Fm khan chapter 5 mod

INTERACTION OF IONIZING RADIATION
A.Harvin Nelson
Intern Medical Physicist
SRMSIMS
Barailly,Bojipura
Utterpradesh

Content
• Introduction
• Ionization
• Photon beam Description
• photon beam attenuation
• Co-efficient
• Interaction of photon with matter
• Relative importance of various type of interaction
• Interaction of charged particle
• Interaction of Neutron

• Ionization:
Ionization is the process by which a neutral atom
acquires a positive or a negative charge.
• Excitation:
If the energy lost by the incident particle is not
sufficient to eject an electron from the atom but is
used to raise the electrons to higher energy levels, the
process is termed excitation.

Photon beam Attenuation in Body

HVL
• The thickness of an absorber required to attenuate the intensity of a
mono- energetic photon-beam to half its original value is known as the
half-value- layer(HVL).

Attenuation Co- Efficient
 This co-efficients 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

• Energy transfer coefficient : The fraction of photon energy transferred
into kinetic energy of charged particles per unit thickness of absorber is
given by the energy transfer coefficient.
-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 : It is defined as 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.
Attenuation Co- Efficient

Coherent Scattering
• Coherent scattering also known as
“classical scattering or Rayleigh
scattering “
• No energy changed into electronic
motion ,
• No energy absorbed in the medium.
• The only effect is the scattering of the
photon at small angles .
• This scattering is probable in high ‘Z’ and
with photons of low energy (10keV).
• This process only of academic interest in
radiation therapy.

Photoelectric Effect
• Definition :
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’.
• The kinetic energy of the ejected
electron (called
the photoelectron) is equal to hν-EB.
• 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
• Energy Range up to 100 keV.

Compton Effect
• The process in which the photon
interacts with an “free” atomic
electron that is, the binding energy
of the electron is much less than the
energy of the bombarding photon.
• The analysis of ‘Compton process’
can be performed in terms of a
collision between two particles,
a photon and an electron, by
applying the laws of conservation
of energy and momentum.
• Following Relationship can be
derived:
where hν0 , hν ', and E are the energies of the incident
photon,scattered photon, and electron, respectively, and α =
hν0 /m0 c2, where m0 c2 is the rest energy of the electron
(0.511MeV).

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 scatter 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
Where α = hν0 /m0 c2
φ
ɵQ

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
φ
ɵ

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.
φ
ɵ

Pair Production
• The photon may interact with matter through the
mechanism of pair production, If the energy of
the photon is greater than 1.02 MeV.
• In this process ,the photon interacts strongly with
the electromagnetic field of an atomic nucleus
and gives up all its energy in the process of
creating a pair consisting of a negative electron
(e-) and a positive electron (e+).
• As the rest mass energy of the electron is
equivalent to 0.51 MeV, a minimum energy of
1.02 MeV is required to create the pair of
electrons.
• Thus, 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
photons, called the annihilation radiation.
Pair Production

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”.
• Because momentum is conserved
in the process , the two photons
are ejected in the right opposite
direction.

• The positron created as a result of pair production process
loses its energy as it traverse the matter by the same type of
interaction as an electron does namely ionization , excitation
and Bremsstrahlung.
• Near the end of this range the slowly moving positron
combines with one of the free electron in its vicinity to give
rise to two annihilation photons , each having 0.51MeV energy.
• Because momentum is conserved in the process ,the two
photons are ejected in opposite directions.
• Annihilation process is utilized in PET imaging (Nuclear
Medicine)
Annihilation Radiation

PHOTO-DISINTEGRATION
• An interaction of a high-energy photon with an atomic nucleus can
lead to a nuclear reaction and to the emission of one or more
nucleons.
• In most cases, this process of photodisintegration results in the
emission of neutrons by the nuclei.
• 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.
• This can be calculated by the definition of threshold energy, namely the
difference between the rest energy of the target nucleus and that of the
residual nucleus plus the emitted nucleon(s).
• Because the rest energies of many nuclei are known for a very high
accuracy, the photodisintegration process can be used as a basis for
energy calibration of machines producing high-energy photons.

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

• 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.
Relative importance of Various types of Interactions

Interactions of particulate radiation:
• Particulate radiation can be classified into two categories:
– charged particles.
– Uncharged particles.
• The charged particles used in radiotherapy are:
– Electron,
– Proton
– Pi – mesons (pions)

 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.
• This difference is due to the higher binding energy of the
electrons and the fewer electrons per gram in higher atomic
number elements .

• The two different modes of interaction and energy transfer of electrons
with matter include:
• Ionization results in the stripping of electrons from atom and may produce
ionization in it’s own turn – when it is called δ rays.
• Electrons are light particles with negligible mass and single negative charge. As a
result they penetrate deeper than other charged particles but at the same time
undergo greater scattering.
• 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 MeV – making these useful in clinical
practice.
• These characteristics make electrons a useful treatment modality for
superficial lesions.

• 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.
• 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.
• The ionization produced by mesons at the end of the track is even more intense
and is often referred to as star formation.
• 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, which complicates overall treatment.
– The large machines necessary for production of these beams often make
it necessary to move the patient instead of the gantry.

Interactions of neutrons:
• 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.
• The most efficient recoil is seen in the hydrogen nucleus and this leads to the
maximum absorption. This is an advantage because most of the soft tissues in
the body contains a large proportion of hydrogen.
• This phenomenon has some practical implications:
– 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.
• The recoil protons, set in motion after interaction with neutrons. further cause
ionization. The dense ionization produced by these particles in the vicinity,
results in high LET values.

Interactions of neutrons:
• Neutrons, being uncharged particles also penetrate deeply into matter
Despite these attractive radiobiological and physical properties, neutrons are
not commonly used in practical radiotherapy, because of technical difficulties
in production of these beams as well as their complicated dosimetry.
• LET has certain important radiobiological implications:
– High LET radiation is more likely to induce lethal damage in the cells
due to the dense ionization they produce.
– The oxygen enhancement ratio nears 1 as the LET increases –
advantage in hypoxic tumors.
– The effect of fractionation reduces as LET increases.
– High LET radiation preferentially increase the repair independent
• damage in the cells.
– High LET radiation also leads to reduced variability in the cell cycle
dependant radiosensitivity of cells.

Conclusions:
• The three major forms of interaction of radiation with matter, which are of
clinical importance in radiotherapy are:
1. Compton effect.
2. Photoelectric effect.
3. Pair production.
• Out of these, the Compton effect is the most important in modern-day
megavoltage radiation therapy.
• The reduced scattering suffered by high-energy radiation as well as the
almost homogeneous tissue dosage is primarily due to the Compton effect.
• The photoelectric effect is of primary importance in diagnostic radiology and
has only historical importance in present day radiotherapy.
• 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 particulate radiation.

Fm khan chapter 5 mod

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