1. Lecture 5
Shahid Younas
INTERACTION OF RADIATION WITH MATTER

2. Introduction
Lecture 5
 Manner of interactions
 Nature of Materials
 Nature of Incident particle
 Interactions are probabilities
 Noble Prize

3. Types and sources of ionizing radiation
Lecture 5
The important types of ionizing radiation
 -rays
 X-rays
 Fast Electrons
 Heavy Charged Particles
 Neutrons

4. -rays
Lecture 04
 Electromagnetic radiation emitted from a
nucleus or in annihilation reactions
between matter and antimatter.
 Practical range of photon energies emitted
by radioactive atoms extends from 2.6 keV
to 7.1 MeV

5. X-rays
Lecture 04
 Electromagnetic radiation emitted by
charged particles (usually electrons) in
changing atomic energy levels (called
characteristic or fluorescence x-rays)
 or in slowing down in a Coulomb field
(continuous or bremsstrahlung x-rays).

6. X-rays
Lecture 04
20 – 120 kV Diagnostic-range x-rays
120 – 300 kV Orthovoltage x-rays
300 kV – 1 MV Intermediate-energy x-rays
1 MV upward Megavoltage x-rays

7. X-rays
Lecture 04
Do you know the name of x-rays of range 0.1 – 20 kV.
Soft x-rays or Grenz rays

8. Fast Electrons
Lecture 04
Do you know the name of electrons if they result from a charged-
particle collision.
Delta Rays
“-rays”

9. Fast Electrons
Lecture 04
 Positrons if positive in charge
 If emitted from a nucleus they are
usually referred to as -rays
(positive or negative)
 Intense continuous beams of
electrons up to 12 MeV available
from Van de Graff generators.

10. Heavy Charged Particles
Lecture 04
 Acceleration by a Coulomb force field.
 Alpha particles also emitted by some
radioactive nuclei.
 Proton – the hydrogen nucleus
 Deuteron – the deuterium nucleus
 Triton – a proton and two neutrons bound
by nuclear force
 Alpha particle – the helium nucleus

11. Heavy Charged Particles
Lecture 04
Do you know the name of some other heavy charged particles
Pions – negative -mesons

12. Neutrons
Lecture 04
 Neutral particles obtained from nuclear reactions [e.g., (p, n) or
fission].
 They cannot themselves be accelerated electrostatically

13. ICRU Terminology
Lecture 04
Directly ionizing radiation
Fast charged particles
Deliver their energy to matter directly, through many small Coulomb-
force interactions along the particle’s track

14. ICRU Terminology
Lecture 04
Indirectly ionizing radiation
 X- or -ray photons or neutrons.
 First transfer their energy to charged particles.
 Resulting fast charged particles then in turn deliver the energy to
the matter as above

15. Specific Ionization
Lecture 04
 Number of primary and secondary ion pairs
produced per unit length of charged particle’s path
is called specific ionization.
 Expressed in ion pairs (IP)/mm.

16. Specific Ionization
Lecture 04
 Increases with electrical charge of particle.
 Decreases with incident particle velocity.
 ~ 7000 IP/mm in air by Alpha particle.

17. Specific Ionization
Lecture 04
Do you know the medical usage of specific ionization?
Radiotherapy

18. Specific Ionization
Lecture 04
7.69 MeV Alpha Particle from Polonium 214.

19. Charged Particle Tracks
Lecture 03
Do you know the difference between “Range” and “Path Length”
for ionizing particles?
Path length is actual distance particle travels whereas range is
actual depth of penetration in matter

20. Charged Particle Tracks
Lecture 03
 Electrons follow tortuous
paths in matter as the
result of multiple
scattering events.
 Ionization track is thin
and non-uniform

21. Charged Particle Tracks
Lecture 03
 Larger mass of heavy
charged particle results in
dense and usually linear
ionization track.

22. Linear Energy Transfer
Lecture 04
 Amount of energy deposited per unit path length is called the
linear energy transfer (LET).
 Expressed in units of eV/cm.
 LET is the product of specific ionization (IP/cm) and the
average energy deposited per ion pair (ev/IP).

23. Linear Energy Transfer
Lecture 04
Radiation LET (keV/ um)
1 MeV Gamma Rays 0.5
100 kVp X-rays 6
20 keV β-particles 10
5 MeV neutrons 20
5 MeV α-particles 50
1 MeV Electron 0.2
100 keV Electron 0.3

24. Linear Energy Transfer
Lecture 04
Can you establish a relation between LET and biological damage?
High LET Radiation (alpha particles, protons etc)are more dangerous to tissue
than low LET radiation (gamma, X-rays, electrons)

25. Linear Energy Transfer
Lecture 04
What is the probability of charged particle passing through a
medium without interaction.
ZERO

26. Linear Energy Transfer
Lecture 04
What is the probability of charged particle passing through a
medium without interaction.
ZERO

27. Scattering
Lecture 04
 Interaction resulting in the deflection of a particle or photon from its original
trajectory.
 Elastic : Billiard Ball
 In-elastic

28. Scattering
Lecture 04
Is there any effect of scattering on image quality.
bone
air
soft
tissue bone
primary
diaphragm
film, fluorescent screen or
image intensifier
primary
radiological
image
intensity at
detector
scattered
radiation
grid
Image taken from: Johns & Cunningham, The Physics of Radiology, 4th Edition

29. Scattering
Lecture 04
Veil over image.
Add patient dose.

30. Particle interactions
Lecture 04
 Energetic charged particles interact with matter by electrical forces and lose
kinetic energy via:
 Excitation
 Radiative losses
 Ionization
 ~ 70% of charged particle energy deposition leads to nonionizing excitation

31. Particle interactions
Lecture 04
A: Excitation (left) and
de-excitation on (right)
B: Ionization and the
production of delta rays

32. INTERACTIONS
Lecture 04
All Interactions are Probabilities.
Different passengers in a car; faces different level of injuries
or enjoy different grades of safety in the situation of an
accident.

33. PHOTON INTERACTIONS
Lecture 04
 Chance of event happening
 Relative predictions can be made,
 energy of photons
 type of matter with which photons are gong to interact

34. Radiative Interactions-Bremsstrahlung
Lecture 04
Path of the electron is deflected / de-
accelerated by the positively charged
nucleus.

35. Radiative Interactions-Bremsstrahlung
Lecture 04
 Angle of emission changes with the incident electron energy.
 Probability of production is ~ Z2 of the absorber.
 Energy emission varies inversely with the square of the mass.
 Protons and alpha particles produce less than one-millionth the amount
of bremsstrahlung radiation as electrons of the same energy.

36. Radiative Interactions-Bremsstrahlung
Lecture 04
Disadvantages:
 two edged sword
 It is not especially useful for therapeutic
 Bremsstrahlung produced by a beta electron is more harmful to the
technologist than the beta particle that produces it, because of the
penetrability of an electromagnetic ray.

37. Radiative Interactions-Bremsstrahlung
Lecture 04
Ratio of electron energy loss by bremsstrahlung production to that lost by
excitation and ionization,
kinetic energy of incident electron * atomic number
820

38. Radiative Interactions-Characteristics
Lecture 04
 If a high speed beta particle approaches
an electron in an inner orbital.
 Ejected from the atom.

39. Neutron interactions
Lecture 04
 Don’t interact with electrons.
 Don’t create direct ionization.
 They do interact with atomic nuclei,
sometimes liberating charged particle.
 Neutrons may also be captured by atomic
nuclei-
Retention of the neutron converts the
atom to a different nuclide (stable or
radioactive)

40. Rayleigh Scattering
Lecture 04
 Incident photon interacts with
and excites the total atom as
opposed to individual electrons.

41. Rayleigh Scattering
Lecture 04
Electric field of the incident
photon’s electromagnetic wave
expands energy.
Causing all of the electrons in the
scattering atom to oscillate in
phase.
Atom’s electron cloud immediately
radiates this energy.
Emitting photon of same energy
but slightly different direction.

42. Rayleigh Scattering
Lecture 04
 Occurs mainly with very low energy diagnostic x-rays, as used
in mammography (15 to 30 keV)
 Less than 5% of interactions in soft tissue above 70 keV; at
most only 12% at ~30 keV.

43. Rayleigh Scattering
Lecture 04

44. Compton Scattering
Lecture 04
 Predominant interaction in the diagnostic energy range
with soft tissue.
 Predominate Energy Region: 26 keV to 30 MeV.
 Most likely to occur between photons and outer
(“valence”) shell electrons.

45. Compton Scattering
Lecture 04
 Electron ejected from the atom.
 Binding energy comparatively small
and can be ignored.
 Photon scattered with reduction in
energy.

46. Compton Scattering
Lecture 04
 Energy of scattered photon can be calculated by,
)cos1(1 2
0
0
0
0


 
cm
E
E
E
EEE
sc
esc

47. Compton Scattering
Lecture 04
 Ionization of the atom.
 Ejected electron lose K.E. by excitation and ionization of atoms
in the surrounding material.

48. Compton Scattering
Lecture 04
 As incident photon
energy increases,
scattered photons and
electrons are scattered
more toward the
forward direction.

49. Compton Scattering
Lecture 04
 In diagnostic imaging (18 to 150 keV), the majority of the incident
photon energy is transferred to the scattered photon.
 In x-ray transmission imaging, these photons are much more likely to
be detected by the image receptor.
 Thus reducing image contrast.

50. Compton Scattering
Lecture 04
 Probability of interaction increases as incident photon energy
increases.
 probability also per atom of the absorber depends on the number
of electrons available as scattering targets and therefore increases
linearly with Z.

51. Compton Scattering
Lecture 04
 Laws of conservation of energy and momentum place limits on
both scattering angle and energy transfer.
 Energy of the scattered electron is usually absorbed near the
scattering site

52. Compton Scattering
Lecture 04
Do you know the angles of maximum energy transfer and scattering
of Compton Electron.
Maximal energy transfer 180-degree- photon backscatter
Maximal Scattering angle is 90 degrees

53. PHOTOELECTRIC EFFECT / ABSORPTION
Lecture 04
 All of the incident photon energy is transferred to an electron, which
is ejected from the atom.
 Kinetic energy of ejected photoelectron (Ec) is equal to incident
photon energy (E0) minus the binding energy of the orbital electron
(Eb).
Ec = Eo - Eb

54. Photoelectric absorption
Lecture 04
Incident photon
energy must be
greater than or
equal to the
binding energy of
the ejected
photon.

55. The Photoelectric Effect / Absorption
Lecture 04
 Atom is ionized, with an inner shell
vacancy.
 Electron cascade from outer to inner
shells- Characteristic x-rays or Auger
electrons

56. The Photoelectric Effect / Absorption
Lecture 04
 Probability of characteristic x-ray emission decreases as Z decreases
 Does not occur frequently for diagnostic energy photon interactions in soft
tissue

57. The Photoelectric Effect / Absorption
Lecture 04
Most likely to occur • With inner-shell electrons
• With tightly bound electrons.
• When the x-ray energy is greater
than the electron-binding energy.

58. The Photoelectric Effect / Absorption
Lecture 04
As the x-ray energy increases
• Increased penetration through
tissue without interaction.
• Less photoelectric effect relative
to Compton effect.
• Reduced absolute absorption.

59. The Photoelectric Effect / Absorption
Lecture 04
 As the atomic number of the
absorber increases
 As mass density of the
absorber increases
• Increases ~ Z3
.
• Proportional increase in
photoelectric effect.

60. The Photoelectric Effect / Absorption
Lecture 04
 Low atomic number target atoms such as soft tissue have low binding
energies.
 Therefore the photoelectric electron is released with kinetic energy nearly
equal to the incident x-ray.
 Higher atomic number target atoms will have higher binding energies.

61. The Photoelectric Effect / Absorption
Lecture 04
 Probability of photoelectric absorption per unit mass is approximately
proportional to,
 No additional non-primary photons to degrade the image.
33
/ EZ

62. The Photoelectric Effect / Absorption
Lecture 04
 1 / E
3
explains why image contrast decreases when higher x-ray energies are used in
imaging process.
 For 1 / E
3
there is an exceptions.
 Absorption Edges (Discontinuities).

63. The Photoelectric Effect / Absorption
Lecture 04
 Photon energy
corresponding to an
absorption edge is the
binding energy of electrons
in a particular shell or
subshell

64. The Photoelectric Effect / Absorption
Lecture 04
• The photoelectric effect predominates when lower energy photons interact
with high Z materials like Lead and Iodine.
• Compton scattering will predominates at most diagnostic photon energies
in materials of lower Z such as tissue and air.

65. Principle of radiological image formation
Lecture 04
Attenuation of an X Ray beam
 Air: negligible
 Bone: significant due to relatively high density (atom mass number of Ca)
 Soft tissue (e.g. muscle ): similar to water
 fat tissue: less important than water

66. Principle of radiological image formation
Lecture 04
Attenuation of an X Ray beam
 lungs: weak due to density.
 bones can allow to visualize lung structures with higher kVp (reducing
photoelectric effect)
 body cavities are made visible by means of contrast products (Iodine,
Barium).

67. Contribution of photoelectric and Compton interactions to attenuation of
X Rays in water (muscle) and Bone
Lecture 04
Water (Muscle) Bone

68. Lecture 04
X Ray penetration in human tissues
60 kV - 50 mAs 70 kV - 50 mAs 80 kV - 50 mAs

69. Lecture 04
X Ray penetration in human tissues
 Higher kVp reduces photoelectric effect
 The image contrast is lowered
 Bones and lungs structures can simultaneously be
visualized
 Note: body cavities can be made visible by means
of contrast media: iodine, barium