Interactions of EM Radiation with Matter

1
Interactions of EM Radiation with Matter

• Manos Papadopoulos
• Nuclear Medicine Department
• Castle Hill Hospital
• Hull East Yorkshire Hospitals NHS Trust

2
ELECTROMAGNETIC RADIATION

• Light is electromagnetic radiation
• a form of energy
• Has both electric and magnetic components
• Characterised by
• wavelength (?)
• frequency (?)

3
WAVE CHARACTERISTICS

• Wavelength (?) The distance between two
  consecutive peaks in the wave

4
WAVE CHARACTERISTICS

• Frequency (?) The number of waves (or cycles)
  per unit time

5
WAVE CHARACTERISTICS

• The product of wavelength (?) and frequency (?)
  is constant

6
PARTICLE CHARACTERISTICS

• Particle-like properties
• Photons or quanta
• ? h? hc/?
• where h is Plancks constant
• For a typical diagnostic X-ray
• ? 210-11 m ? photon energy is 62 keV

7
ELECTROMAGNETIC SPECTRUM
8
ELECTROMAGNETIC SPECTRUM
A triangular prism dispersing light
9
ELECTROMAGNETIC SPECTRUM
Name ? (m) ? (Hz) Interesting Facts
Radio/TV 10-1 10-4 109 104 Low ??are reflected from earths atmosphere
Microwaves 10-3 10-1 1011 109 Cellular phones, Radar
Infrared 10-7 10-3 1014 109 Heat radiation
Visible 410-7 710-7 7.51014 4.31014 1/40 of total spectrum
Ultraviolet 10-8 7x10-7 1016 1014 Burning rays of sun
X-rays 10-11 10-8 1019 1016 tissue damage, ionisation
Gamma rays lt10-11 gt1019 tissue damage, ionisation
10
ATMOSPHERIC OPACITY
11
GENERAL PROPERTIES

• Intensity (I) of a beam of radiation
• rate of flow of energy per unit area (A)
  perpendicular to the beam
• Reduction in intensity by
• the inverse square law
• attenuation by interaction with matter

12
INVERSE SQUARE LAW

• The intensity of a beam of radiation decreases as
  the inverse of the square of the distance (r)
  from that source
• where E is the rate of energy emission of the
  source
• Applies to all radiations under defined
  conditions
• for a point source
• in the absence of attenuation

13
INVERSE SQUARE LAW
14
PHOTON ATTENUATION

• The removal of photons from a beam of photons
• as it passes through matter
• Attenuation is caused by
• absorption
• scattering

of primary beam
15
ATTENUATION COEFFICIENT

• Linear Attenuation Coefficient (µ) is defined as
• the fraction of photons removed from a beam of X-
  or ?- rays per unit thickness
• n number of photons removed from the beam
• N number of photons incident on the material
• ?x thickness of the material (cm)

16
ATTENUATION COEFFICIENT
Linear attenuation coefficients (in cm-1) for a
range of materials at ?-ray energies of 100-,
200- and 300 keV
Absorber 100 keV 200 keV 500 keV
Air 0.000195 0.000159 0.000112
Water 0.167 0.136 0.097
Carbon 0.335 0.274 0.196
Aluminium 0.435 0.324 0.227
Iron 2.72 1.09 0.655
Copper 3.8 1.309 0.73
Lead 59.7 10.15 1.64
17
PHOTON ATTENUATION
18
HALF-VALUE LAYER

• The half-value layer (HVL) is defined as
• the thickness of material required to reduce the
  intensity of a beam to one half of its initial
  value
• µ and HVL are related as follows
• HVL is a function of
• photon energy
• attenuating material
• geometry

19
HALF-VALUE THICKNESS
20
INTERACTIONS WITH MATTER

• Rayleigh scattering
• Compton scattering
• Photoelectric effect
• Pair production

21
RAYLEIGH SCATTERING

• Incident photon interacts with and excites an
  atom
• Atom is excited ? emission of a photon
• Emitted photon
• same energy
• different direction ? scattered photon

22
RAYLEIGH SCATTERING
23
RAYLEIGH SCATTERING
24
RAYLEIGH SCATTERING
25
RAYLEIGH SCATTERING

• Electrons are not ejected
• no ionisation
• In medical imaging
• detection of scattered photons
• impairs image quality
• Scattering angle increases as the photon energy
  decreases
• Occurs with very low-energetic diagnostic X-rays
• Low probability of occurrence in diagnostic
  energies
• 12 of interactions at 30 keV
• 5 of interactions above 70 keV

26
COMPTON SCATTERING

• Inelastic scattering
• Photon interacts with an outer-shell (valence)
  electron
• scattered photon reduced energy
• Compton electron
• Through conservation of energy

27
COMPTON SCATTERING
28
COMPTON SCATTERING

• Compton electron loses its kinetic energy through
• excitation and ionisation of surrounding atoms
• Scattered photon may traverse the medium
• without interaction or
• may undergo subsequent interactions
• Scattered photons detected by image receptor
• image quality is impaired

29
COMPTON SCATTERING

• Incident photon energy increases
• scattered photons
• Compton electrons
• For higher energy incident photons
• majority of energy transferred to scattered
  electron
• Probability of a Compton interaction
• increases with the incident photon energy (E)
• is independent of atomic number (Z)

scattered more towards the forward direction
30
PHOTOELECTRIC EFFECT

• Photon interacts with orbital
• electron
• Electron absorbs all of photon energy
• Electron is ejected
• now called a photoelectron
• Through conservation of energy

31
PHOTOELECTRIC EFFECT
32
PHOTOELECTRIC EFFECT

• The incident photon energy must be
• to the binding energy of the ejected electron
• Following a photoelectric interaction
• the atom is ionised
• a vacancy is created ? electron cascade
• Characteristic X-rays or Auger electrons
• Probability of a photoelectric interaction
• decreases with increasing photon energy (E)
• increases with atomic number (Z)

33
PAIR PRODUCTION

• X- or ?-ray photon interacts with electric field
  of nucleus
• energy of photon transformed into an
  electron-positron pair
• Pair production has a threshold energy
• equal to 1.022 MeV - the rest mass energies of
  the ß-particles
• The beta particles lose their kinetic energy via
• excitation and ionisation
• When the positron comes to rest
• interacts with an electron ? annihilation
  radiation

34
PAIR PRODUCTION
35
DOMINANT REGIONS
36
SUMMARY I

• Light is electromagnetic radiation
• energy propagated as a pair of electric and
  magnetic fields
• Duality of light
• wave-properties
• particle-properties
• Reduction in intensity by
• the inverse square law
• attenuation by interaction with matter

37
SUMMARY II

• Interactions of photons with matter
• Rayleigh scattering
• incident photon excites the entirety of the atom
• Compton scattering
• part of the incident photons electron absorbed
  by free electron
• Photoelectric effect
• all of incident electron absorbed by inner-shell
  electron
• Pair production
• X- or ?-ray photon interacts with electric field
  of nucleus
• electron positron pair created

38
THE END

• Any questions
• ?