l5 interaction of radiation with matter

5/19/2018 L5 Interaction of Radiation With Matter

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IAEAInternational Atomic Energy Agency

RADIATION PROTECTION INDIAGNOSTIC AND

INTERVENTIONAL RADIOLOGY

L 5: Interaction of radiation with matter

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology


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5: Interaction of radiation with matter

Topics

Introduction to the atomic basic structure

Quantities and units

Bremsstrahlung production

Characteristic X Rays

Primary and secondary ionization

Photo-electric effect and Compton scattering

Beam attenuation and half value thickness

Principle of radiological image formation


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Overview

To become familiar with the basicknowledge in radiation physics and image

formation process.


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Part 5: Interaction of radiation withmatter

Topic 1: Introduction to the atomic basic structure



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Electromagnetic spectrum

1041031021013 eV

0.0010.010.1110

0.12 keV

100

1.5

Angstrm

keV

Xand raysUVIR light

E

40008000

IR: infrared, UV= ultraviolet


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The atomic structure

The nuclear structure protons and neutrons = nucleons

Z protons with a positive electric charge

(1.6 10-19 C)

neutrons with no charge (neutral) number of nucleons = mass number A

The extranuclear structure Z electrons (light particles with electric

charge) equal to proton charge but negative

The atom is normally electricallyneutral


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Topic 2: Quantities and units



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Basic units in physics (SI system)

Time: 1 second [s]

Length: 1 meter [m]

Mass: 1 kilogram [kg]

Energy: 1 joule [J] Electric charge: 1 coulomb[C]

Other quantities and units

Power: 1 watt [W] (1 J/s) 1 mAs = 0.001 C



Quantities and units

electron-volt [eV]:1.603 10-19 J

1 keV = 103 eV

1 MeV = 106 eV 1 electric charge: 1.6

10-19 C

mass of proton: 1.67210-27 kg



Atom characteristics

A, Z and associated quantities

Hydrogen A= 1 Z= 1 EK= 13.6 eV

Carbon A= 12 Z= 6 EK= 283 eV Phosphor A= 31 Z= 15 EK= 2.1 keV

Tungsten A= 183 Z= 74 EK= 69.5 keV

Uranium A= 238 Z= 92 EK= 115.6 keV


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Topic 3: Bremsstrahlung production




Electron-nucleus interaction (I)

Bremsstrahlung: radiative energy loss (E) by electrons

slowing down on passage through a

material is the deceleration of the incident

electron by the nuclear Coulomb

field radiation energy (E) (photon) is

emitted.


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IAEA 5: Interaction of radiation with matter

Electrons strike the nucleus

N N

n(E) E

E1

E2E3

n1

n3n2

E1

E2E3

n1E1

n2E2

n3E3

E

Emax

Bremsstrahlung

spectrum


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Electron-nucleus interaction (II)

With materials of high atomic number the energy loss is higher

The energy loss by Bremsstrahlung

> 99% of kinetic E loss as heat production, it increaseswith increasing electron energy

X Rays are dominantly produced byBremsstrahlung


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Bremsstrahlung continuous spectrum

Energy (E) of Bremsstrahlung photons may takeany value between zero and the maximum

kinetic energy of incident electrons

Number of photons as a function of E isproportional to 1/E

Thick target continuous linear spectrum


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Bremsstrahlung spectra

dN/dE (spectral density)dN/dE

From a thin target EE0

EE0

E0= energy of electrons, E = energy of emitted photons

From a thick target


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X Ray spectrum energy

Maximum energy of Bremsstrahlung photons kinetic energy of incident electrons

In X Ray spectrum of radiology installations: Max (energy) = Energy at X Ray tube peak voltage

BremsstrahlungE

keV50 100 150 200

Bremsstrahlung

after filtration

keV

I i ti d i t d


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Ionization and associated energy

transfers

Example: electrons in water

ionization energy: 16 eV (for a water molecule

other energy transfers associated to ionization

excitations(each requires only a few eV) thermal transfers(at even lower energy)

W = 32 eV is the average loss per ionization

it is characteristic of the medium

independent of incident particle and of its energy


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Topic 4: Characteristic X Rays


Spectral distribution of characteristic


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X Rays (I)

Starts with ejection of e- mainly from k shell (alsopossible for L, M,) by ionization

e- from L or M shell fall into the vacancy created in

the k shell Energy difference is emitted as photons

A sequence of successive electron transitionsbetween energy levels

Energy of emitted photons is characteristic of theatom



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L

K

M

NOP

Energy

(eV)

654

3

2

0

- 20- 70- 590

- 2800

- 11000

- 69510 0 10 20 30 40 50 60 70 80

100

80

60

40

20

L

L

L

K1

K

2

K

2

K1

(keV)


X Rays (II)


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Topic 5: Primary and secondary ionization



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Stopping power

Loss of energy along track through both collisions andBremsstrahlung

The linear stopping power of the medium

S = E / x [MeV.cm-1] E: energy loss

x: element of track

for distant collisions: the lower the electron energy, thehigher the amount transferred

most Bremsstrahlung photons are of low energy

collisions (hence ionization) are the main source ofenergy loss

except at high energies or in media of high Z


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Linear Energy Transfer

Biological effectiveness of ionizing

radiation

Linear Energy Transfer (LET): amount

of energy transferred to the medium perunit of track length of the particle

Unit: e.g. [keV.m-1]


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Topic 6: Photoelectric effect and Compton

scattering



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Photoelectric effect

Incident photon with energy h

all photon energy absorbed by a tightly boundorbital electron

ejection of electron from the atom Kinetic energy of ejected electron: E = h - EB

Condition: h > EB (electron binding energy)

Recoil of the residual atom Attenuation (or interaction) coefficient

photoelectric absorption coefficient

Factors influencing photoelectric


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Factors influencing photoelectric

effect

Photon energy (h) > electron binding energy EB The probability of interaction decreases as h

increases

It is the main effect at low photon energies The probability of interaction increases with Z3(Z:atomic number)

High-Z materials are strong X Ray absorber


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Compton scattering

Interaction between photon and electron

h = Ea + Es (energy is conserved) Ea: energy transferred to the atom

Es: energy of the scattered photon

momentum is conserved in angular distributions

At low energy, most of initial energy is scattered

ex: Es> 80% (h) if h


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Compton scattering and tissue

density

Variation of Compton effect according to: energy (related to X Ray tube kV) and material

lower E Compton scattering process 1/E

Increasing E decreasing photon deviation angle Mass attenuation coefficient constant with Z

effect proportional to the electron density in the medium

small variation with atomic number (Z)


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Topic 7: Beam attenuation and Half value

thickness


Exponential attenuation law of


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Exponential attenuation law ofphotons (I)

Any interaction change in photon energy and ordirection

Accounts for all effects: Compton, photoelectric,

dI/I = - dx

Ix= I0 exp (-x)

I: number of photons per unit area per second [s-1]

: the linear attenuation coefficient [m-1]

/[m2.kg-1]: mass attenuation coefficient

[kg.m-3]: material density


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Attenuation coefficients

Linear attenuation depends on:

characteristics of the medium (density )

photon beam energy

Mass attenuation coefficient: /[m2kg-1] /same for water and water vapor (different )

/similar for air and water (different )

Attenuation of an heterogeneous


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Attenuation of an heterogeneousbeam

Various energies No more exponentialattenuation

Progressive elimination of photons through the

matter Lower energies preferentially

This effect is used in the design of filters

Beam hardening effect


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Half Value Layer (HVL)

HVL: thickness reducing beam intensity by 50%

Definition holds strictly for monoenergetic beams

Heterogeneous beam hardening effect

I/I0 = 1/2 = exp (- HVL) HVL = 0.693 / HVL depends on material and photon energy

HVL characterizesbeam quality

modification of beam quality through filtration HVL (filtered beam) HVL (beam before filter)


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Photon interactions with matter

Annihilation photon

Incident

photons

Secondary

photons

Secondaryelectrons

Scattered photon

Compton effect

Fluorescence photon

(Characteristic radiation)

Recoil electron

Electron pair

E > 1.02 MeV

Photoelectron

(Photoelectric effect)

Non interacting photons

(simplified

representation)


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Dependence on Z and photon energy

Z < 10 predominating Compton effect higher Z increase photoelectric effect

at low E: photoelectric effect predominates in bone comparedto soft tissue

(total photon absorption)

contrast products photoelectric absorption

high Z (Barium 56, Iodine 53)

use of photoelectric absorption in radiation protection

ex: lead (Z = 82) for photons (E > 0.5 MeV)


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Topic 8: Principle of radiological image formation


X Ray penetration and attenuation in


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X Ray penetration and attenuation inhuman tissues

Attenuation of an X Ray beam: air: negligible

bone: significantdue to relatively highdensity (atom mass number of Ca)

soft tissue (e.g. muscle,.. ): similarto water fat tissue: less important than water

lungs: weakdue to density bones can allow to visualize lung structures with higher kVp

(reducing photoelectric effect) body cavities are made visible by means of contrast products

(iodine, barium).


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X Ray penetration in human tissues

60 kV - 50 mAs 70 kV - 50 mAs 80 kV - 50 mAs


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Improvement of image contrast (lung)

X R i i h i


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Improvement of image contrast (bone)

X R t ti i h ti


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70 kV - 25 mAs 70 kV - 50 mAs 70 kV - 80 mAs

X R t ti i h ti


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X R t ti i h ti


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P f i t t di


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Purpose of using contrast media

To make visible soft tissues normally transparentto X Rays

To enhance the contrast within a specific organ

To improve the image quality

Main used substances Barium: abdominal parts

Iodine: urography, angiography, etc.

X Ray absorption characteristics of


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X Ray absorption characteristics ofiodine, barium and body soft tissue

100

20 30 40 50 60 70 80 90 100

10

1

0.1(keV)

XRayATT

ENUATIONCOEFF

ICIENT(cm2g

-1)

Photoelectric absorption and


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Photoelectric absorption andradiological image

In soft or fat tissues (close to water), at lowenergies (E< 25 - 30 keV)

The photoelectriceffect predominates

main contributor to image formation onthe radiographic film

Contribution of photoelectric and Compton interactions


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to attenuation of X Rays in water (muscle)

20 40 60 80 100 120 140

10

1.0

0.1

0.01

Total

Compton + CoherentPhotoelectric

(keV)XRayA

TTENUATIONCOE

FFICIENT(cm2g

-1)



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to attenuation of X Rays in bone

20 40 60 80 100 120 140

10

1.0

0.1

0.01

Total

Compton + Coherent

Photoelectric(keV)

XRayATTENUATIONCOE

FFICIENT(cm2g

-1)

X R t ti i h ti


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Higher kVp reducesphotoelectric effect

The image contrast is lowered

Bones and lungs structures can

simultaneously be visualized

Note: bodycavities can bemade visible by means ofcontrast media: iodine, barium

Effect of Compton scattering


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Effect of Compton scattering

Effects of scattered radiation on:

image quality

patient absorbed energy

scattered radiation in the room

Summary


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Summary

The elemental parts of the atom constitutingboth the nucleus and the extranucleusstructure can be schematically represented.

Electrons and photons have different types ofinteractions with matter

Two different forms of X Rays productionBremsstrahlungand characteristicradiation

contribute to the image formation process. Photoelectric and Compton effects have asignificant influence on the image quality.

Where to Get More Information (1)


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Part 2: Lecture on Radiation quantities and Units Attix FH. Introduction to radiological physics and

radiation dosimetry. New York, NY: John Wiley &

Sons, 1986. 607 pp. ISBN 0-47101-146-0. Johns HE, Cunningham JR. Solution to selected

problems form the physics of radiology 4th edition.

Springfield, IL: Charles C. Thomas, 1991.



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Wahlstrom B. Understanding Radiation.Madison, WI: Medical Physics Publishing,

1995. ISBN 0-944838-62-6.

Evans RD. The atomic nucleus. Malabar,FL: R.E. Kriege, 1982 (originally 1955) ISBN

0-89874-414-8.

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