Radiation oncology , Interactions of Photons with matter

1. INTERACTION OF
PHOTONS WITH
MATTER
- Dr Ankita
Singh
JR, RT&RM
IMS-BHU

2. RADIATION
 The term radiation applies to the emission and propagation
of energy through space or a material medium.
Types
Electromagnetic
Particulate

3. ELECTROMAGNETIC RADIATION
A. Wave model

4. B.
Quantum model
-Considered electromagnetic radiation as particle
-The amount of energy carried by such a packet of energy or
Photon,

5. Eletromagnetic spectrum

6. ionizing
radiation
Non ionizing
radiation
Directly
ionising
Indirectly
ionising
Particulate
• Electron
• Proton
• Neutron
• Alpha
particle
• Heavy
EM waves
X rays Gamma
rays
• Ultraviolet
• Infrared
• Visible
light
• Microwave
s
• Radio
waves

7. INTERACTIONS OF PHOTONS

8. A. Excitation
&
Deexcitation with subsequent
release of EM radiation
B. Ionisation
&
the production of Delta
rays

9. EXCITATION &
IONIZATION
Excitation is the transfer of some of
the incident particle's energy to
electrons in the absorbing material,
promoting them to electron
orbitals farther from the nucleus
(i.e., higher energy levels).
 In excitation, the energy
transferred to an electron does not
exceed its binding energy.
 the electron will return to a lower
energy level, with the emission of
the excitation energy in the form of
If the transferred energy exceeds the
binding energy of the electron,
ionization occurs, whereby the electron
is ejected from the atom .
 The result of ionization is an ion pair
consisting of the ejected electron and
the positively charged atom.
Sometimes the ejected electrons
possess sufficient energy to produce
further ionizations called secondary
ionization.
 These electrons are called delta rays

11. Photon Beam
Attenuation When radiation passes through any material, a reduction in the
intensity of the beam occurs, This is known as attenuation.
 Attenuation occurs exponentially, i.e. a given fraction of the
photons is removed for a given thickness of the attenuating

13.  The reduction in the number of photons (dN) is
proportional to the number of incident photons (N) and
to the thickness of the absorber (dx).
• Where μ is called Proportionality constant
• This equation can be written in terms of intensity
 If thickness x is expressed as a length, then μ is called the “linear

14.  The greater the thickness of material , the greater the
attenuation.
 For a given thickness, the greater the atomic number and/or
the density of the material, the greater the attenuation.
 The greater the photon energy , the smaller the attenuation
produced by a given thickness of a particular material.

15. Half-value-layer (HVL)-
 The thickness of the absorber material
required to decrease (attenuate) the intensity
of a monoenergetic photon-beam to half of its
original value
This reflects the quality or the penetrating power
of an x-ray beam.
From the equation,
I (x) =I e  x

16. ATTENUATION CO-EFFICIENTS
 This coefficients depends on the energy of the photons and the nature of
material.
• Since the attenuation produced by a thickness x depends on the number
of electrons present in that thickness, μ depends on the density of the
material.
 Mass attenuation coefficient: Attenuation coefficient per unit density ρ is
called mass attenuation coefficient.
/ (cm2/g)
 Electronic attenuation coefficient: The absorber thickness can also be
expressed in units of electrons/cm2 .
(/) (1/NO) (cm2/electron)
where N0 is the number of electrons per gram

17.  Energy transfer coefficient :
The fraction of photon energy transferred into kinetic energy of
charged particles per unit thickness of absorber.
- where Etr is the average energy transferred into kinetic energy of
charged
particles per interaction.
- The mass energy transfer coefficient is given by tr/ .
 The energy absorption coefficient en : The product of energy transfer
coefficient and (1 - g) where g is the fraction of the energy of
secondary charged particles that is lost to bremsstrahlung in the
material.
en = tr (1 - g)

18. 1. can penetrate the section of matter without
interacting.
2. It can interact with the matter and
be completely absorbed by depositing its
energy.
3. It can interact and be scattered or deflected
from its original direction and deposit part of
its energy.

19. SCATTERING
 Scattering refers to an interaction resulting in the deflection of a
particle or photon from its original trajectory,
Elastic
scattering
Inelastic
Scattering
 The total kinetic
energy of the
colliding particles
is unchanged
 Attenuation
without absorption
 When scattering occurs with
a
loss of kinetic energy i.e., the
total kinetic energy of the
scattered particles is less than
that of the particles before the

20. Coherent scattering/classical/ Rayleigh
scattering
EM waves passing near the electron
Setting it into oscillation
Irradiates the energy at the same frequency as the
incident EM wave.

21.  Scattered X rays have the same wavelength as the incident
beam.
 No energy is changed into electronic motion & no energy is
absorbed into medium
 This process involves bound electron, coherent scattering
occurs more in high atomic number materials and with low
energy radiations.
 Important in X-ray crystallography: to know about the structure
of materials.

22. PHOTOELECTRIC EFFECT
 The process in which a photon is
absorbed by an atom, and as a result one
of its orbital electrons is ejected is called
‘Photoelectric effect’.
 In this process, the entire energy (hν) of
the photon is first absorbed by the atom
and then essentially all of it is transferred
to the atomic electron.
 The kinetic energy of the ejected electron
(called the photoelectron) is equal to
hν - EB.
where EB is the binding energy of the

23.  The ionized atom regains electrical neutrality by rearrangement of
the other orbital electrons.
 The electrons that undergo the these rearrangements surrender
some of the energy in form of a photon known as the characteristic

25.  Absorption of the characteristic radiation internally in the atom may
result in emission of Auger electrons.
 These electrons are monoenergetic in nature
 Dominant interaction at energies of 10- 26 KeV
 The probability of the photoelectric effect occurring is strongly
dependent on the atomic number of the material traversed and on the
energy of the incident photon
Probability ~ Z3/E3
 The mass photoelectric attenuation coefficient (τ/ρ) is directly
proportional to the cube of the atomic number and inversely
proportional to the cube of the radiation energy.
τ/ρ = k Z3/ E3

26. Absorption edges
The sudden increase in attenuation of the radiation occur at photon
energies equal to the binding energies of the different shells due to
increased probability of PE absorption.

27.  Graph of mass photoelectric
attenuation
coefficients plotted against photon
energy, & for different materials.
• The graph for lead has discontinuities
at
about 15 and 88keV.
• These are absorption edges, &
correspond to the binding energies of L
& K shells.
• A photon with energy less than 15 keV
does not have enough energy to eject an
L electron.
• Thus, below 15 keV, the interaction is

28. Clinical application-
In diagnostic imaging
PE absorption – white areas on the radiograph
Transmitted X rays - grey areas on radiograph
1. As it provides clear differentiation between tissues with different
atomic number
(Eg - bone ,muscle, fat) amplifies differences in Xray absorption due to
differences in Z.
2. Z dependence is also exploited when using contrast materials such as
Barium for the greater appreciation of structures that would otherwise not
visible clearly.
3. Benefit of photoelectric absorption in x-ray transmission imaging is
that there are no nonprimary photons to degrade the image.( radiograph –
too black)

29. The probability of photoelectric interaction is proportional to 1/E3
explains, why image contrast decreases when higher x-ray energies are
used in the imaging process
KV imaging is better soft tissue visibility and contrast that MV imaging

30. COMPTON SCATTERING
Also known as incoherent scattering, modified scattering
 Compton process involves transfer of a part of the energy
of the incoming photon to a “free electron”.
Predominant at 100 KeV - 1 MeV
 Electron receives some energy and ejected at an angle
and photon with reduced energy (increased wavelength)
scattered at an angle
Since the Compton process involves these free electrons,
the process is independent of the atomic number of the
medium in which the interaction takes place.

33.  If the angle by which the electron is ejected is θ and the angle by
which the photon is scattered is Φ,
then theformula describes the change in the wavelength (δλ) of the
photon
λ2 – λ1 = δλ = 0.024 ( 1- cos θ) Å

34. The Compton process can be analysed in terms of collison
between 2 particles , a Photon & an Electron
By applying Laws of conservation of Energy & Momentum,
hν0 , hν ', and E are the energies of the incident photon, Scattered
photon, and electron, respectively,
α = hν0 /m0 c2, where m0 c2 is the rest energy of the electron (0.511MeV).

35. DIRECT HIT
If a photon makes a direct hit with the electron, the electron will travel
forward (θ = 0 degrees) and the scattered photon will travel backward
(φ = 180 degrees) after the collision.
• In such a collision, the electron will receive maximum energy Emax
and the scattered photon will be left with minimum energy hν I min.
• Emax and hν I min can be calculated by substituting
cos φ = cos 180o = -1

36. APPLICATION
Interaction with low
Energy incident Photon
Interaction with High
Energy Incident Photon
 Compton scattered
Photon have approx. the
same energy as the
original photons, only
small part is imparted to
the electron.
 Scattered photon carry
away only a small fraction
of initial energy
 Compton effect causes a
large amount of energy
absorption as compared
to tat with low energy
photons.

37. Average proportion of Photon energy transmitted to secondary electron during
Compton process
 Energy transmitted to the secondary electrons increases with increase
in energy of incident photon

38. GRAZING HIT
If a photon makes a grazing hit with the electron, the electron will be
emitted at right angles (θ = 90 degrees) and the scattered photon
will go in the forward direction (φ = 0 degrees).
By substituting cos φ = cos 0o = 1
Substituting these above values in the equations we get ,
Emax = 0
hν ' = hν0

39. 90-DEGREE PHOTON SCATTER
If a photon is scattered at right angles to its original direction
(φ = 90 degrees)
• Emax and hν ' can be calculated from acquired equations by
substituting
cos φ = cos 900 = 0
• The angle of the electron emission in this case will depend on α.

40. DEPENDENCE OF COMPTON
EFFECT ON E & Z
 Compton effect decreases with increase in
Energy
 Independent of Atomic number
 Depends only on Electron density i. e number
of electron per gram
• Number of electron per gram decreases slowly
but systemically with atomic number
• Most materials except Hydrogen have approx.
same electron density
• c nearly same for all material.

41. CLINICAL APPLICATION
 The probability of the Compton interaction depends on the density of
electrons in a material, which varies as Z/A.
This ratio is almost constant for elements except hydrogen
So the Compton effect can be considered to be independent of the atomic
number of the material the photons pass through and is dependent only on
the electron density.
1. Medical imaging with megavoltage photons leads to poorer contrast than
imaging with kilovoltage photon beams.
2. A benefit for radiotherapy to tumors as a dependence on atomic number
would lead to higher absorbed dose being delivered to bone than soft tissue.

42.  As the incident photon energy increases, a higher proportion of its energy
is transferred to the electron.
have implications for radiotherapy and radiation dosimetry.
1. For kilovoltage photon beams, electrons set in motion through Compton
interactions & deposit their energy very close to the site of interaction,
2. For megavoltage photons, these interactions produce high energy
secondary electrons which will travel a significant distance.
observed skin-sparing effect of absorbed dose deposition in tissue by
megavoltage photon beams, as electrons set in motion near the skin surface
deposit their energy over a significant depth.

43. PAIR PRODUCTION

44.  The threshold energy for the pair production process is 1.02 MeV.
 The photon energy in excess of this threshold is shared between the
particles as kinetic energy.
 The total kinetic energy available for the electron-positron pair is given
by
(hν = 1.02) MeV.
 The particles tend to be emitted in the forward direction relative to the
incident photon.
 The pair production process is an example of an event in which energy
is converted into mass, as predicted by Einstein's equation
E = mc2
 The reverse process, namely the conversion of mass into energy, takes
place when a positron combines with an electron to produce two

45. VARIATION OF PAIR PRODUCTION
WITH E & Z
 Pair production results from interaction with
electromagnetic field of nucleus,
 probability increases rapidly with atomic number
Pair atomic attenuation coefficient
aП α Z2
Pair electronic attenuation coefficient
П α Z
Pair mass attenuation coefficient
П α Z

46.  The likelihood of pair production increases as the logarithm of the
incident photon energy , above the threshold energy.
 For energies upto 20 MeV , curves are coincident for all materials indicating ,
aП α Z2
 For Higher energies , the curve for high Z materials fall below the low Z
materials because of screening of nuclear charge by orbital electron

47.  In water (and soft tissue), pair-production only becomes
significant at photon energies above approximately 10 MeV so
accounts for very little of the absorbed dose to a patient
undergoing radiotherapy.
Mass attenuation coefficients, showing the relative
contributions from the photoelectric effect, Compton effect
and pair- production in
water (effective Z = 7)

48. ANNIHILATION RADIATION
 Two photons of energy 0.51 MeV are
produced when positron generated in Pair
Production combines with electron after
many interactions
These photons are called as “Annihilation
photons”.
 Due to momentum conservation of energy
the direction of propagation these photons
becomes opposite

49. PHOTODISINTEGRATION
 This reaction occurs when the photon has energy
greater than the binding energy of the nucleus
itself.
 In this case, it enters the nucleus and ejects a
particle from it.
 The photon disappears altogether, and any
energy possesses in excess of that needed to
remove the particle becomes the kinetic energy of
escape of that particle.

50.  An example of such a reaction is provided by the nucleus of
63Cu bombarded with a photon beam:
The above reaction has a definite threshold, 10.86 MeV
 Because of the production of neutrons , it is important to consider
for neutron shielding in RT bunker where Energy of Photon is
above 10 MeV

51. Relative importance of Various types of
Interactions
The Total mass attenuation coefficient () is the sum of four individual
coefficients for these processes:
Where.,
• -Total mass attenuation co-efficient
• coh  -Coherent scattering
•   -Photoelectric effect
• c-Compton effect
• -Pair production

53.  The mass attenuation coefficient is large for low energies and high-atomic
number
media because of the predominance of photoelectric interactions under
these conditions.
 The attenuation coefficient decreases rapidly with energy until the photon
energy far exceeds the electron-binding energies and the Compton effect
becomes the predominant mode of interaction.
 In the Compton range of energies, the  of lead and water do not differ
greatly, since this type of interaction is independent of atomic number.
 The coefficient, however, decreases with energy until pair production
begins to become important.
 The dominance of pair production occurs at energies much greater than
the threshold energy of 1.02 MeV.