Interaction of Electromagnetic Radiation with Matter

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Interaction of Electromagnetic Radiation with
Matter

• N E 162 Lecture 5
• Chapter 8 of Text book
• Jasmina Vujic

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Electromagnetic Radiation

• Its Nature and Properties Dual Nature
• Wave properties ???m??????1/s???c (m/s) ??
• Particle Properties m 0, E h?, p E/c
  h?/c h/?
• h? 6.63 x 10-34 J s
• Photons Neutral
• cannot steadily lose E as they penetrate matter
• can travel some distance d before interacting
  with any atom
• Photon can be absorbed - and it disappears,
• Photon can scatter, changing its direction w/ or
  w/o E-loss

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Electromagnetic spectrum
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Interaction of Photons with Matter

• Photoelectric Effect
• Compton Scattering
• Pair Production
• Photonuclear Reactions
• In addition, there are two processes with very
  small energy transfer
• Thomson (elastic) scattering on a free
  electron, redirection of low energy photon
  without change in energy
• Raleigh (coherent) scattering results from
  combined (coherent) action of an atom as a whole.

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Photoelectric Effect

• Photoelectron is detected whenever a metal is
  illuminated by light of a frequency ??which is
  greater than a critical threshold frequency,
  irrespective of the intensity of light.
• This is in direct conflict with prediction based
  on the wave nature of light (if light is
  classical wave the electron should absorb E
  continuously and at any intensity, it should be
  just a matter of time until electron has
  sufficient E to escape. Thus there should be no
  threshold frequency).
• Photoelectric effect could be explained if one
  assumed that the E carried by the incoming light
  came in discrete amounts. This amount only
  depends of ? and not on the intensity I
• Einstein in 1905 -gtNobel Prize in 1921

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Photoelectric Effect
Microscopic cross section Linear attenuation
coefficient
Mass attenuation coefficient
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Compton Scattering
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Compton Scattering

• ? 00, h?max h?, Tmin 0
• T 1800, h?min h?????? h??mc2)
• Tmax h????h?2?h?????mc2/h??

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The Klein-Nishina Formula for differential
scattering cross section

• Per electron and per atom

cm²/sr (electron
cm²/electron
cm²/atom
cm-1
Linear attenuation coefficient
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The Klein-Nishina Formula coefficients

• Differential Klein-Nishina energy-transfer cross
  section
• Differential Klein-Nishina energy-scattering
  cross section
• The total Compton attenuation coefficient

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Pair Production
Linear attenuation coefficient
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Photonuclear Reactions

• (?,n),(?,p), (?,2n), (?,?), (?,f) etc.
• These are the threshold reactions - photon must
  have enough energy to overcome the binding energy
  of the ejected nucleon.
• Need at least several MeV of photon energy.
• 206Pb(?,n)205Pb, Emin 8 MeV

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Total linear attenuation coefficient

• Total mass attenuation coefficient

??? ???????????????? (cm2/g)
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• Total mass energy transfer coefficient
• g takes into account the Bremsstrahlung
  contribution by the electrons freed in each
  process

Total Mass Energy-Absorption Coefficient
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(No Transcript)
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Photon attenuation
I(x)
?
I(d)
I0
dI -?I(x)dx I(x)I0 exp(-?x)
d
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Photon with Attenuation
Fluence
Linear attenuation coefficient
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Photon fluence for monoenergetic beam
Collimated beam
Broad beam
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Half-Value layer
http//www.ndt-ed.org
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Point Source
d
r
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Tables of X-Ray Mass Attenuation Coefficients

• http//physics.nist.gov/PhysRefData/XrayMassCoef/c
  over.html
• Tables of X-Ray Mass Attenuation Coefficients and
  Mass Energy-Absorption Coefficients from 1 keV to
  20 MeV for Elements Z 1 to 92and 48 Additional
  Substances of Dosimetric Interest
• J. H. Hubbell and S. M. Seltzer Ionizing
  Radiation Division, Physics Laboratory National
  Institute of Standards and Technology
  Gaithersburg, MD 20899

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Tables of X-Ray Mass Attenuation and Mass Energy
Absorption Coefficients

• Abstract Tables and graphs of the photon mass
  attenuation coefficient ??? and the mass
  energy-absorption coefficient ?en/? are presented
  for all of the elements Z 1 to 92, and for 48
  compounds and mixtures of radiological interest.
  The tables cover energies of the photon (x-ray,
  gamma ray, bremsstrahlung) from 1 keV to 20 MeV.
  The ??? values are taken from the current photon
  interaction database at the National Institute of
  Standards and Technology, and the ?en/? values
  are based on the new calculations by Seltzer
  described in Radiation Research 136, 147 (1993).
  These tables of ??? and ?en/? replace and extend
  the tables given by Hubbell in the International
  Journal of Applied Radiation and Isotopes 33,
  1269 (1982).

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http//physics.nist.gov/PhysRefData/XrayMassCoef/E
lemTab/z08.html
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http//physics.nist.gov/PhysRefData/XrayMassCoef/E
lemTab/z19.html
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http//physics.nist.gov/PhysRefData/XrayMassCoef/E
lemTab/z82.html
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http//physics.nist.gov/PhysRefData/XrayMassCoef/C
omTab/bone.html
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Bone, Cortical (ICRU-44) - Composition

• Z/A 0.51478 I (eV)112.0 ?1.920E00 (g/cm3)
• I - mean excitation energy
• H-1 0.034000
• C-6 0.155000
• N-7 0.042000
• O-8 0.435000
• Na-11 0.001000
• Mg-12 0.002000
• P-15 0.103000
• S-16 0.003000
• Ca-20 0.225000

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http//physics.nist.gov/PhysRefData/XrayMassCoef/C
omTab/tissue.html
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Tissue, Soft (ICRU Four-Component)

• Z/A 0.54975 I(eV)74.9 ? 1.000E00
  (g/cm3)
• 1 0.101174
• 6 0.111000
• 7 0.026000
• 8 0.761826

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http//physics.nist.gov/PhysRefData/XrayMassCoef/C
omTab/water.html