1. Radiation can interact with matter through ionization or excitation. Ionization removes an electron from an atom, while excitation raises an electron to a higher energy state. 2. Radiation is classified as either non-ionizing or ionizing. Ionizing radiation can directly or indirectly ionize matter and includes photons, electrons, protons, alpha particles, neutrons, and other heavy charged particles. 3. The interaction of radiation with matter depends on the type of radiation. Electromagnetic radiation can undergo processes like the photoelectric effect, Compton scattering, and pair production. Charged particles lose energy through ionization and excitation of atoms. Neutrons can elastically or inelastically scatter

1. 1
Interaction of Radiation with Matter
SEEMA SHARMA
Medical Physicist
Dept. of Radiotherapy
I.R.C.H. AIIMS, New Delhi

2. 2
Classification of radiation
Energy from radiation is transferred to matter in two ways:
Ionisation and Excitation.
Ionisation is the process of removal of an electron from an atom leaving the atom with a net
positive charge.
In excitation, the energy of incoming radiation raises an outer electron to a higher energy state
from which it returns very rapidly (10-8s) to its original state emitting a photon of light in the
process.

3. 3
● Non-ionizing radiation (cannot ionize matter).
● Ionizing radiation (can ionize matter either directly or indirectly):
—Directly ionizing radiation (charged particles): electrons, protons, a particles and heavy ions.
—Indirectly ionizing radiation (neutral particles): photons (X rays and g rays), neutrons.
Directly ionizing radiation deposits energy in the medium through direct Coulomb interactions
between the directly ionizing charged particle and orbital electrons of atoms in the medium.
Indirectly ionizing radiation (photons or neutrons) deposits energy in the medium through a two step
process:
● In the first step a charged particle is released in the medium (photons release electrons or positrons,
neutrons release protons or heavier ions);
● In the second step the released charged particles deposit energy to the medium through direct
Coulomb interactions with orbital electrons of the atoms in the medium.
Classification of radiation

4. 4
IONIZING RADIATION
• Electromagnetic
• Particular
X-rays (produced extra-nuclearly)
γ-rays (produced intra-nuclearly)
Electrons
Protons
α-Particles
Neutrons
Deuterons
Heavy charged particles

5. 5
Interactions of Radiation with Matter
Electromagnetic Radiation &
its interaction with Matter
1) Elastic scattering
2) Compton effect
3) Photo-electric effect
4) Pair production
5) Photonuclear interactions
6) Auger effect
7) Scattered radiation
8) Secondary electrons
9) Linear energy transfer
10) Range versus energy
Interaction of sub atomic
particles with matter.
1. Ionisation and excitation due
to charged particles
2. Electrons
a) collision loss
b) radiative loss
c) stopping power due to each and
total stopping power,
d) Particle range
e) Bragg peak
3. Bremsstrahlung
4. Neutrons - elastic and inelastic
collisions.
5. Protons, ionisation profile
6. Elementary knowledge of pions and
heavy ions.

6. 6
absorption
scattering
transmission
energy deposition
Photons interaction with matter

7. 7
Coherent scattering
 Also known as unmodified, Rayleigh, classical or elastic
scattering, is one of three forms of photon interaction which occurs
when the energy of the x-ray or gamma photon is small in relation to
the ionisation energy of the atom. It therefore occurs with low energy
radiation.
 There is no energy deposition and thus no dose resulting from
coherent scattering. The only change is a change of direction (scatter)
of the photon, hence 'unmodified' scatter. Coherent scattering is not a
major interaction process encountered in radiography at the energies
normally used.

8. 8
photon
characteristic
radiation
electron
PHOTOELECTRIC EFFECT

9. 9

10. 10
bone
air
soft
tissue bone
primary
diaphragm
film, fluorescent screen or
image intensifier
primary
radiological
image
intensity at
detector
scattered
radiation
grid

11. 11
X Ray penetration in human tissues
60 kV - 50 mAs 70 kV - 50 mAs 80 kV - 50 mAs

12. 12
X Ray penetration in human tissues
Improvement of image contrast (lung)

13. 13
X Ray penetration in human tissues
Improvement of image contrast (bone)

14. 14
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

15. 15
Auger Electron

16. 16
photon
electron
scattered
photon
COMPTON PROCESS

17. 17
• The incident photon interact
with free electron of the
atom.
• The cross-section is
independent of atomic
number of the material.
• It decreases with energy.
• Some of the energy is given
to recoil electron while rest
of the energy is scattered.
COMPTON PROCESS

18. 18
PAIR PRODUCTION

19. 19
ANNIHILATION
+ + e-
(511 keV)
(511 keV)
+ (1-3 mm)
Radionuclide

20. 20
Photonuclear reaction / Photodisintegration
Photodisintegration are of
concern in high energy
radiotherapy treatment
rooms because of the
neutron production
through the (x, n) reactions
and because of the
radioactivity that is
induced in the treatment
room air and in machine
components through the (x,
n) reaction.

21. 21
PHOTON INTERACTION
0
10
20
30
40
50
60
70
80
90
100
0,01 0,1 1 10 100
Photon energy (MeV)
Photoelectric
effect
Compton
process
Pair
production
The dominating photon absorption process in different materials of different atomic numbers
Photon energy (MeV)
Atomic number (Z)

22. 22
Photon Energy Effect
• Low energy photon
 Mainly photoelectron effect
 Emission of single electron
Localized electron ionization
• Medium energy photon
 Compton scattering
 Photons undergo series of scattering events
 Produce spatially discrete energetic electrons
• High energy photon
 Pair production with Compton

23. 23
Charged particles interaction with matter
heavy
light

24. 24
1. “soft collision” when b >> a
2. “hard collision” when b ~ a
3. “Coulomb-force interactions with
the external nuclear field” when b << a
Charged
particle
b
a
Undisturbed
trajectory
Interactions characterised by:
“impact parameter, b” vs “atomic radius, a”
Interaction of Charged Particles with Matter – Energy Loss
Collisional Energy Loss
Radiative Energy Loss

25. 25
Interaction of Charged Particles with Matter – Energy Loss
Soft Collisions (b >> a): Excitation and Ionisation
The electric field of the charged particle interacts with atomic electrons
causing them to accelerate and gain energy.
Passing charged particle
1.
Ejected electron
2.
1. Excitation: If the gain in electron energy
is equal to the difference in energy between its own energy
level and a higher energy level, then the electron is
excited to the higher energy level.
2. Ionisation: If the gain in energy is greater than the binding
energy for the electron, then an electron is removed
from its orbital. The atom is “ionised”.
Net effect: transfer of a small amount of energy (few eV) to atom of
absorbing medium

26. 26
Interaction of Charged Particles with Matter – Energy Loss
Soft Collisions (b >> a)
Large b more probable than small b
 “soft” collisions more likely than any other type of interaction
 approx. 1/2 particle energy transferred to absorbing medium
Cherenkov radiation
in the core
of a reactor
Two additional effects:
1. Polarisation of atoms in absorbing medium
(more important for the
physicist!)
2. Cherenkov radiation = emission of
bluish light (< 0.1 % of particle
energy spent in this way.
Unimportant in RT physics)

27. 27
Cherenkov radiation

28. 28
Interactions of Charged Particles with Matter – Energy Loss
Hard Collisions (b ~ a): Ionisation, d-rays, char. X-rays + Auger e-
When b ~ a, more likely for CP to interact with single atomic e-
 “hard” collisions result in ejection of e-
 e- emitted with large K.E. = d-ray
 d-rays have sufficient energy to ionise other atoms
 d-rays dissipate energy along separate track = spur
d-ray
Incoming
radiation
Bremsstrahlung
Main e- track
Ejected electron

29. 29
Hard Collisions (b ~ a): Ionisation, d-rays, char. X-rays + Auger e-
 char. X-rays and Auger electrons also emitted
 some energy transferred to medium by d-rays, char. x-rays and
Auger e- transported away from primary particle track
 no. of hard collisions is small
 BUT fraction of energy spent in hard + soft collision comparable
Interactions of Charged Particles with Matter – Energy Loss
Incoming charged
particle
K radiation
E - hnk
Ejected
electron
K
L
M
L-shell to K-shell = Ka radiation
M-shell to K-shell = K radiation

30. 30
Bremsstrahlung
Photon
Electron
Electron

31. 31
X-ray production
• High energy electrons hit a (metallic) target
where part of their energy is converted into
radiation
target
electrons
X-rays
Low to
medium
energy
(10-400keV)
High
> 1MeV
energy

32. 32
X-Ray Tube for low and medium X-ray production

33. 33
Megavoltage X-ray linac
target
electrons
X-rays

34. 34
The resulting X-Ray spectrum
Unfilteredradiation(in vacuum)
20 40 60 80 100 120
INTENSITY
PHOTONENERGY(keV)
Characteristic
X-rays
Bremsstrahlung
Spectrum after
filtration
Maximum electron energy

35. 35
Mean Energy Expended per Ion Pair, W
In measuring the energy absorbed extensive use is made of ionisation.
Mean energy expended to form an ion pair: W = E/N
where E = initial K.E. of the charged particle
N = mean no. of ion pairs formed when all energy is
used
EXAMPLE: W for dry air is 34 eV
Interaction of Charged Particles with Matter – Energy Loss

36. 36
Coulomb-force interactions with the external nuclear field” (b << a):
Bremsstrahlung
When charged particle comes very close to nucleus, its electric field interacts with that of the nucleus.
 Most important for electrons because: Prob.  Z2 , 1/m2
 Most cases, elastic scattering results i.e. electron changes direction
but loses no energy
 2-3% of cases, charged particle decelerates thereby losing energy and
changing direction
 Up to 100 % particle energy lost as X-rays = Bremsstrahlung
 continuous spectrum of Bremsstrahlung radiation
Incoming charged
particle
Bremsstrahlung,
hn
E - hn
Interaction of Charged Particles with Matter – Energy Loss

37. 37
• The furthest distance radiation travels in a medium is called “the
range”.
A
B
Medium
Range
Incoming
Radiation
A: starting point for
secondary e-
B: stopping point for
secondary e-
Electrons follow tortuous paths undergoing many interactions before
coming to a stop.

38. 38

39. 39

40. 40
Pions (= Pi Mesons)
Symbols: P-,P0, P+
Pions are the lightest of the Mesons (0.15 x Mp,N)
Mesons exist inside the nucleus i.e. they are sub-atomic
particles which experience the strong nuclear forces.
Pions hold the nucleus together .
Pions are produced as a result of high energy collisions in
a particle accelerator e.g. protons colliding with a C or Be
target.
Pions live for 26 billionths of a second.
Pions

41. 41
Pions
Pions (P-) in radiotherapy:
• When the P- reaches the tumour it has slowed down so much that a
nucleus captures it.
• The nucleus is now unstable and breaks up violently into smaller
fragments.
• These fragments damage surrounding cells within a small radius

42. 42
Properties of Neutrons:
Mass = 1.67 e-27 kg
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
All neutrons are initially Fast Neutrons which lose kinetic energy through
interactions with their environment until they become thermal neutrons which
are captured by nuclei in matter
Interaction of Neutron with Matter

43. 43
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
Interaction of Neutron with Matter

44. 44
• 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
Neutron, E’
Recoiling
Nucleus
Incoming
Neutron, Eo
Nucleus
Total energy
unchanged
Elastic Scattering

45. 45
• 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
Elastic Scattering

46. 46
• Neutron momentarily captured by nucleus
• Neutron re-emitted with less energy
• Nucleus left in excited state
• Nucleus relaxes by emitting -rays or charged particles
(adds to dose)
Emitted
Neutron
-ray
Incoming
Neutron
Nucleus
Inelastic Scattering

47. 47
• 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 - E
E = Energy of the neutron after collision
Eo = Initial energy of the neutron
Inelastic Scattering

48. 48
Neutron captured by nucleus of absorbing material
• Only -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
-ray
Nucleus
Na23 Na24
3. Neutron Capture

49. 49
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
Where are neutrons useful?

50. 50
 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
Neutrons for Radiotherapy

51. 51

52. 52
Reaction with water
• Radicals are
formed by the
interaction of
radiation with
water
 Radicals drive
reactions

53. 53
Radiation interaction with water
HOH+ + e-
HOH
HOH + e-  HOH-
HOH+ H+ + OH•*
HOH-
OH- + H•
radiation
Ion pair (H+, OH-)
HOH
Free Radicals (H•, OH•)
The cell is composed of 80% water. The ultimate result of radiation
interaction with water molecule is the formation of an ion pair and free
radicals. Free radicals have an unpaired electron in their outer shell, a state
which confers a high degree of reactivity.

54. 54

55. 55
Biological Effects
Time Event
10-18 seconds Absorption of Ionizing Radiation
10-16 seconds Ionization, Excitation
10-12 seconds Radical formation, bond breakage
10-12 to 10-6 seconds Radical reaction
Min. to Hrs. Cellular Processes
Hrs. to Months Tissue Damage
Years Clinical effects
Generations Genetic Effects

56. 56
X-ray photon fast electron (e) ion radical
free radical chemical changes
biologic effect

57. 57
Narrow- and Broad-Beam Geometries

58. 58
N N e d
   
0

d: absorber thickness
:attenuation coefficient
HVL: half value layer TVL: tenth value layer
TRANSMISSION-PHOTONS

59. 59
Attenuation is the removal of photons from a beam of x- or gamma
rays as it passes through matter

60. 60
Linear Energy Transfer
• The LET is the rate at which energy is transferred to the medium
and therefore the density of ionisation along the track of the radiation.
• LET also referred to as “restricted stopping power” (LD)
• LET is expressed in terms of keV per micron
Radiation LET keV/m
1 MeV -rays
100 kVp X-rays
20 keV -particles
5 MeV neutrons
5 MeV aparticles
0.5
6
10
20
50
dX
dE
LET 

dE = energy lost by radiation
dX = length of track
• Radiation that is easily
stopped has a high LET and
vice versa

61. 61
Mass Attenuation Coefficient
• For given thickness, probability of interaction is
dependent on number of atoms per volume
 Dependency can be overcome by normalizing
linear attenuation coefficient for density of
material:
 Mass attenuation coefficient usually expressed in
units of cm2/g
)
(
Material
of
Density
)
(
t
Coefficien
n
Attenuatio
Linear
)
/
(
t
Coefficien
n
Attenuatio
Mass



 

62. 62
Mass Attenuation Coeff. (cont.)
• In radiology, we usually compare regions of an image
that correspond to irradiation of adjacent volumes of
tissue
 Density, the mass contained within a given volume,
plays an important role

63. 63
Radiograph of Ice Cubes in Water