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INTERACTION OF RADIATION
DR ARNAB BOSE
Dept. of Radiotherapy
NRS Medical College, Kolkata
Part 1 : Introduction
Importance of the knowledge of the
fundamentals of interaction with matter
3. Forms the basis of Radiobiology
4. Forms the basis of Radiation protection
5. Forms the basis of Radiation detection
6. Ensures safe and effective methodologies in
Radiology and Radiotherapy
The term radiation applies to the emission and
propagation of energy through space or a
material medium .
Radiation may be
When radiation passes through matter it may
interact with the material , transferring some or
all of its energy to the atoms of that material.
Constitutes the mode of energy propagation for
such phenomena as light waves, heat waves,
radio waves, u v rays, x rays and γ rays .
Spectrum of electromagnetic radiation ranges
from 107 m (radio waves) to 10-13 m (ultra high
energy X rays) .
X rays and γ rays are the two major forms of
electromagnetic radiation used in modern day
An X ray beam emitted from a target or a γ ray
emitted from a radioactive source consists of a
large number of photons , usually with a variety
of energies .
Electromagnetic Wave with respect to Electric & Magnetic Field
Refers to the energy propagated by traveling
corpuscles – which have definite rest mass ,
definite momentum and a defined position at any
Elementary atomic particles are electrons (charge
– 1) , protons (charge + 1) and neutrons (zero
Some common subatomic particles are positrons
(charge + 1) , neutrinos (zero charge) and
Part 2 : Interaction of Photons with
When an X ray or γ ray beam passes through a
medium , interaction occurs between the photon
and the matter and energy is transferred to the
If the absorbing medium consists of body tissues
sufficient energy may be deposited within the
cells destroying their reproductive capacity .
Fate of the Photon Beam
The photon beam may undergo the following four
processes – Attenuation , Absorption , Scattering
and Transmission .
Attenuation refers to the removal of radiation
from the beam by the matter . Attenuation may
occur due to scattering and absorption .
Absorption refers to the taking up of energy from
the beam by the irradiated material .It is the
absorbed energy which is important in producing
the radiobiological effects .
Scattering refers to the change in the direction of
photons and it contributes to both attenuation
and absorption .
Any photon which does not suffer the above
processes is transmitted .
Attenuation Coefficient (1)
Fraction of photons removed from a mono
energetic beam of x-ray or gamma ray per unit
thickness of material is called linear attenuation
coefficient (µ), typically expressed in cm-1 .
Number of photons removed from the beam
traversing a very small thickness ∆x:
n = N∆
where n = number removed from beam,
N = number of photons incident on the material,
and minus sign is placed before μ to indicate that
no. of photons decreases as the absorber
Attenuation Coefficient (2)
For a mono energetic beam of photons incident
on either thick or thin slabs of material, an
exponential relationship exists between number
of incident photons (N0 ) and those transmitted
(N) through thickness (x) without interaction:
N =N e 0
The number of photons indicate the Intensity of
the beam and can also be written as ( I ).
Attenuation Coefficient (3)
Total Linear attenuation coefficient is the sum of
individual linear attenuation coefficients for each
type of interaction:
µ = µRayleigh + µphoto + µCompton + µpair
For a given thickness of material , probability of
interaction depends on number of atoms the x
ray or gamma ray encounter per unit distance.
The density (ρ ) of material affects this number.
Linear attenuation coefficient is proportional to
the density of the material.
Mass Attenuation Coefficient
For a given thickness probability of interaction is
dependent on the number of atoms per volume.
This dependency can be overcome by
normalizing linear attenuation coefficient for
density of material –
Mass Attenuation Coefficient (μ / ρ ) =
Linear attenuation coefficient
Density of the material
Mass attenuation coefficient is independent of
density of the material.
Half Value Layer (1)
Half value layer (HVL) defined as thickness of
material required to reduce intensity of an x-ray
or gamma-ray beam to one-half of its initial
It is an indirect measure of the photon energies
(also referred to as quality or penetrability) of a
beam of radiation.
For mono energetic photons, the probability of
attenuation remains the same for each additional
HVL thickness placed in the beam.
Relationship between μ and HVL:
HVL = 0.693/μ
List of Interactions
Attenuation of a photon beam by an absorbing
material is caused by 5 major types of
2. Coherent Scattering
3. Photoelectric Effect
4. Compton Effect
5. Pair Production
6. Photonuclear Effect
X-rays passing close to the atom cause the bound
electrons to vibrate momentarily at a frequency
equal to that of the radiation. These in turn emit
radiation of the same frequency in all directions .
The energy is taken up from the beam and
scattered in all direction, but none of the energy
is absorbed. Thus this is a form of attenuation
without absorption .
This interaction is of little importance in practical
radiotherapy, but is important in X-ray
Since it involves bound electrons, it occurs more
in higher atomic number materials, and also
more with low-energy radiations.
Photoelectric Effect (1)
All of the incident photon energy is transferred to
an electron, which is ejected from the atom.
Kinetic energy of ejected electron called the
photoelectron (EC ) is equal to incident photon
energy (EO ) minus the binding energy of the
orbital electron (EB )
EC =EO - EB
Photoelectric Effect (3)
Incident photon energy must be greater than or
equal to the binding energy of the ejected
The ionized atom regains electrical neutrality by
rearrangement of the other orbital electrons. The
electrons that undergo these rearrangements
surrender some of the energy in form of a photon
known as the characteristic radiation of the atom.
Absorption of these characteristic radiation
internally in the atom may result in emission of
Auger electrons . These electrons are mono
energetic in nature.
Photoelectric Effect (4)
Probability of photoelectric absorption per unit
mass is approximately proportional to
Energy dependence explains, in part, why image
contrast decreases with higher x-ray energies.
Process can be used to amplify differences in
attenuation between tissues with slightly different
atomic numbers, improving image contrast.
Photoelectric Effect (5)
Graph of probability of photoelectric effect, as a
function of photon energy, exhibits sharp
discontinuities called absorption edges .
Photon energy corresponding to an absorption
edge is the binding energy of electrons in a
particular shell or sub shell .
The phenomena of absorption edges is important
for two different reasons:1) At these absorption
edges, low-energy photons are less attenuated
and therefore more penetrating than high energy
photons. 2)A substance is relatively transparent
to its own characteristic radiation. This effect is
important when filters are considered as the
filters will be “transparent” to their own
Compton Effect (1)
Photon interacts with an atomic electron as
though it were a free electron. Practically this
means that energy of the incident photon must
be large compared with the electron binding
The electron receives some energy from the
photon and is emitted at an angle Φ while the
photon with reduced energy is scattered at an
angle θ .
Compton Effect (4)
As incident photon energy increases, scattered
photons and electrons are scattered more toward
the forward direction.
Probability of interaction increases as incident
photon energy increases.
Probability also depends on electron density ( no.
of electrons per gram of matter ).
Electron density fairly constant in tissue.
Probability of Compton scatter/unit mass
independent of Z.
Compton Effect (5)
Maximum energy of photons with 90° scatter is
0.511 M e V while that for 180° scatter ( i.e..
Back scatter) is 0.255 M e V.
Energy of the photons scattered at angles <90 °
will be more than 0.511 M e V and will gradually
approach the incident photon energy.
Energy of the scattered radiation is independent
of the incident beam energy .This implies that as
the photon energy increases there is a
corresponding increase in the forward scatter of
the beam. This results in better dose distribution.
Direction of the scatter depends on the energy of
the incident photon beam . This means that
higher beam energies allow greater absorption of
the dose in the body with less scattering of
Compton Effect (6)
If the angle by which the electron is scattered is
θ and the angle by which the photon is scattered
is Φ, then the following formula describes the
change in the wavelength ( δ λ )of the photon:
λ 1 – λ 2 = δ λ = 0.024 ( 1- c o s Φ) Å
Thus the wavelength change depends neither on
the material being irradiated nor on the radiation
energy, but only upon the angle through which
the radiation is scattered.
The Compton effect results in both attenuation
and absorption .
Compton Effect (7)
Laws of conservation of energy and momentum place
limits on both scattering angle and energy transfer.
Maximal energy transfer to the Compton electron
occurs with a 180-degree photon backscatter.
Scattering angle for ejected electron cannot exceed 90
Energy of the scattered electron is usually absorbed
near the scattering site.
Pair Production (1)
When the photon with energy in excess of 1.02
M e V passes close to the nucleus of an atom,
the photon disappears, and a positron and an
electron appear. This effect is known as pair
Pair production results in attenuation of the beam
The positron created as a result loses its energy
by interaction with an electron to give rise to two
annihilation photons, each having 0.511 M e V
energy. Again because momentum is conserved
in the process two photons are rejected in
opposite directions. This reaction is known as an
Pair Production (3)
Pair production results from an interaction with
the electromagnetic field of the nucleus and as
such the probability of this process increases
rapidly with the atomic number (Z 2 ).
In addition, the likelihood of this interaction
increases as the photon energy increases, in
contrast to the Compton effects and the
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 possessed in excess
of that needed to remove the particle becomes
the kinetic energy of escape of that particle.
The threshold energy for this effect is 10.8 M e V.
The main use of this reaction is for energy
calibration of machines producing high energy
Relative Importance of the
The relative importance of the 3 principal modes of
interaction pertinent to radiation therapy- the
Photoelectric , Compton and Pair production processes
- as a function of Incident beam energy and Atomic
number of absorber matter shows -
For an absorber with Z approximately equal to that of
soft tissue - 7 , and for mono energetic photons ,
Photoelectric effect is the dominant interaction below
about 30 k e v.
Above 30 k e v Compton effect remains dominant and
Until about 24 M e v , after which Pair Production effect
becomes dominant .
Relative Importance of the
Various Processes (3)
In a graph plotted for total mass attenuation
coefficient vs. photon energy it is seen that -
The μ /ρ is large for low energies and high Z
media (eg. Lead ) because of the predominance
of Photoelectric interactions under these
conditions. The μ /ρ decreases rapidly with
energy until the photon energies far exceed the
electron binding energies and Compton effect
becomes the predominant mode of interaction.
In the range of Compton effect the μ /ρ of lead
and soft tissues do not vary greatly as Compton
effect is independent of Z .The μ /ρ however
decrease with energy until Pair production
becomes important .
Plot of total mass att. Coef.
As a function of photon energy
Part 3 : Interaction of Particle
Radiation with Matter
Interaction of Electrons with matter –
The two different modes of interaction and energy
transfer of electrons with matter include:
Collision between the particle and the electron
cloud resulting in ionization and excitation ( more
important in low atomic number elements). This
is called Collisional loss .
Collision between the nucleus and the particle
resulting in bremsstrahlung radiation (more in
high atomic number elements). This is called
Radiative loss .
The ionization pattern produced by a beam of
electrons is characterized by a constant value
from the surface to a depth equal to about half
the range, followed by a rapid falling off to almost
zero at a depth equal to the range . The
bremsstrahlung radiation produced when
electrons slow down contributes to an
insignificant dose beyond the range of any
electron. This is specially seen in electrons in the
energy range of 6 -15 M e V .
These characteristics make electrons a useful
treatment modality for superficial lesions.
Proton & Pi Meson Interactions
Protons and pi mesons are charged particles that
are being used in experimental set-ups only.
These particles have a very high linear energy
transfer (LET) that is they have a very high
ionization density( Amount of energy deposited
per unit path length is called the linear energy
transfer (LET) ).
Further, these charged particles also exhibit the
phenomena of Bragg’s peak which refers to the
increased ionization occurring near the end of the
track with little effect beyond.
• Neutrons are indirectly ionizing uncharged
radiations, which interact only with the nucleus
in two ways:
By recoiling protons from hydrogen and the
nucleus in other elements.
Nuclear disintegration , which contribute to
~30% of the total dose in tissues.
Part 4 : Practical Implications
The three major forms of interaction of radiation
with matter, which are of clinical importance in
Out of these, the Compton effect is the most
important in modern-day megavoltage
The reduced scattering suffered by high-energy
radiation as well as the almost homogeneous
tissue dosage is primarily due to the Compton
Coherent scattering is of little importance in
practical radiotherapy, but is important in X-ray
The photoelectric effect has several important
implications in practical radiology:
In diagnostic radiology , the primary mode of
interaction is photoelectric. It is also responsible
for the contrast effect.
In therapeutic radiology , low-energy beams in
orthovoltage irradiation causes excessive
absorption of energy in bone.
The attenuation produced by the Compton effect
is described by the mass scattering coefficient
( σ / ρ ), and is practically same for all
substances except hydrogenous material, like
water and soft tissue, where the Compton effect
is greater (because of the higher electron
Attenuation does not depend on the atomic
number of absorber matter in Compton effect.
Thus concrete is as good as lead in shielding of
The absorption in bones does not exceed that
produced in the soft tissues – unlike in PE effect
seen in orthovoltage radiation era.
Port films produced in megavoltage equipment
have very little detail.
The low mass of the electron leads to greater
scattering. This is of practical importance as
radioactive isotopes which are produce high
energy beta radiation are better stored in low
atomic number materials e.g. plastics as they will
lead to lesser bremsstrahlung radiation.
Also higher atomic number elements are better
for x ray production. The amount of radiative loss
is proportional to the square of the atomic
number of the material This leads to the
phenomenon of greater ionization in soft tissues
relative to bones. Ionization and excitation are
more for low atomic materials.
Protons and Heavy particle beams have the
ability to concentrate dose inside the target
volume and minimize dose to surrounding normal
tissues because of the Bragg peak effect and
However there are several practical and
theoretical difficulties with the use of these
charged particles. Some of them include:
The narrow Bragg peak makes a homogenous
Tumor Dose difficult.
Generation of these charged particles requires
expensive and large machines.
The method of the production ensures that the
field size is very narrow. So, for treatment of
cancers the beam has to be scanned back and
forth across the treatment area.
Hydrogenous materials like fats absorb neutrons
more than heavier materials and thus there is a
20% greater absorption in fat relative to muscle.
Lower atomic materials (e.g. fats and paraffin)
are better for neutron shielding as compared to
lead as greater absorption occurs.
Neutrons, being uncharged particles also
penetrate deeply into matter.
But neutrons are not commonly used in practical
radiotherapy, because of technical difficulties in
production of these beams as well as their
Part 5 : Conclusion
Despite several decades of research, photon-
beam still constitute the main therapeutic
modality in radiotherapy, because of several
unresolved technical problems with the use of