Nuclear activation analysis

1. Presented By: Dr. Vandana
Junior Resident,
Dept. of Radiotherapy
CSMMU, Lucknow
Moderator: Mr. Teerthraj

2. The term radiation applies to the emission and
propagation of energy through space or a
material.

3.  no mass or physical form
 travel at speed of light (c) in a vacuum (or
air)
 c = 3 x 108 m/s
 travel in a linear path (until interaction
occurs)
 unaffected by
 electric or magnetic fields
 gravity
3

4.  obeys the wave equation
c = λ ν
 In Passing through the matter, the intensity is reduced
(attenuation), because of absorption &
scattering.
 obeys the inverse square law
I1d1
2
= I2d2
2
Radiation intensity is inversely proportional to the square of the distance from 4

5.  dual nature: wave vs.
particle
 Wave: continuously
changing force fields
 energy travels as sine
WAVE
 macroscopic level
 Particle: photon or
quanta
 small packet of energy
acting as a PARTICLE
 microscopic level
5

6. Ionizing
ionizes [strips
electrons from] atoms
Non-Ionizing
many other modes of
interaction
Electro Magnetic Spectrum (EMR)

7. Radiation that has enough energy to
move atoms to vibrate, but not
enough energy to remove electrons.
The process by which a neutral atom
acquires a positive or a negative charge
is known as Ionization.
Removal of an orbital electron leaves the
atom positively charged, resulting in an
ion pair.
• molecule with a net positive
charge
• free electron with a negative
charge
c
Non-Ionizing Vs Ionizing
Radiation

8.  Atom - The smallest indivisible part of
an element.
 Nuclei + Orbital e-
= Atom
 Nucleus  protons and neutrons
 Atoms are specified as ZXA
where Z =
atomic number, and A = mass number.

9. Fig : Bohr’s model of the
atom
Fig : Energy level diagram
(Hydrogen Nucleus)
 According to Niels Bohr, electrons
revolve in specific orbits around the
nucleus. These orbits are named as K,L,M
etc; K being innermost orbit.
 These electron orbits are synonymous
with energy levels.
 Higher the atomic number, greater is this
binding energy.

10.  Amount of Energy required to remove an
electron completely from an atom
 B.E. α Z (Atomic Number )
the greater Atomic Number, the greater
binding energies
10

11.  When an x-ray or γ ray beam passes through a medium,
interactions occur between the beam and the matter.
 Initially the electrons are ejected from the atoms of the absorbing
medium which in turn, transfer their energy by producing
ionization and excitation of the atoms along their path.
 If the absorbing medium consists of body tissues, sufficient
energy may be deposited within the cells, destroying their
reproductive capacity.
However, most of the absorbed energy is converted into
heat, producing no biologic effect.

12. Process Definition
Attenuation Removal of radiation from the beam by the matter.
Attenuation may occur due to scattering and absorption
Absorption The taking up of the energy from the beam by the irradiated
material. It is absorbed energy, which is important in
producing the radiobiological effects in material or soft
tissues.
Scattering refers to a change in the direction of the photons and its
contributes to both attenuation and absorption
Transmission Any photon, which does not suffer the above processes is
transmitted.

13. High Speed
Electrons
Photon

14.  When mono-energetic (mono-
chromatic) 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 material. Fig : Semilog plot showing exponential
attenuation of a monoenergetic photon
beam.

15. • Half-value-layer (HVL)- The thickness of the absorber material required
to decrease (attenuate) the intensity of a monoenergetic photon-beam to
half its original value.
• This shows the quality or the penetrating power of an x-ray beam.
2nd
HVL
1st
HVL

16.  Linear attenuation coefficient (μ) : The fractional reduction (in any
monoenergetic photon-beam) for any given material per unit
thickness.
 μ : is the probability of the photon being removed by a given
material.
μ = 0.693 / HVL
 The linear attenuation coefficient depends upon the density of the
material. As compression of a layer of material to one half of the
thickness will not affect its attenuation.
 To circumvent this problem, the mass attenuation coefficient is used
which is defined as:
Mass attenuation coefficient = μ / ρ

17. Attenuation of a photon beam by an absorbing material is
caused by five major types of interactions :

18. • Elastic scattering, Thomson scattering, unmodified scattering,
classical scattering, Rayleigh scattering, etc.
• X-rays cause the bound electrons to vibrate. These in turn emit
radiation of the same frequency in all directions.
• Wave nature of radiation
•
Attenuation without absorption- The energy is scattered in all
direction, but none of the energy is absorbed.
• Little importance in practical
radiotherapy, but is important
in X-ray crystallography.

19. Inelastic scattering, Modified, incoherent.
An incident photon interacts with an orbital electron to produce a recoil
electron and a scattered photon of energy less than the incident photon.
-
-
-
Incoming photon
Collides with
electron
-
-
-
-
Electron is
ejected from
atom
-
Scattered
Photon
Before
interaction
After
interaction

20.  The photon collides with electron and hands over part of its
energy to it. The angle through which the photon is scattered,
the energy handed over to the electron, and energy lost by the
photon are interconnected.
If the angle by which the electron is
scattered is Φ and the angle by which
the photon is scattered is θ, then the
following formula describes the
change in the wavelength (δλ)of the
photon:
λ2 – λ1 = δλ = 0.024 ( 1- cos θ) Å

21. • The Compton effect results in both attenuation and absorption.
• The attenuation caused here is dependent upon the Electron
density 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
density).
• It does not depend on Atomic Number.
• It is measured as mass scattering coefficient (σ/ρ),

22. Material Density (g/cm3
) Atomic Number Number of Electrons
per Gram
Hydrogen 0.0000899 1 6.00 × 1023
Carbon 2.25 6 3.01 × 1023
Oxygen 0.001429 8 3.01 × 1023
Aluminum 2.7 13 2.90 × 1023
Copper 8.9 29 2.75 × 1023
Lead 11.3 82 2.38 × 1023
Effective Atomic
Number
Fat 0.916 5.92 3.48 × 1023
Muscle 1.00 7.42 3.36 × 1023
Water 1.00 7.42 3.34 × 1023
Air 0.001293 7.64 3.01 × 1023
Bone 1.85 13.8 3.00 × 1023
Data from Johns HE, Cunningham JR. The physics of radiology. 3rd ed. Springfield, IL:
Charles C Thomas, 1969.

23.  The photoelectric effect is a phenomenon in
which a photon interacts with an atom and
ejects one of the orbital electrons from the
atom.
 The photon transfers all its energy to the
atom. This is used to overcome the binding
energy as well as to provide the kinetic
energy to the photo-electron.
hν - W + ½ mν2
W = The binding energy of the electron and
½ mν2
is the kinetic energy of the photo
electron.
 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 monoenergetic in nature.
Fig. : The photo electric effect

24.  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
• As the graph on the right shows,
there are discontinuities in the
attenuation coefficient at specific
photon energies.
• The absorption edges,
correspond to the binding
energies of the electrons in
different shells.

25. • 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 caused excessive absorption of
energy in bone.

26.  Pair Production: When the photon with energy in excess of 1.02
MeV passes close to the nucleus of an atom, the photon
disappears, and a positron and an electron appear.
 Annihilation: These two particles collide, converting to 2 photons
with equal energy of 511 kev.

27.  Thus, the energy absorbed from the beam (with incident energy,
E) is given by:
Eabsorbed = E - 1.02 MeV
 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 (Z2
).
 In addition, the likelihood of this interaction increases as the
photon energy increases.
 The pair production coefficient (π) is directly proportional to Z2
and log of incident photon energy.
π = k Z2
log (E)

28.  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.
 In most cases, this process results in the emission of
neutrons by the nuclei.
 This has a threshold of 10.86 MeV.
 Now a days, the main use of this reaction is for energy
calibration of machines producing high energy photons.
For this the following reaction is used:
29Cu63
+γ  29Cu62
+ 0n1

29.  The Total Mass attenuation coefficient is the sum of three
individual coefficients; photoelectric coefficient, mass scattering
coefficient and pair production coefficient:
(μ/ρ) = (τ/ρ)+(σ/ρ)+(π/ρ)
 At low energies the photo electric attenuation coefficient is
larger.
 In between the ranges of 200 KeV- 4 MeV, Compton scattering is
the predominant mode of interaction.
 In the ranges above, pair production is dominant.

31. Photon Energy
(MeV)
Relative Number of Interactions (%)
P.E. (τ/ρ) Compton (σ/ρ) Pair Prod. (π/ρ)
0.01 95 5 0
0.026 50 50 0
0.060 7 93 0
0.150 0 100 0
4.00 0 94 6
10.00 0 77 23
24.00 0 50 50
100.00 0 16 84
Data from Johns HE, Cunningham JR. The physics of radiology. 3rd ed. Springfield,
IL: Charles C Thomas, 1969.

32. Figure: Plot of total mass attenuation coefficient (μ/ρ) as a function
of photon energy for lead and water. (from Johns HE, Cunningham JR.
The physics of radiology, 3rd ed.)
Energy
Range
Dominant Effects
Up to 50KeV PE (Photo Electric)
effect is important
60 KeV - 90
KeV
Both PE & Compton
effect
200 KeV - 4
MeV
Compton effect
Beyond 20
MeV
Pair Production

33. Radiation type Direction
Recoil electrons Travels forward, angle not more than 90°.
Photoelectrons and electron
pairs
Travels forward
Characteristic and annihilation
radiation
Isotropic i.e. equally in all directions
Coherent scattered photons Isotropic
Compton scatter photons In forward direction, small angle of
scattering, lesser scattering for greater
incident energy

34.  Most of electrons set in motion by the above interactions lose
energy by inelastic collisions with the atomic electrons of the
material.
 Some electrons also loose energy by Bremsstrahlung
interactions with the nuclei.
Beta
Particle
-
Bremsstrahlu
ng Photon
+ +
Nucleus

35.  Thus, the energy absorption coefficient(μen) is defined as the
product of the energy transfer coefficient(μtr) and (1-g) where g
is the fraction of energy of secondarily charged particles lost to
bremsstrahlung in the material.
μen = μtr (1-g)
 In most interactions involving the soft tissues, the
bremsstrahlung component is negligible , and the energy
absorption coefficient is equal to the energy transfer coefficient
under these conditions.

36.  The relationship between the mass attenuation
coefficients and the mass absorption coefficient varies
as per the radiation energy as follows:
Photon energy
Mass
coefficient
100 KeV 1 MeV 10 MeV
91%
1
5
%
4
6
%
7
1
%
9
6
%
10 KeV
% of attenuated energy absorbed
μen
μ/ρ

37.  The mass absorption coefficients are practically identical
for most biological materials .
 In this energy range, the absorption per gram is maximum
for hydrogen, because of its higher electron density.
 However in very high and very low energy ranges the high
atomic number materials e.g. Bone absorb more radiation
with several unfortunate consequences.
 The energy absorption coefficient is an important quantity in
radiotherapy since it allows the evaluation of energy absorbed
in the tissues, a quantity of interest in predicting the biologic
effects of radiation.
Absorption (contd.)

38.  Particulate radiation can be classified into two categories:
◦ Ionizing or charged particles - Electron, Proton
◦ Uncharged particles.
 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. This is called Collisional loss.
◦ Collision between the nucleus and the particle resulting in
bremsstrahlung radiation. This is called Radiative loss.

39.  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.
 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.

40.  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.
 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

41.  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.

42.  Cellular damage may occur directly when the radiation interacts
with the atom directly ( e.g. neutrons) or indirectly when
interaction occurs by secondary electrons (e.g. photon beams).
 Electrons produced by the ionizing events lead to further
ionizations as they move inside biological material --> these lead
to the formation of highly reactive free radicals like OH-
, H-
radicals which in turn lead to chemical changes by breaking
chemical bonds.
 Some of these reactions are potentially damaging to the cell,
others effectively inactivate the radicals.
 The reactions that most commonly lead to cell damage usually
occur at the level of the DNA although they may occur at the level
of cell membranes, proteins etc.

43.  The main effect of radiation is to cause ionisation of the atoms in
the absorbing medium.
 Thus, when cells are irradiated, it is likely that ionisation of one or
more of the atoms on some of the DNA molecules will occur.
 This can lead to a number of consequences for the affected
molecule. These effects include
◦ breakage of the chains of molecules comprising the DNA, and
◦ breakage of the links between chains.

44.  The human body (about 60%) is
made up of water, and the
ionizing effects of radiation on
water can lead to an indirect
attack on DNA. The direct
attack of radiation on the
structure of DNA is not the
only means by which radiation
can affect cells.
 The radiation produces H2O2
after reaction with Water.
Hydrogen peroxide is a
chemically active and is
capable of reacting with DNA
to damage cells and the
genetic information contained

45.  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.

46.  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.

47. Any Questions?