.
  • Ʈ is proportional to  z3/E3, where Ʈ is the photoelectric attenuation coefficient 
  • Difference in z between muscle/bone/fat amplifies difference in X-ray absorption improving image contrast
  • Photoelectric process predominates when lower energy photons interact with high Z materials (like radiographic contrast agents)


COMPTON EFFECT

  • Incoherent and inelastic scattering 
  • First elucidated by H. Compton in 1923
  • Strictly a photon-electron interaction
  • The photon interacts with an atomic electron as though it were a “free” electron
  • The binding energy of the electron is much less than the energy of the bombarding photon 
  • It is the most dominating interaction in the field of Radiation therapy





SPECIAL CASES

  • Grazing hit or glancing blow
  • The scattered photon has the same energy as incident
  • where, ϕ=0 and ϴ=90
  • Direct hit 
  • where, ϕ=180 and ϴ=0
  • Electron receives max energy and photon loses max energy leading to drastic change in wavelength


ENERGY DISTRIBUTION

  • High energy - most of goes to electro
  • Lower energy – most is scattered
  • High energy radiation are less likely to be scattered because the energy removed by scattered photon is smaller fraction of the total
  • They also yield smaller scatter radiation intensity

Relative probability of compton scattering And scattering angle for different incident Photon energies