rtfi-ri-1

NDT Training & Certification

Radiography

Part 1.

A Brief Radiation History

• 1895: Roentgen - X-rays• 1896: Becquerel – radioactivity• 1898: Pierre & Marie Curie• 1904: C. Dally died of X-ray exposure• 1920s: Radium Dial Painters• 1938: Nuclear Fission• 1942: the Manhattan Project and Health Physics• 1945: Hiroshima & Nagasaki• 1946 on: Atmospheric testing & regulation

Principles of Radiography

X or Gamma radiation is imposed upon a test

object

Radiation is transmitted to varying degrees

dependant upon the density of the material

through which it is travelling

Thinner areas and materials of a less density show

as darker areas on the radiograph

Thicker areas and materials of a greater density

show as lighter areas on a radiograph

Applicable to metals,non-metals and composites

Penetrating radiation is absorbed as it passes through matter. The extent to which it is absorbed depends upon three factors:

• The thickness of the absorber.• The physical characteristics of the

absorber (in particular its density and atomic number).

• The wavelength or “photon energy” of the radiation itself.

X - Rays

Electrically generated

Gamma Rays

Generated by the decay of unstable atoms

Industrial Radiography

Radiographic Testing

X-Ray RadiographyX-Rays are produced form electrical equipment referred to as x-ray tubes or x-ray tube heads

X-Ray Radiography

X-Rays are produced form electrical equipment referred to as x-ray tubes or x-ray tube heads

Gamma Ray Radiography

Gamma rays are produced from artificial isotopes, example Cobalt 60, Iridium 192.

Source

Radiation beam

10fe16

Image quality Indicator (IQI)

Test specimenRadiographic film


Source

Radiation beam Image quality indicator

Radiographic film with latent image after exposure

Test specimen

10fe16


10fe16

Thinner areas and materials of a less density show as darker areas on the radiograph

Thicker areas and materials of a greater density show as lighter areas on a radiograph

Note that the radiograph cannot be used to determine the through thickness position of the voids.

For example, suppose that a chosen radiographic technique is capable of detecting a thickness difference of say 0.5 mm in 50 mm of steel.

The gas pore will readily be detected because A - (B + C) = 3 mm.

The lack of side fusion will not appear as an image on the radiograph because A - (D + E) = 0.01 mm which is much too small to be detected by thetechnique used.

Advantages of Radiography

Permanent record

Internal flaws

Can be used on most materials

Direct image of flaws

Real - time imaging

Disadvantages of Radiography

Health hazard

Sensitive to defect orientation

Access to both sides required

Limited by material thickness

Skilled interpretation required

Relatively slow

High capital outlay and running costs

Electromagnetic Radiation

Waves of energy associated with electrical and

magnetic fields

Electrical and magnetic fields at right angles to

each other and to the direction of propagation

What is radiation?

Electromagnetic Spectrum

10-10 10-8 10-6 10-4 10-2 1cm 102 104 106 108

Wavelength

Electric Waves

TV

Microwaves

Infra redUltra violet

Industrial radiography

Prefixes Definition

Symbol

1012 Tera T

109 Giga G

106 Mega M

103 Kilo K

102 Hecto h

10 Deca da

10-1 Deci d

10-2 Centi c

10-3 Mili m

10-6 Micro µ

10-9 Nano n

10-12 Pico p

NUMERICAL

• Travels at the speed of light• Travels through a vacuum• Travels in a straight line• No electric charge or mass• Intensity proportional to 1/D2 where D is

the distance from the source

Properties of Electromagnetic Radiation

Properties of Electromagnetic Radiation

Shorter Wavelength = Increased Energy

Shortening Wavelength

Natural Background Radiation

Cosmic = about 28 mrem /year

Radon = about 200 mrem /year

Internal Sources = about 40 mrem /year

Terrestrial = about 28 mrem/year

Man-Made Radiation

Medical Radiation = about 40 mrem/year

Diagnosis and Therapy = about 14 mrem/year

Consumer Products = about 10 mrem/year

Atmospheric Testing = less than 1 mrem/year

•

Properties of x-ray and gamma rays

They have no effect on the human sense

They have adverse effects on the body

They penetrate matter

They travel at the speed of light

They obey the inverse square law

They may be scattered

They affect photographic emulsion

They may be refracted and diffracted

Radiography

X-ray production

http://www.orau.org/ptp/collection/dosimeters/chalkriver.htm

X-Ray Production

• A source of electrons.

• A target, constructed from a suitable high melting point material.

• A means of accelerating electrons toward the target.

In order to produce x-rays three things are required:

High velocity electrons cannot travel far in air, therefore the process of acceleration must take place

in a high vacuum.

Electronaccelerated

Fin (heat dissipated)

Production of X-Ray

Vacuum

Anode +ve

Cathode- ve

Target(tungsten)

Filament

Focusingcup

X-rays

Berrylium

window

(97-99% heat)(1-3% X-ray)

X-Ray Tube (Evacuated Glass Bulb)

‘window’ In the form of a beryllium insert or a thinned section of copper which permits x-rays to exit without unduly increasing ‘inherent filtration’.

Inherent filtration Term used to describe removal of x-rays from the primary beam due to absorption by the materials used in x-ray head construction. Beryllium has a very low absorption factor and this minimises inherent filtration whilst still affording the tube walls protection from stray electrons.

Nearly all anodes are ‘hooded’ the hood is a high conductivity copper shroud which is designed to intercept stray electrons and to prevent them from hitting the tube walls.

99 % will changed into heat and light

(Bremsstrahlung)

Continuous X-ray

(Industrial radiography)

Polychromatic ray

Characteristic X-ray

(Monochromatic ray)

Lower velocity electron

Higher velocity electron

Higher velocity electron

Atomic structure of Tungsten ( Anode)

X-ray spectrum

• The two characteristic peaks are caused by target material inner shell electrons jumping to a higher energy level, then falling back to their equilibrium state.

• Relatively low energy, long wavelength and are little used in the industrial radiography of metallic components

• It can cause a problem known as diffraction mottling (artefacts).

Characteristic X- ray

X-Ray ProductionX-Ray Production

1. Electron Source : Tungsten Filament

Current

Heating the filament produces a cloud of loosely bound, low kinetic energy electrons in close proximity to the filament.

This process is known as “thermionic emission.”


2. Accelerating Electron : Potential

Difference

-ve +ve

Focusing cup concentrates electrons into a beam


2. Accelerating Electron : Potential Difference

-ve +ve

Tungsten Target

X-rays / Bremsstrahlung

ProblemsProblems• Electrons travel for only short distances through gasses• Kinetic Energy converted into 97% heat and 3% X-rays• Tungsten has a very high melting point (3370°C). This

reduces the chances that it will be vaporised by the large amount of heat generated.

• Sometimes the target is constructed from Tantalum (melting point 2996°C)

Tungsten has a high atomic number and therefore a large number of electrons.

X-Ray Production - HEATX-Ray Production - HEAT

In any X-ray tube around 95% of the energy generated is in the form of heat

For typical 200kV portable equipment around 1kW of heat has to be dissipated

For a 300kV constant potential laboratory unit heat generation is typically 7.5kW

X-ray tubes of all types therefore require a cooling system in order to prevent overheating and increase duty cycle

Older type sets having glass envelope tubes are generally oil or gas cooled

X-Ray Production - HEATX-Ray Production - HEAT

A rotating anode may be used in order to help dissipate heat - this type of arrangement is generally limited to X-ray units intended for medical use.

Modern X-ray units have so-called “metal-ceramic” envelopes. The use of such envelopes makes it practical to have a much higher potential difference between the electrodes and the envelope than was the case with glass.

This in turn permits the use of “grounded anodes”.

Such anodes are at zero volts and can therefore be cooled directly by water

X-Ray Production - AnodesX-Ray Production - Anodes

Directional Type


PANORAMIC


ROD-ANODE

X-Ray Production - AnodesX-Ray Production - AnodesROTATING-ANODE

USED MAINLY FOR LOW kV, VERY HIGH

TUBE CURRENT, EQUIPMENT IN

MEDICAL APPLICATIONS


• Tube current (mA)

• Tube voltage (kV)

- controls the amount or intensity of radiation

- controls the “quality” or penetrating ability of the radiation

KV’s Reduced

• Electron Flow Reduces

• Wave Length Increases

• Reduction In Penetration

• Increase In Contrast

KV’s Increased

• Electron Flow Increases

• Wave Length Shortens

• Increase In Penetration

• Reduction In Contrast

The Effects of Kilo Volts

Conventional x-ray tubes, as used in industrial radiography, are capable of being

operated in the range from below 50 to 400 kV.

If greater penetrating power is required high energy x-ray sources such as betatrons, linear accelerators or Van der Graaf

generators can be used to provide x-ray energies of up to 30 or even 40 MeV.

The Conservation of EnergyThe law states that energy can neither be created nor destroyed although it is possible to change it to one form to another.In the case of x-rays a stream of quickly moving particles (usually electrons) strike a target material (usually tungsten) and are brought to a rapid halt. A portion of this energy is give off as packets of electromagnetic radiation called photons. The photons can vary in energy which is determined by

1. The original energy of the electrons.

2. How rapid the electrons are decelerated.

3. The atomic number of the target material.

This process is known as bremsstrahlung

X-ray - Bremsstrahlung

NUCLEUS+

CHARGEDPARTICLE

b Beta

g CHARGED PARTICLE LOSES ENERGY IN THE FORM OF ELECTROMAGNETIC RADIATION AS A RESULT OFCHANGE IN VELOCITY and DIRECTION OF TRAVEL.

A.C. Circuit

+

The effect of a.c. on the direction of current flow.In an x-ray tube x-rays can only be produced when the current is travelling from the cathode (-ve) to the anode (+ve).

+

Half Wave rectified Circuit

In the half wave rectified circuit the anode is only positive every half cycle, therefore the electrons will only flow from the filament during that time. The x-rays are only produced during the positive half cycle.

+

Constant Potential Circuit

The introduction of separate rectifiers into the circuit, produces a constant electron flow from the cathode to anode and therefore a relatively constant output of x-rays. This circuit is know as a Greinacher circuit.

Advantages

More commonly used on site

More robust

Portable/lighter

Disdavantages

Low output/unit time

Longer exposure times

Low duty cycle 50%

Advantages

High output/unit time

100% duty cycle

Shorter exposure times

Disdavantages

Bulky equipment

Expensive

X-Ray Set Circuits

Constant Potential Half Wave Rectified

Radiography

Gamma ray production

Source assembly in fully shielded position of radiographic exposure device

Source assembly and remote control cable connectors

Sealed source in the exposure mode

Sealed source in transit mode

All atoms are composed of the 3 basic particles:

Atomic structure

1. PROTON – Has a positive charge & relatively heavy

2. NEUTRON – About the same size and weight as the proton but has NO electrical charge

3. ELECTRON – Very light particle, about 1/1840 of the weight of proton & it has a negative charge

The NUCLEUS – contains NEUTRON + PROTON (packed together in the center of the atom).

THREE BASIC PARTICLES

Proton

Electron

* In nucleus

* +1 charge

* Number of protons determines the element

* In nucleus

* No charge

* Needed for stability in nucleus

* Outside of nucleus

* -1 charge

Atomic structure

Atomic structureN SHELL

M SHELL

L SHELL

K SHELL

Proton + ve charge

Neutron no charge

Electron –ve charge

Nucleus

Atomic Structure

Atomic Mass Number (A)The number of protons + neutrons, this can be altered in order to make artificial isotopes.

A COMPLETE ATOMS must have an equal number of protons & electrons therefore:

Number of protons = Number of electrons

Atomic Number (Z) – The number of protons only in the nucleus of an atom.This determines the type of a basic element. All atoms of particular element have the same atomic number,

THREE BASIC PARTICLESAtomic structure

2

He4

Atomic Number• No. of electrons• No. of protons

Element/Symbol

Atomic Mass (AMU)

Atomic structure ELECTRONS: -Ve Charge

NEUTRONS: No Charge

PROTONS: +Ve Charge

Atomic number (Z) : 2Atomic mass (A) : 4

The atom carries no overall charge.

Helium Atom

CHARGE OF THE ATOM

The Stable Atom

A Positive Charge

A Negative Charge

Electrons = 2

Protons = 2

Electrons = 1

Protons = 2

Electrons = 3

Protons = 2

Ionization

Definitions:

The removal of electrons from an atom.

The essential characteristic of high energy radiations when interacting with matter

This effect is the reason why ionizing radiation is hazardous to health, and provides the means by which radiation can be detected

8+

Oxygen atom

8 +ve protons8 -ve electronsno overall charge

Protons & Neutrons

Electrons

8 +ve protons7 -ve electrons1 +ve charge

Ionising Radiation

8+

Ejected electron

8+ 8 +ve protons9 -ve electrons

1 -ve charge

Negative oxygen ionPositive oxygen ion

Ionization

IONIZING VS NON-IONIZING RADIATION

Non-Ionizing

Radiation

Ionizing Radiation

Ion Pair

NON-IONIZING RADIATION

• Types of non-ionizing radiation include:

– Microwaves– Radio waves– Visible light– Heat– Infrared

Radioactive Isotopes

Some isotopes are stable others are not

Unstable isotopes transform into another element

and in so doing emit radiation in 3 forms

Alpha (particles)

Beta (particles)

Gamma (rays)

Isotopes Specific Activity

ALPHA PARTICLES2 NEUTRONS AND 2 PROTONSVERY LOW PENETRATING

GAMMA RAYS EMMITTED AFTER BETA OR ALPHA PARTICLES.Photons of energy they are not particles.

BETA PARTICLES EJECTED AS ELECTRONS-Ve CHARGE

ISOTOPE

RADIOACTIVE AREASTHE GREATER THE AMOUNT THE GREATER THE SPECIFIC ACTIVITY

NEUTRONSTHERMAL & FAST

ISOTOPES

Protium H or H-1Protium H or H-11111

Deuterium H or H-2Deuterium H or H-21122

Tritium H or H-3Tritium H or H-31133

ProtonProton

ElectronElectron

NeutronNeutron

Rate of Decay

Curie = 3.7 x 1010 disintegration / second Becquerel = 1 disintegration / second 1 Curie = 37 Gbq

The amount of gamma radiation – the number of photons, produced by an isotope is controlled by the number of disintegrations (atomic fissions) per unit time.

The “source strength” of an isotope is usually expressed in curies (Ci) or becquerels (Bq).

“Source strength” may also be referred to as “source activity.”

HALF-LIFE

After OneHalf Life

The activity is now half of what

it was

Half Life = Time taken for the activity of an isotope to reduce by a half

Neutron FluxStable cobalt - 59 Unstable cobalt - 60

Nuclear Reactor

Inserted Removed

Each Co 59 Nucleus

contains :27 protons

32 neutrons

Each Co 60 Nucleus

contains :27 protons

33 neutrons

Production Of Artificial Isotopes

Only a relatively few Co 59 atoms become Co 60 depending on the time in the reactor and the magnitude of the neutron flux

Isotope Half-Life

PrincipleEmissions

(MeV)

Equivalent x-ray

Kilovoltage(kV)

PenetratingPower in

mmOf Steel

Iridium (Ir) 192 74.4 days0.31,0.47,

0.60400 75

Cobalt (Co) 60 5.3 years 1.17,1.33 1200 200

Thulium (Tm) 170

127 days 0.052,0.084 80 4

Ytterbium (Yb) 169

32 days 0.17,0.20 145 10

Selenium (Se) 75

118.5 days

0.121, 0.136,0.265, 0.28,

0.401

217(low energy beam components improve sensitivity)

30

Gamma line spectrum (discrete energies), the wave length is not of a fixed nature. A number of frequencies will be emitted for most sources.

Re

lati

ve

Inte

ns

ity

Me

v.

Long Short

Co 601.17 to1.3 Mev

Ir 1920.3 to 0.47 MevYb 169

0.06 to 0.2 Mev

Wavelength l

Wavelengths

Rayleigh scattering Occurs at very low energies

In this process, photons are deflected by outer electrons

with no change in energy

Photoelectric effect Occurs at low energies

The complete absorption of a photon of energy by an atom with the emission of an electron

ABSORPTION AND SCATTERING

Compton effect Occurs at higher energies

The interaction of a photon of energy by an electron resulting in the ejection of an electron from its atom with a certain amount of energy. The remaining energy is scattered this is known as COMPTON SCATTER

Pair production Occurs at very high energies

The simultaneous formation of an positron (+ve electron) and a electron as a result of the interaction of a photon with the nucleus of the atom. The particles are soon afterwards destroyed thus creating photons this is known as Annihilation

1. Rayleigh Scattering

Soft radiationθ

The primary photon is scattered by the orbital electrons without removing any electrons . The photon is deflected but does not change the energy

Scattering process

Absorption process

1. Photoelectric Process

Low Energy X-rayEjected electron

(total energy beam absorbed by this electron)

Low energy level - Below 0.3 Mev

Moderate Energy ( 0.3 - 3.0 Mev)

Most commonly happen in radiography industry using Ir 192

Absorption process

1. Compton Effect

Energy level-(0.3 - 3.0 Mev)Ejected electron

Scattered radiation

photon X-ray

Absorption process

3. Pair Production

Energy level (Above 3.0 Mev)

Thick material using Co 60

Ejected electron

High Energy X-ray

Ejected positron

Scattered radiation

Measuring Radiation

WAVELENGTH: New: Nanometers (nm) 1nm = 10-9

Old: Angstroms (Å) 1Å = 10-10 m

RADIATION EXPOSURE: New: Coulomb/kilogram (C/kg)Old: Roentgen

ABSORBED DOSE: New: Gray (Gy)1 Gy = 1 joule/kilogramOld: Rad 100 rads = 1 Gy

BIOLOGICAL EFFECT: New: Sievert (Sv) 1 Sv = 1 joule/kilogram Old: Rem 100 rems = 1 Sv

Gamma ray VS X-ray

• No electrical or water supplies needed

• Equipment smaller and lighter (More portable)

• Equipment simpler and more robust

• More easily accessed

• Less scatter

• Equipment initially less costly

• Greater penetrating power

Advantages

• Poorer quality radiographs

• Exposure times can be longer

• Sources need replacing

• Radiation cannot be switched off

• Poorer geometric unsharpness

• Remote handling necessary

Disadvantages

SPECIFIC ACTIVITY

Units measured - ‘curies per gram’ (Ci/g)

source activity

weight of the source.specific activity =

Formulae:

Inverse Square Law

D2

D1

I1

I2

D12

D22

=

I2

I1

Inverse Square Law CalculationsExample: 1 An x-ray tube emits 40 msv/h of radiation at

an auto-monitored distance of 1m. What is the distance where safety barriers are to be erected at 7.5 sv/h?

I1 = I2 = D1 =D2 =

Answer D2 = 73 m

X D12

I2I1D2 =

X 12

7.540000=D2

40 msv/h (X 1000)7.5 µsv/h1m?

D12

D22

=

I2

I1Formulae:

Example: 2 An emergency is when an unshielded isotope emits 6.4 sv/h at the barriers at 45m distance. What will be the exposure at 1m?

Answer I2 = 12960µsv/h

X I1D22D12

2I =

X 6.412

452

I2 =

6.4 µsv/h?45m1m

D12

D22

=

I2

I1Formulae:

I1 = I2 = D1 =D2 =

Determine the intensity of radiation at a distance of 1m if a survey meter reveals 0.02 mr/h at 35m.

Answer I1 = 24.5 mr/h

X I2D12D22

1I =

X 0.0212

352

I1 =

?0.02 mr/h1m35m

D12

D22

=

I2

I1Formulae:

I1 = I2 = D1 =D2 =

Example: 4 The intensity of radiation on a survey meter is 333sv/h at 15m. What distance is between the meter and radiation source if the meter shows 75 msv/h?

Answer D2 = 0.999 m

X D12

I2I1D2= X 15

75000333=D2

333 µsv/h75 msv/h (X 1000)15m?

D12

D22

=

I2

I1Formulae:

I1 = I2 = D1 =D2 =

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