Interaction of Radiation with Matter

Interaction of radiation with matter Dr Santam Chakraborty Junior resident Department of radiotherapy. PGIMER Moderator: Dr. T.S. Kehwar

Nature of radiation ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Nature of matter Fig 1 : Bohr’s model of the atom Fig 2 : Energy level diagram (Hydrogen Nucleus)

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Radiation Interaction (overview): Fig. 3 Interaction of photons with matter Photon Matter Photoelectric effect Compton Scatter Pair production High Speed electrons Matter Ionization Excitation Heat Recombination X rays Chemical effects Biological effects

Attenuation: ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Fig 5: Semilog plot showing exponential attenuation of a monoenergetic photon beam. 2 nd HVL 1 st HVL

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Attenuation Coefficients: ,[object Object],Lead 500 kV- 2 MV Copper Aluminum Cellophane Material 120-600kV 30-150 kV ≤ 30kV Generation Energy

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Processes causing attenuation: Coherent scattering: ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Photoelectric effect: Fig. 6 : The photo electric effect

[object Object],Photoelectric effect: ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

[object Object],[object Object],Compton effect: Fig. 7 : Compton’s experiment ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

[object Object],Compton effect (contd.): Practical Implications: 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 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 ,[object Object],[object Object],[object Object],[object Object],Attenuation doesn't depend on the atomic number

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Pair production: π = k Z 2 log (E)

[object Object],[object Object],[object Object],[object Object],[object Object],Photo nuclear reaction:

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Relative importance the reactions:

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Absorption: Photon energy Mass coefficient 100 KeV 1 MeV 10 MeV 91% 15% 46% 71% 96% 10 KeV % of attenuated energy absorbed μ en μ / ρ

[object Object],[object Object],[object Object],[object Object],Absorption (contd.): Spatial distribution of secondary radiation: ,[object Object],[object Object],[object Object],[object Object],[object Object]

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Interactions of particulate radiation:

Practical Implications (electrons): 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

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Interactions of particulate radiation:

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Interactions of neutrons:

[object Object],[object Object],[object Object],[object Object],Biological correlates: DNA damage/ misrepair M Premitotic death (? Membrane damage – interphase) Senescence Clonogenic cell surviving after mitosis ,[object Object],[object Object],[object Object],[object Object],Mitotic death – apoptotic / necrotic ,[object Object],[object Object],[object Object],[object Object],Breakage and rejoining leads to -> dicentrics, rings, acentric fragments, translocations etc.

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Conclusions:

Interaction of Radiation with Matter

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