Attenuation & Buildup of Radiations 1
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Transcript of Attenuation & Buildup of Radiations 1
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Muhammad Afzal Nagrah
SSO, CNS
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Interaction of radiations with matter Charged particles interaction
-ray interaction
Mechanisms of interaction with matterBuildup of secondary radiations
Attenuation/Absorption of primary
radiationsExamples calculation
Quiz
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There are the four basic interactions forces:
1. Gravitational
2. Electromagnetic
3. Nuclear strong
4. Nuclear weak
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Passage of radiation through matter
depends on:
Type of radiation charged particles (e.g., electrons, protons, etc.)
high energy photons or x-rays
Energy of radiation (e.g., keV or MeV)
Nature of matter being traversed (atomic numberand density)
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Passage of charged particles throughmatter Gradual loss of particles energy
Energy transferred to nearby atoms andmolecules
Charged particle interaction
mechanisms Ionization Excitation
Bremsstrahlung
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Excitation
Energy is transferred to an orbital electron, but not
enough to free it.
Electron is left in an excited state
and energy is dissipated in
molecular vibrations,
atomic emission of infrared,
visible or uv radiation, etc.
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Ionization Interaction between charged particle and orbital
electron
Energy transferred from passing particle to electron If E > ionization potential, electron is freed
Ionization potential for gasesare in the range of 10-15 eV.
Ejected electrons energetic
enough to cause secondary
ionizations are called rays
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Bremsstrahlung Some particles will interact with the nucleus.
The particle will be deflected by the strong
electrical forces exerted on it by the nucleus. The particle is rapidly decelerated and loses
energy in the collision.
The energy appears as
a photon of
electromagnetic
radiation.
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Interaction mechanisms
Photoelectric effect
Compton scattering Pair production
Coherent (Rayleigh) scattering
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1- Photoelectric Effect An atomic absorption process in which an atom
absorbs all the energy of an incident photon.
Z is atomic number of thematerial,
E is energy of the incident photon,
and is the density of
the material.
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Photoelectric Effect and Buildup of
Secondary Radiations In case of photoelectric effect, the photon disappears after
transferring all of its energy to a bound electron, which getsejected from the atom.
When the vacancy left in the shell by the removed electron gets
filled by an electron dropping into it from a higher energy level,
the difference in energy between the two transition states may
appear as a fluorescent photon. These photons are characteristically low in energy, but some may
be capable of reaching the dose point inside or outside the
shielding material.
Ejected electron further may excite/ionize other atoms
(fluorescent photons)
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2- Compton Scattering Collision between a photon and a loosely
bound outer shell orbital electron.
Interaction
looks
like a collision
between the
photon and a
free electron.
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Compton Scattering and Buildup of
Secondary Radiations Only a portion of the photon energy is transferred to
an electron, and a scattered photon moves away fromthe interaction site, often in a direction different from
that of the original photon.
This scattered photon may find its way to a dose point
of interest inside or outside the attenuating material.OR further interaction with matter
Vacancy left in the shell by the recoiled electron gets
filled by an electron from a higher energy level, result
in as a fluorescent photon.
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3- Pair Production Pair production occurs when a photon (E 1.022
MeV) interacts with the electric field of an atomic
nucleus or charged particle. Photon energy is converted into an electron-positron
pair and kinetic energy.
Buildup of Secondary Radiations
Positron will eventually interact with a free electronand produce a pair of 511 keV annihilation photons.
These two gamma rays can escape or interact with
matter through the Compton scattering or
Photoelectric effect.
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4- Coherent or Rayleigh Scattering Scattering interactions that occur between a
photon and an atom as a whole.
Because of the great mass of an atom very little
recoil energy is absorbed by the atom. The photon
is therefore deflected with essentially no loss of
energy.
Coherent scattering is only important at energies
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Thus, any of the common gamma
interaction processes may result in
secondary photons that have a finite
probability of reaching the dose point.The extent to which such secondary
photons add to the fluence or dose at the
dose point is usually described through the use of an appropriate
buildup factor.
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Eo Eo
Buildup of Secondary
radiations
Energy spectrum of incident-ray bean
Energy spectrum of-rayemerging from shield
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The buildup factor is a dimensionless quantity that
represents the ratio of total flux (including the secondary
photons) at a point to primary photon flux at the same
point.
where is the uncollided flux and bis the buildup flux.
The buildup factor, B, accounts for the amount of forward
scattering by the shield; B is a function of material and -ray energy as well as geometry.
Magnitudes of buildup factors vary widely, ranging from a
minimum of 1.0 to very large values, depending on
source and shield characteristics.
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Monodirectional beam of 2-MeV of
intensity 106 rays/cm2-sec is incident
on a lead shield 10 cm thick. At the rear
side of the shield calculate the: Uncollided flux
Buildup flux
For lead, the linear attenuation coefficientat 2 MeV is 0.518 cm1.
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Uncollided flux o = 10
6 rays/cm2-sec
a = 0.518 cm-1 x 10 cm = 5.18
o = 106 e-5.18 = 5.63x103rays/cm2-sec
Buildup flux The buildup factor at 2 MeV for a = 5.18
See Table 10.1 from Lamarsh, Bm = 2.78= 2.78 x 5.63x103
b =1.56 x104rays/cm2-sec
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For Point Isotropic Source
Unshielded Flux
Uncollided flux
Buildup flux
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First, determine the unshielded flux 5 cm
from a 100-mCi point source that emits a
0.5 MeV gamma ray for each decay.
Second, if a 10-cm diameter, spherical
lead shield encapsulates the point source,
determine the uncollided gamma flux on
the surface of the shield.For lead, the linear attenuation coefficient
at 0.5 MeV is 1.64 cm1.
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The unshielded flux at a radius of 5 cm from the point
source is:
The uncollided flux on the surface of the Pb shield is:
Buildup flux The buildup factor at 0.5 MeV for R = 8.2
See Table 10.2 from Lamarsh, Bp = 2.108
= 2.108 x 3.235x103 b =6.819 x103rays/cm2-sec
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When a photon passes through a
thickness of absorber material, the
probability that it will experience an
interaction (i.e., photoelectric, Compton
scatter, or pair production) depends on the
energy of the photon and on the
composition and thickness of the absorber.
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Under conditions of narrow beam geometry thetransmission of a monoenergetic photon beamthrough an absorber is described by anexponential equation:
where I(0) is the initial beam intensity, I(x) is thebeam intensity transmitted through a thickness x ofabsorber, and is the total linear attenuationcoefficient of the absorber at the photon energy of
interest.
The linear attenuation coefficient is expressed inunits of cm-1.
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There are three basic components to the
linear attenuation coefficient: due to the
photoelectric effect; due to Compton
scattering; and due to pair production.The exponential equation can also be
written as:
atten = + + = absorption + scattering
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Since photon attenuation does not mean that all
the photon energy is absorbed (e.g., consider
Compton scattering in which only a fraction of
the photon energy is liberated to an electron), itis necessary to introduce another quantitythe
energy absorption coefficient,a.
In comparing the photon attenuation versus
absorption coefficientattenuation absorption
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Without collimation, scattered photons
cause artificially high counts to be
measured, resulting in smaller measured
values for the attenuation coefficients.
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Linear attenuation coefficient l depends on photon energy
depends on material composition
depends on material density
dimensions are 1/length (e.g., 1/cm, cm-1)
Mass attenuation coefficient m m= l/ ( = density of material yielding l)
does not depend on material density dimensions arelength2/mass (e.g., cm2/g)
The mean free path which is the average distance that a photon moves
between interactions, is mfp = 1/.
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Half-value thickness is the amount of
material needed to attenuate a photon flux
by 1/2 (attenuation factor = 0.5).
Tenth value thickness is given by
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Values for Lead and Water
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What thickness of lead is required toattenuate 99% of 511 keV photons? 99% attenuated = 1% surviving
Using the exponential attenuation formula
Alternatively, if the TVT is known (1.35 cm),doubling the TVT results in two consecutive layerswhich each transmit 1/10 of photons, or a totaltransmission of 1/100 or 1%.
2 * 1.35 cm = 2.7 cm.
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What fraction of 140 keV photons will
escape unscattered from the middle of a
30 cm cylinder?
Medium water
The photons must travel through 15 cm of water.
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Attenuation coefficient depends on?
Buildup factor depends on?
Pair production is prominent at low E (t/f)
l/ is energy attenuation coefficient (t/f)A material having greater buildup factor and low
attenuation/absorption is better for dose
shielding (t/f)
atten ab (t/f) Magnitudes of buildup factors vary widely,
ranging from a minimum of 0.1 to 1 (t/f)