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 (λ)
frequency (ν)

3. WAVE CHARACTERISTICS
Wavelength (λ): The distance between two consecutive peaks in the wave

4. WAVE CHARACTERISTICS
Frequency (ν): The number of waves (or cycles) per unit time

5. WAVE CHARACTERISTICS
The product of wavelength (λ) and frequency (ν) is constant

6. PARTICLE CHARACTERISTICS
Particle-like properties
Photons or quanta
Ε = hν = hc/λ
where h is Planck’s constant
For a typical diagnostic X-ray
λ = 2·10-11 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 earth’s atmosphere
Microwaves
10-3 – 10-1
1011 – 109
Cellular phones, Radar
Infrared
10-7 – 10-3
1014 – 109
“Heat” radiation
Visible
4·10-7 – 7·10-7
7.5·1014 – 4.3·1014
~ 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
<10-11
>1019

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 γ- rays per unit thickness
n: number of photons removed from the beam
N: number of photons incident on the material
Δx: thickness of the material (cm)

16. ATTENUATION COEFFICIENT
Linear attenuation coefficients (in cm-1) for a range of materials at γ-ray energies of 100-, 200- and 300 keV
Absorber
100 keV
200 keV
500 keV
Air
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 In medical imaging
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 γ-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 MeV - the rest mass energies of the β-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 photon’s electron absorbed by free electron
Photoelectric effect
all of incident electron absorbed by inner-shell electron
Pair production
X- or γ-ray photon interacts with electric field of nucleus
electron – positron pair created

38. THE END
Any questions ?