L05 Interaction

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

IAEA 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


Overview

• To become familiar with the basic knowledge in radiation physics and image formation process.


Part 5: Interaction of radiation with matter

Topic 1: Introduction to the atomic basic structure



Electromagnetic spectrum

1041031021013 eV

0.0010.010.1110

0.12 keV

100

1.5

Angström

keV

X and raysUVIR light

E

40008000

IR: infrared, UV = ultraviolet


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 electrically neutral



Topic 2: Quantities and units



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: 1.672

10-27 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



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.


Electrons strike the nucleus

N N

n(E) E

E1

E2E3

n1

n3

n2

E1

E2E3

n1E1

n2E2

n3E3

E

Emax

Bremsstrahlungspectrum


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 increases

with increasing electron energy

• X Rays are dominantly produced by Bremsstrahlung


Bremsstrahlung continuous spectrum

• Energy (E) of Bremsstrahlung photons may take any value between “zero” and the maximum kinetic energy of incident electrons

• Number of photons as a function of E is proportional to 1/E

• Thick target continuous linear spectrum


Bremsstrahlung spectra

dN/dE (spectral density) dN/dE

From a “thin” target EE0EE0

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

From a “thick” target


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


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



Topic 4: Characteristic X Rays



Spectral distribution of characteristic X Rays (I)

• Starts with ejection of e- mainly from k shell (also possible 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 transitions

between energy levels • Energy of emitted photons is characteristic of the

atom


LL

KK

MMNNOOPP

Energy(eV)

65432

0

- 20- 70- 590- 2800- 11000

- 695100 10 20 30 40 50 60 70 80

100

80

60

40

20

L L L

K1

K2

K2

K1

(keV)

Spectral distribution of characteristic X Rays (II)



Topic 5: Primary and secondary ionization



Stopping power

• Loss of energy along track through both collisions and Bremsstrahlung

• The linear stopping power of the mediumS = E / x [MeV.cm-1]

• E: energy loss• x: element of track

• for distant collisions: the lower the electron energy, the higher the amount transferred

• most Bremsstrahlung photons are of low energy• collisions (hence ionization) are the main source of

energy loss • except at high energies or in media of high Z


Linear Energy Transfer

• Biological effectiveness of ionizing radiation• Linear Energy Transfer (LET): amount

of energy transferred to the medium per unit of track length of the particle• Unit: e.g. [keV.m-1]



Topic 6: Photoelectric effect and Compton scattering



Photoelectric effect

• Incident photon with energy h • all photon energy absorbed by a tightly bound

orbital 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 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


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 <1 keV

• Increasing Z increasing probability of interaction. Compton is practically independent of Z in diagnostic range

• The probability of interaction decreases as h increases


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)



Topic 7: Beam attenuation and Half value thickness



Exponential attenuation law of photons (I)• Any interaction change in photon energy and or

direction

• 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


Attenuation coefficients

Linear attenuation depends on:• characteristics of the medium (density )• photon beam energyMass attenuation coefficient: / [m2kg-1]• / same for water and water vapor (different )• / similar for air and water (different µ)


Attenuation of an heterogeneous beam

• Various energies No more exponential attenuation

• Progressive elimination of photons through the matter

• Lower energies preferentially• This effect is used in the design of filters

Beam hardening effect


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 (-µ HVL) HVL = 0.693 / µ• HVL depends on material and photon energy• HVL characterizes beam quality modification of beam quality through filtration HVL (filtered beam) HVL (beam before filter)


Photon interactions with matter

Annihilation photon

Incidentphotons

Secondaryphotons

Secondaryelectrons

Scattered photonCompton effect

Fluorescence photon(Characteristic radiation)

Recoil electron

Electron pairE > 1.02 MeV

Photoelectron(Photoelectric effect)

Non interacting photons

(simplified representation)


Dependence on Z and photon energy

• Z < 10 predominating Compton effect• higher Z increase photoelectric effect

• at low E: photoelectric effect predominates in bone compared to soft tissue

• (total photon absorption)

• contrast products photoelectric absorptionhigh Z (Barium 56, Iodine 53)

• use of photoelectric absorption in radiation protectionex: lead (Z = 82) for photons (E > 0.5 MeV)



Topic 8: Principle of radiological image formation



X Ray penetration and attenuation in human tissuesAttenuation of an X Ray beam:• air: negligible• bone: significant due to relatively high

density (atom mass number of Ca)• soft tissue (e.g. muscle,.. ): similar to water• fat tissue: less important than water• lungs: weak due 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).


X Ray penetration in human tissues

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



Improvement of image contrast (lung)



Improvement of image contrast (bone)



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






Purpose of using contrast media

• To make visible soft tissues normally transparent to 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 iodine, barium and body soft tissue

100

20 30 40 50 60 70 80 90 100

10

1

0.1

IodineIodine

(keV)

X R

ay A

TTEN

UA

TIO

N C

OEF

FIC

IEN

T (c

m2 g

-1)

BariumBariumSoft Tissue

Soft Tissue


Photoelectric absorption and radiological image

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

• The photoelectric effect predominates • main contributor to image formation on the

radiographic film


Contribution of photoelectric and Compton interactions 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)

X R

ay A

TTEN

UA

TIO

N C

OEF

FIC

IEN

T (c

m2 g

-1)


Contribution of photoelectric and Compton interactions to attenuation of X Rays in bone

20 40 60 80 100 120 140

10

1.0

0.1

0.01

Total

Compton + CoherentPhotoelectric

(keV)

X R

ay A

TTEN

UA

TIO

N C

OEF

FIC

IEN

T (c

m2 g

-1)



• Higher kVp reduces photoelectric effect

• The image contrast is lowered

• Bones and lungs structures can simultaneously be visualized

Note: body cavities can be made visible by means of contrast media: iodine, barium


Effect of Compton scattering

Effects of scattered radiation on:

• image quality

• patient absorbed energy

• scattered radiation in the room


Summary

• The elemental parts of the atom constituting both the nucleus and the extranucleus structure can be schematically represented.

• Electrons and photons have different types of interactions with matter

• Two different forms of X Rays production Bremsstrahlung and characteristic radiation contribute to the image formation process.

• Photoelectric and Compton effects have a significant influence on the image quality.


Where to Get More Information (1)

• 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.


Where to Get More Information (2)

• 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.

L05 Interaction

Education

image formation

human tissues

linear energy

ray attenuation

photoelectric

compton scattering

ray penetration

image contrast