This document discusses the interactions of ionizing radiation with matter. It begins by introducing the objectives of understanding how radiation interacts with matter and the effects on physical, chemical, and biological levels. It then covers the three main interactions of photons (photoelectric effect, Compton scattering, pair production) and how they lead to attenuation. Key points include how probability depends on photon energy and atomic number, and the byproducts of each interaction. It also discusses the interactions of particulate radiation like electrons, protons, neutrons, and how they ionize matter through excitation, ionization, and bremsstrahlung. Factors like linear energy transfer and specific ionization are addressed. The document is intended to provide background knowledge on radiation detection and

1. Interactions of Radiation
With Matter

2. Objectives
We have focused in previous lectures on
the source of radiation and the types of
radiation. We are now in a position to
consider what happens when this
radiation interacts with matter
(mechanisms of energy transfer and its
effects on the physical-chemical- and
biological level)

3. Objectives
Our main reason for doing this is to find
out what happens to the radiation as it
passes through matter and also to set
ourselves up for considering how it
interacts with living tissue and understand
the chain of events leading to radiation
injury. This knowledge also forms the
bases of radiation therapy and diagnostic
radiology.

4. Objectives
Also, Since all radiation detectors are
made from some form of matter it is
useful to first of all know how radiation
interacts so that we can exploit the
effects in the design of such detectors
and know how it works.

5. Ionizing radiation
Ionizing radiation is radiation that has
sufficient energy to remove electrons
from atoms, creating ions.
Ionizing radiation can be classified into
two groups: photons (gamma and X-
rays) and particles (alpha, beta, and
neutrons (

6. A -Basic Concepts Of Interaction of
photons with matter
Three possible occurrences when x or
gamma photons in the primary beam
pass through matter:
 No interaction at all
 Known as transmission
 Absorption
 Scatter
 The latter two are methods of attenuation

7. Attenuation Of An X-Ray
Photon

8. The Three main Interactions Of X
and Gamma Rays With Matter
Photoelectric effect
 Very important in diagnostic radiology
Compton scatter
 Very important in diagnostic radiology
Pair production
 Very important in therapeutic & diagnostic
radiology

9. Photoelectric Effect
All of the energy of the incoming photon is
totally transferred to the atom
 Following interaction, the photon ceases to exist
The incoming photon interacts with an orbital
electron in an inner shell – usually K
The orbital electron is dislodged
To dislodge the electron, the energy of the
incoming photon must be equal to, or greater
than the electron’s energy

10. Photoelectric Effect
The incoming photon gives up all its energy,
and ceases to exist
The ejected electron is now a photoelectron
This photoelectron now contains the energy
of the incoming photon minus the binding
energy of the electron shell
This photoelectron can interact with other
atoms until all its energy is spent

11. Photoelectric Effect
A vacancy now exists in the inner shell
To fill this gap, an electron from an outer
shell drops down to fill the gap
Once the gap is filled, the electron releases
its energy in the form of a characteristic
photon
This process continues, with each electron
emitting characteristic photons, until the
atom is stable
The characteristic photon produces relatively
low energies and is generally absorbed in
tissue

12. Photoelectric absorption (I-131)

13. The Byproducts of the
Photoelectric Effect
Photoelectrons
Characteristic photons

14. The Probability of Occurrence
Depends on the following:
 The energy of the incident photon(E)
 The atomic number of the irradiated object(Z)
 It increases as the photon energy decreases, and
the atomic number of the irradiated object
increases
 The probability of photoelectric absorption, is
roughly proportional to (Z/E)3
 This type of interaction is prevalent in the
diagnostic keV range – 30 - 150

15. What Does This All Mean?
Bones are more likely to absorb
radiation(Higher Z)
 This is why they appear white on the film
Soft tissue allows more radiation to pass
through than bone (Lower Z)
 These structures will appear gray on the film
Air-containing structures allow more radiation
to pass through
 These structures will appear black on the film

16. Compton Scattering
An incoming photon is partially absorbed in
an outer shell electron
The electron absorbs enough energy to break
the binding energy, and is ejected
The ejected electron is now a Compton
electron
Not much energy is needed to eject an
electron from an outer shell
The incoming photon, continues on a
different path with less energy as scattered
radiation

18. Byproducts Of Compton
Scatter
Compton scattered electron
 Possesses kinetic energy and is capable of ionizing
atoms
 Finally recombines with an atom that has an
electron deficiency
Scattered x-ray photon with lower energy
 Continues on its way, but in a different direction
 It can interact with other atoms, either by
photoelectric or Compton scattering
 It may emerge from the patient as scatter

19. Probability Of Compton
Scatter Occurring
Increases as the incoming photon energy
increases up to certain limit then decreases
as the photon energy increases.
Independent on Z of the absorber.
The Compton process is most important for
energy absorption for soft tissues in the
range from 100 keV to 2MeV.

20. Pair Production
Incoming photon must have an energy of at
least 1.02 MeV
This process is a conversion of energy into
matter and then matter back into energy
Two electrons are produced in this interaction

21. Pair Production
An incoming photon of 1.02 MeV or greater
interacts with the nucleus of an atom
The incoming photon disappears
The transformation of energy results in the
formation of two particles
Negatron
 Possesses negative charge
Positron
 Possesses a positive charge

22. Pair Production

23. Positrons
Considered antimatter
Do not exist freely in nature
Cannot exist near matter
Will interact with the first electron they
encounter
An electron and the positron destroy each
other during interaction
 Known as the annihilation reaction
This converts matter back into energy
Both the positron and electron disappear
Two gamma photons are released with an

24. Pair Production
The produced gamma photons may
interact with matter through pair
production or Compton scatter
Pair production is used for positron
emission tomography, a nuclear
medicine imaging procedure
It is also used in radiation therapy

25. Pair production probability
· Increases with increasing photon
energy
· Increases with atomic number
approximately as Z2

26. Photoelectric effect
dominant
Pair production
dominant
Compton effect
dominant
0.01
0.1 1 10 100
Z of absorber
Photon Energy, in Mev

27. B -Interactions Of Particulate
Radiation With Matter
Alpha particles
ionize by attracting
an electron from an
atom
Beta particles ionize
by repelling an
electron from an
atom

28. Particle interactions
Energetic charged particles
interact with matter by
electrical forces and lose kinetic
energy via:
 Excitation
 Ionization
 Radiative losses

29. Charged Particle Tracks
Electrons follow tortuous paths in matter as
the result of multiple scattering events
 Ionization track is sparse and nonuniform
Larger mass of heavy charged particle results
in dense and usually linear ionization track
Path length is actual distance particle travels;
range is actual depth of penetration in matter

30. Path lengths vs. ranges

31. Linear Energy Transfer
Amount of energy deposited per unit path
length is called the linear energy transfer
(LET)
Expressed in units of eV/cm
LET of a charged particle is proportional to
the square of the charge and inversely
proportional to its kinetic energy(velocity)
High LET radiations (alpha particles, protons,
etc.) are more damaging to tissue than low
LET radiations (electrons, gamma and x-rays)

32. Specific Ionization
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
Increases with electrical charge of
particle
Decreases with increase incident
particle velocity

33. Bremsstrahlung(Radiative losses)
Probability of bremsstrahlung
production per atom is proportional to
the square of Z of the absorber
Energy emission via bremsstrahlung
varies inversely with the square of the
mass of the incident particle (Z/M)2
 Protons and alpha particles produce less
than one-millionth the amount of
bremsstrahlung radiation as electrons of
the same energy

34. Bremsstrahlung

35. Properties of Neutrons:
No Charge
Indirectly Ionising Radiation
Neutron half-life ~ 10.3 minutes
Types of Neutron:
 Thermal neutrons, E < 0.5 eV
 Intermediate-energy neutrons, 0.5 eV < EN
< 10 keV
 Fast neutrons, E > 10 keV
Interaction of Neutrons with
Matter

36. Interaction of Neutrons with
Matter
Some sources of neutrons
• Spontaneous fission of isotopes
• Photonuclear interactions
• Neutron generator
Interactions of neutrons:
• Collisions with atomic nuclei often in a ‘billiard-ball’ type
interaction.
• Rare events, because neutron and nucleus are tiny compared
to atom.
• So, neutrons can travel long distances through matter before
interacting.
Types of neutron interaction:
1. Elastic scattering
2. Inelastic scattering
3. Neutron capture

37. 1. Elastic Scattering
• Neutron collides with atomic nucleus
• Neutron deflected with loss of energy E
• E given to recoiling nucleus
• Energy of recoiling nucleus absorbed by medium.
The recoil nuclei quickly become ion pairs and loose energy through excitation and
ionisation as they pass through the biological material. This is the most important
mechanism by which neutrons produce damage in tissue.
• Struck atoms can also lose orbital electron
Interaction of Neutrons with
Matter – Elastic Scattering
Neutron, E’
Recoiling
Nucleus
Incoming
Neutron, Eo
Nucleus
Total energy
unchanged

38. Interaction of Neutrons with
Matter – Elastic Scattering
• Conservation of Energy and Momentum:
E = energy of scattered neutron
Eo =initial energy of neutron
M = mass of the scattered nucleus
m = mass of neutron
 Energy transferred to nucleus  as target mass  neutron mass.
 Hydrogen good for stopping neutrons e.g. fat better than muscle.
• Elastic scattering important at low neutron energies (few MeV) and
not effective above 150 MeV
2









m
M
m
M
E
E o

39. Interaction of Neutrons with
Matter – Inelastic Scattering
2. Inelastic Scattering
• Neutron momentarily captured by nucleus
• Neutron re-emitted with less energy
• Nucleus left in excited state
• Nucleus relaxes by emitting g-rays or charged particles
(adds to dose)
Emitted
Neutron
g-ray
Incoming
Neutron
Nucleus

40. Interaction of Neutrons with
Matter – Inelastic Scattering
• Interaction probability  as: neutron energy 
target size 
 Important at high neutron energies in heavy materials
• Energy transferred to the target nucleus and emitted energy:
E = Eo - Eg
E = Energy of the neutron after collision
Eo = Initial energy of the neutron

41. Interaction of Neutrons with
Matter- Neutron Capture
3. Neutron Capture
• Neutron captured by nucleus of absorbing material
• Only g-ray emitted.
• Probability of capture is inversely proportional to the energy of the
neutron.
 Low energy (=thermal neutrons) have the highest probability for
capture.
Slow
Neutron
g-ray
Nucleus
Na23 Na24

42. Interaction of Neutrons with
Matter
Where are neutrons useful?
1. Cancer Therapy
2. To produce radioactive isotopes for radiotherapy or imaging
3. To analyse composition and structure of unknown elements
4. Bomb detectors in airports
5. Construction of electronic devices
6. Nuclear energy
Image from: A. L. Galperin, Nuclear Energy/Nuclear Waste.
Chelsea House Publications: New York, 1992

43. Interaction of Neutrons with
Matter
Neutrons for Radiotherapy
• Neutrons have good tumour killing capabilities
• Tissue damage is primarily by nuclear interactions
• Neutrons are high LET radiation + have high B.E.
 Lower chance of tumour repair
 Often lower dose required
 Good for radioresistant tumours

44. Question 1
What are the factors affecting The
Probability of Occurrence of :
Photoelectric effect
Compton scattering
Pair production
Bremsstrahlung

45. Question 2
What are the byproducts of :
Photoelectric effect
Compton scattering
Pair production
Bremsstrahlung

46. Question 3
Give the scientific reason for:
Bones appear more clear than soft
tissues in the radiographic film?
Lead is used for sheading gamma
emitters?
Plastic is preferred than lead for
sheading Beta emitters?

47. How many interactions does a 1 MeV
electron typically undergo before
coming to a stop?
A: 100,000

48. Of these two sub-atomic particles,
which has the largest LET?
Photon? Neutron?
A: Neutron

49. Which of these is a form of
DIRECTLY ionising radiation?
Electron? Neutron?
A: Electron

50. What is produced when an electron and
a positron annihilate?
A: Two g-rays

51. In which material do electrons of the
same energy have the longest
range?
Bone? Fat?
A: Fat

52. Radiation that is easily stopped in
matter, has a HIGH or LOW LET?
A: High

53. What is the probability that a
charged particle will pass
through a medium without
interaction?
A: Zero