Attenuation & Buildup of Radiations 1

7/28/2019 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)

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Attenuation & Buildup of Radiations 1