Radiography l2 Notes

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RADIOGRAPHYSankara Narayanan.V Level 2 Notes

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1. PRINCIPLES -A source of penetrating radiation is placed on one side of a specimen and a detector of radiation on the other side. Thepenetrating radiation is absorbed selectively so that more radiation is absorbed through the thicker part of the specimen and lesthrough the thinner part. The variations in thickness, the macroscopic flaws in the specimen, such as cavities, cracks, inclusionetc. contribute to the selective absorption.Source- A gamma emitter or an X-ray machine.Detector- A photographic film, or a radiation counter, or an ionization chamber, or a fluorescent screen, or a semi conductor /photodiode material.The quality of the recorded image depends upon the nature and the size of the radiation, the atomic number of the specimen,the nature of the recording medium and the geometrical aspects.

2. PROPERTIES OF IONISING RADIATIONS-2.1 Nature of X-Rays and Gamma-RaysX-rays and gamma rays are forms of electromagnetic radiation having wavelengths in the range 10

-10to 10

-7cm. X-rays are

produced by allowing a beam of high energy electrons to hit a target and they originate from the electron orbits of the targetatoms. Gamma rays are emitted from the nucleus of radioactive elements.The properties are-a) They travel at the speed of lightb) They travel in straight linesc) They pass through materials of low density more readily than through high density materialsd) The penetrating ability depends upon the wavelengthe) They are invisible.f) They cause ionization of the matter as they pass throughg) They can affect photographic emulsionsh) They cause fluorescence in certain salts such as calcium tungstate

The energy of the electromagnetic radiation is given by:

E=h = hc/Where h is Planks constant and is the frequency of the radiation, c is the velocity that is 3x10 10c.m. per second and is thewavelength. It is sometimes more convenient to consider the electromagnetic radiation as corpuscular than wave-motion and thterm photon is used instead of quantum. The wavelength of the radiation is often given in Angstrom units(A). 1A=10

-8c.m. and

energy of the radiation is measured in electron-Volts (eV), which is the energy acquired by an electron when accelerated throua potential difference of one volt.The electron-volt describes the quality of the radiation and this term is particularly used in the case of gamma rays. In the caseof the X-rays, the term kilovoltage is conveniently used.

2.2 Atomic StructureElement- that cannot be broken down chemically into simpler substances.Atom- It has a positively charged nucleus surrounded by negatively charged electrons moving in orbits. The number of theelectrons determines the chemical properties.Nucleus- Contains protons and neutrons with the exception of hydrogen which has a proton and no neutron. The electrons hav

discrete energy levels in orbits K, L, M, N, etc. The K-shell, being closest to the nucleus contains electrons of the highest enerand thus these are tightly bound. The electrons in the outer orbits are loosely bound.Atomic Number= Number of protons, indicated by Z.Mass Number= the total number of protons and neutrons, indicated by A. The protons and neutrons are approximately equal innumber.Isotopes are varieties of the same chemical element having the same number of protons and different numbers of neutrons andso different mass numbers. Many of the isotopes are radioactive since unstable and these are called radioisotopes. When anelectron is removed from an atom, this becomes unstable and an electron from outer orbit jumps into the vacant orbit; thedifference in energy between the electron orbits is emitted as a quantum of X-rays, characteristic X-rays. The shortest

wavelength of Uranium K radiation is 0.11A i.e., 0.113MeV

Isobars - Elements of same mass number but different number of protons.Isomers - Elements of same number of protons and neutrons but different modes of radioactive decay.

ELEMENT K L M N O P Q1H1 14He2 2

7Li3 2 1

9Be4 2 2

12C6 2 4

27AL13 2 8 3

59Co27 2 8 15 2

59Ni28 2 8 16 2

137Ba56 2 8 18 18 8 2

192Ir77 2 8 18 32 15 2

U92

2 8 18 32 21 9 2


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2.3. Generation of X-RaysWhen a filament is heated, it emits electrons. These electrons when accelerated by a potential difference are made to hit atarget material of high melting point will produce X-rays. The kinetic energy of an electron accelerated by a potential difference V volts is:

mv2= eV x 10

7

where m is the mass of the electron, e is its charge, and v is the velocity (from zero) the electron is accelerated to.An electron that stops completely in one collision on entering the target undergoes a complete energy conversion from kinetic tother forms of energy; gives the shortest wavelength x-ray. This energy is called the quantum limit. The wavelength for the

quantum limit is=1.234/kV whereis in nanometers.

The electron is decelerated in the electric field of the nucleus and the kinetic energy is partially transformed into a quantum of

radiation whose minimum wavelength is,min=he/eV = 12.375/V Angstroms. This radiation is sometimes calledbremsstrahlung (braking ray).h is the Plancks constant = 6.624 x 10

-28erg-s; c is the velocity of light = 2.998 x 10

10cm/s; e is the charge on the electron =

4.8 x 10-10

esu; m is the mass of the electron = 9.11 x 10-28

gm= 1/1840 times the mass of a proton.A part of the energy of the electron is used up in removing an orbital electron. The X-ray photon thus has less energy than theoriginal kinetic energy of the electron. Due to multiple collision of the speeding electron with the target, a continuous spectrum oX-rays is emitted which is also called white radiation.2.4 Gamma-RaysRadioisotopes emit three types of radiation, namely, alpha, beta and gamma.

Alpha radiation- These are positively charged helium ions,2He4.

Beta radiation- These are negatively charged electrons.Gamma radiation- These are electromagnetic radiation without any charge.A radioisotope may emit one or more of the above radiation until it becomes stable.

2.5 Absorption of RadiationIncludes a multiplicity of processes which can be summarized within absorption and scatter, which are interdependent.An incident photon (gamma or X-ray) either transfers its whole energy to the target particle and becomes extinct or transfers paof its energy and then is reflected in another direction. In the second case, the photon is said to be scattered. In both the casesthe target particle which is normally an electron, is dislodged from its orbit and this results in another burst of radiation. Thescattered photon suffers another scatter if it has excess energy or else it is absorbed, and it is finally absorbed. The totalabsorption of the electromagnetic radiation can be expressed by:

I = I0 e- x

Where I0is the incident intensity, I is the emergent intensity through the thickness X of the medium and is a constant which i

called the linear absorption coefficient.depends upon Z, density and energy of the medium. The mass absorption coefficient

is/,whereis the density of the absorbing material. The atomic absorption coefficient a,is the linear absorptioncoefficient divided by the number of atoms per unit volume. The term cross section is also used for various absorption andscattering processes, where the probability that a photon will be affected by the process in traversing a slab of material is called

the cross-section for that process, expressed by the unit barn (1 barn = 10-24

cm2

).Scattered Radiation-A photon undergoes absorption by one or more of the following processes.

Photoelectric absorptionRaleigh scatteringCompton scatteringPair production

The total absorption is the sum of the above components.Photoelectric absorption-The incident photon is completely absorbed as a result of the interaction and an orbital electron leaves the atom orreabsorbed, and any energy in excess of the binding energy imparts kinetic energy to the electron. This type of absorptionoccurs when the photon energy is proximate to the binding energy of the K and L shells of the target atoms.For materials of low atomic number and photon energies greater than 100KeV, the photoelectric effect has almost negligibleeffect in absorption. The atom that has lost an electron is said to be ionized and the ejected electron loses energy by producinga series of ion pairs as it travels through the material.

At low photon energy,That is 50 KeV for Al

500 KeV for Pb

The cross section (probability of occurrence) of the event is1/E7.5

Z5; E being the energy of the photon.

Raleigh Scattering-At low photon energies with high atomic number targets, the photon is reflected rather than absorbed without loss of energy inthe forward direction.Compton Scattering-The photon is inelastically scattered and the electron recoils out of the atom with the photon still travelling in the forward but atan inclination to the primary photon direction at a lower energy. For a high photon energy, there will be multiple Comptonscattering ending with photoelectric absorption. Large secondary radiation is associated with Compton scattering.The Compton scattering is predominant in the industrial radiography.

Probability Z/E


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Pair Production-For photon energies more than 1.02 MeV. In this process, the incident photon disappears, and an electron-positron pairs areproduced of 0.51 MeV each and any excess energy is imparted as kinetic energy to the pair. The positrons have short life, andtheir annihilation is accompanied by the emission of two photons of 0.5MeV each, traveling in opposite directions. The electronmay produce bremsstrahlung (braking ray).

ProbabilityZ(Z+1)/log (E0 1.02)For Steel

3 MeV , Pair production.

I=I0Be-x,where B is the buildup factor which is equal to (1+ Is/Id) where Isis the intensity of the scatter under the broad

beam condition and Idis the intensity of the direct radiation. Buildup factor has great significance in the radiographic sensitivity.is also called the total absorption coefficient, as it is the sum of the absorption coefficients due to the three effects.

Half Value Thickness-It is the thickness of the material that will reduce the intensity of the beam to half its value.

HVT= 0.693/Tenth Value Thickness-The thickness of the material that will reduce the intensity by a factor of 10.

Tenth VT = 2.303/In practice, or HVT is not constant for a particular beam due to inhomogeniety from scatter and hence the thickness of theabsorber at which the measurement is made must be stated. For instance, the HVT for steel under practical broad beamconditions is 12.7 mm while the same under narrow beam conditions is 33mm maximum.Scattered Radiation and Radiographic Sensitivity-

x = 2.3 D (1+Is/Id)

GDx is the minimum thickness that can be detected.

Dis the minimum density differenceGDis the film gradient

x/xis called the thickness sensitivity. This equation is of fundamental importance in industrial radiography.

Gamma-ray sources-Ir-192, Co-60, Tm-170, Cs-137 are a few of the industrial isotopes commonly used.

Iridium-192 Cobalt-60 Thulium-170 Cesium-137 Selenium-75

Particles emitted , , , ,

Principal energies(MeV)

0.296,0.31,0.32,0.47()

1.17,1.33() 0.968,0.884(

) 0.52,1.17

() 0.136,0.265

Half life 74.4 days 5.3 years 127 days 30 years 114

Useful thicknessrange in steel

up to 100mm 100-200mm up to 6mm 40-100mm 0-35 mm

Half life o f other sou rces: Se-75 - 114 daysYt-169 - 32 daysAm 241 Be - 452 yearsRadium-226 - 1590 years

Disintegration Mechanism-

1. Emission of an alpha particle2. Emission of a beta particle3. Emission of a gamma ray4. Electron capture.

Radioisotope decay-

N = N0 x e- t where N is the number of nuclei at time t and , the disintegration constant.

This can be expressed in terms of intensities of radioactivity, that is, in curie strength.

I = I0X e- t

Half life T1/2= 0.693/ Unit of Radioactivity-Curie = No. of disintegrations per second = 3.7x 10

10dps

SI Unit Becquerel (Bq), is equivalent to reciprocal second (s-1

),1 Bq = 2.703 x 10

-11curies

1 Curie = 3.7x1010

bequerels


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Rhm = Roentgens per hour at one meter per curie = Radiation output; this is of practical use in industrial radiography. The valuis 0.5 for Ir-192, 1.3 for Co-60, 0.37 for Cs-137 and 0.0025 for Tm-170.K-factor = The no. of roentgens per hour at 1 cm from a 1mCi source of a gamma-ray emitter. This is also called specificgamma-ray emission.Specific activity - Measured in curies per gram. Higher specific activity means smaller physical size of source.Other properties of industr ial radioisotopes-

Ir-192 Co-60 Tm-170 Cs-137

Chemical form Ir Co Tm2O3 CsCl

Density 22.4 8.9 4 3.5

Practical Sp. Activity 350 50 1000 25

Practical Curies/cc 8000 450 4000 90Practical Rhm/cc 4400 600 10 33

PRODUCTION OF GAMMA-RAY SOURCES-Naturally available radioisotope - Radium-226 (half-life 1590 years).

Artificial sources are produced by (n,reaction in a nuclear reactor.

e.g., 27Co59

+ 0n1 27Co

60+

Other (n, ) reactions are, 77Ir191

(n, ) 77Ir192

; 69Tm169

(n, ) 69Tm170

.Cs-137 is a fission product.

Approximate HVL for Various Materials When Radiation is from a Gamma Source

Half-Value Layer, mm (inch)

Source Concrete Steel Lead Tungsten Uranium

Iridium-192 44.5 (1.75) 12.7 (0.5) 4.8 (0.19) 3.3 (0.13) 2.8 (0.11)

Cobalt-60 60.5 (2.38) 21.6 (0.85) 12.5 (0.49) 7.9 (0.31) 6.9 (0.27)

Approximate Half-Value Layer for Various Materials When Radiation is from a X-ray Source

Half-Value Layer, mm (inch)

Peak Voltage (kVp) Lead Concrete

50 0.06 (0.002) 4.32 (0.170)

100 0.27 (0.010) 15.10 (0.595)

150 0.30 (0.012) 22.32 (0.879)

200 0.52 (0.021) 25.0 (0.984)

250 0.88 (0.035) 28.0 (1.102)

300 1.47 (0.055) 31.21 (1.229)

400 2.5 (0.098) 33.0 (1.299)

1000 7.9 (0.311) 44.45 (1.75)

Note: The values presented on this page are intended for educational purposes. Other sources of information should beconsulted when designing shielding for radiation sources.

The approximate half and tenth thickness values for various shielding materials for

75

Se

ntinel

sourcecapsules was calculated using the MCBEND code. These values are shown in table 3.

1/2 Thickness Value 1/10th

Thickness Value Shielding Material

(mm) (inches) (mm) (inches)

Aluminum 27 1.06 80 3.15

Steel 8 0.315 27.5 1.08

Lead 1 0.0394 4.75 0.187

Tungsten 0.8 0.0315 4.0 0.157

Concrete 30 1.18 90 3.54

Approximate 1/2 and1/10

thThickness Values for Various Shielding Materials for

75Se

ntinelSources


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APPROXIMATE RADIOGRAPHIC EQUIVALENCE FACTORSTo radiograph 0.5 inch of Cu at 220 kV, multiply 0.5 inch by the factor 1.4, obtaining an equivalent thickness of 0.7 inch of steeTherefore, give the exposure required for 0.7 inch of steel.

APPROXIMATE RADIOGRAPHIC EQUIVALENCE FACTORS

Material 50 kV 100kV 150kV 220kV 400kV 1000kV 2000kV 4 to25MeV

Ir 192 Co60

Magnesium 0.6 0.6 0.5 0.08

Aluminium 1.0 1.0 0.12 0.18 0.35 0.352024 AlAlloy

2.2 1.6 0.16 0.22 0.35 0.35

Titanium 0.45 0.35

Steel 12 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

SS 304 12 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

Copper 18 1.6 1.4 1.4 1.3 1.1 1.1

Zinc 1.4 1.3 1.3 1.2 1.1 1.0

Brass * 1.4 1.3 1.3 1.2 1.2 1.2 1.1 1.1

Inconel X 16 1.4 1.3 1.3 1.3 1.3 1.3 1.3 1.3

Zirconium 2.3 2.0 1.0

Lead 14 12 5 2.5 3 4 2.3

Uranium 25 3.9 12.6 3.4

ALUMINIUM is taken as the standard metal at 50 kV and 100 kV and STEEL at higher kV.

Tin and lead alloyed in the brass will increase these factors.

IQI SELECTION (Ref: Table T-276 of ASME Sec.V- 2001 Edition)

Nominal Single-Wall IQIMaterial Thickness Range(mm) Source Side Film Side

Hole-Type Wire-Type Hole-Type Wire-TypeDesignation Essential-Wire Designation Essential-Wire

Up to 6.4, incl. 12 5 10 4Over 6.4 through 9.5 15 6 12 5Over 9.5 through 12.7 17 7 15 6Over 12.7 through 19.0 20 8 17 7Over 19.0 through 25.4 25 9 20 8Over 25.4 through 38.1 30 10 25 9Over 38.1 through 50.8 35 11 30 10Over 50.8 through 63.5 40 12 35 11Over 63.5 through 101.6 50 13 40 12

WIRE IQI DESIGNATION, WIRE DIAMETER AND WIRE IDENTITYSet A Set B Set C Set D

Diameter Identity Diameter Identity Diameter Identity Diameter Identity

0.08 1 0.25 6 0.81 11 2.54 160.1 2 0.33 7 1.02 12 3.20 170.13 3 0.41 8 1.27 13 4.06 180.16 4 0.51 9 1.60 14 5.08 190.20 5 0.64 10 2.03 15 6.35 200.25 6 0.81 11 2.54 16 8.13 21

Radiographic sensitivity and sensitivity to defects are NOT the same. Better radiographic sensitivity will allow adequate defectimages to be seen more readily and/or clearly, but will NOT allow inadequate defect images to be seen.

The adequacy of image quality in respect of defect detection depends upon both the nature and orientation of the defect. Forvolumetric defects, defect size and the difference in absorption between the defect and the background control the adequacy o


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the image. For planar defects, defect size and orientation dominate in determining the adequacy of an image and indeedwhether an image is formed. Thus large planar defects oriented at unsuitable angles relative to the beam, can give no visibleimage even with very high radiographic sensitivity.

IQI is measure of the photographic sensitivity of the radiograph (not sensitivity to defects).

EN 462 WIRE DIAMETER

WireNo.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

WireDia(mm)

3.2 2.5 2.0 1.6 1.25 1.0 0.8 0.625 0.5 0.4 0.32 0.25 0.2 0.16 0.12 0.1 0.08 0.062


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GEOMETRIC UNSHARPNESS LIMITATIONS

Geometric Unsharpness = Ug =

Focal Spot Size X b/a

Effect of Source size and Distance on Penumbra-

Material Thickness (mm) Ug, Max. (mm)Under 50.8 0.51

50.8 to 76.2 (incl) 0.7676.2 to 101.6 (incl) 1.02Greater than 101.6 1.78Material thickness is the thickness on which the IQI is placed.


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FACTORS INFLUENCING SENSITIVITY

CONTRAST

Contrast is the difference in density between adjacent areas on a radiograph. The greater the difference in densities, the greatethe contrast.

Subject contrast is the ratio of beam intensities transmitted by two selected portions of a specimen. It depends on the nature ofthe specimen, the energy of the radiation used, and the intensity and distribution of the scattered radiation, but is independent otime, mA or curie, and distance and of the characteristics or treatment of the film.

Film contrast refers to the slope of the characteristic curve of the film. It depends on the type of film, the processing it receives,and the density. It also depends on whether the film is exposed with lead screens (or direct) or with fluorescent screens. Filmcontrast is independent, for most practical purposes, of the wavelengths and distribution of the radiation reaching the film, andhence is independent of subject contrast.

DEFINITIONDefinition is the sharpness of the dividing line between areas of differing density. It is a function of film, type of screen used, theradiation energy and geometry of image formation, and can be assessed by measuring the width on the film of the region overwhich the density change occurs.

Film unsharpness can be assessed by taking a radiograph of a sharp edge as in a step wedge with the beam accurately alignewith the step, and then measuring the variation in density with a micro-line densitometer. Density can be plotted againstdistance.


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DENSITY

Transmittance

(I0/It)

PercentTransmittance

Film

Density

Log(I0/It)

1.0 100% 0

0.1 10% 1

0.01 1% 2

0.001 0.1% 3

0.0001 0.01% 40.00001 0.001% 5

0.000001 0.0001% 6

0.0000001 0.00001% 7

FILM UNSHARPNESS VALUES

Radiation Uf (mm)

100 kV X-rays 0.05

200 kV X-rays 0.10400 kV X-rays 0.152 MV X-rays 0.328 MV X-rays 0.60Iridium-192 0.17Cobalt-60 0.35Yb-169 0.07-0.13

RADIOGRAPHIC FILMSCONSTRUCTIONGenerally coated with Silver halide both sides of the base.


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Base-Cellulose nitrate is flammable and hence not used now. Instead, polyester is used; this has good transparency, high tensilestrength and dimensional stability.

Substratum (subbing layer)-It is the adhesive between the emulsion and the base. It is composed of emulsion and base solvent. Prevents separation of thebase from the emulsion.

Emulsion-It consists of silver halides suspended in gelatin. Silver halides are produced by adding a solution of silver nitrate to a solution opotassium bromide, potassium iodide, potassium chloride or some combination of these. This is then mixed with gelatin. Silverhalides from minute crystals or grains throughout the gelatin as it solidifies. The rate and method of mixing with the gelatindetermines the speed and characteristics of the emulsion. The silver halide crystals are produced with a negatively chargedbromide ions, a bromide barrier. During exposure, the barrier is broken or weakened and free electrons are produced eitherwithin the emulsion or specks of metallic silver which form development centers. Each development center consists of a fewatoms of metallic silver and therefore cannot be seen. This is known as the latent image.

The metallic silver of the latent image acts as a catalyst, causing the developer to convert the rest of the grains containing thessilver specks to metallic silver. This process is aided by the breakdown or weakening by the radiation, of the bromide barrier

around these grains.

Supercoat-This is a protective layer of clear gelatin. A hardener may also be added to provide abrasion resistance.

Film Characteristic Curve (also called Sensitometric Curve)-

Slope of the Curvegives the gradient or the contrast of the film; steeper the slope, better the contrast.

The film characteristic curve is independent of the quality of radiation.


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SCREEN TYPE FILMSUses fluorescent salt screens short exposures used in the medical field

DIRECT TYPE FILMKnown also as non-screen type. Used with or without lead screens requires long exposure records fine details

FILM STORAGEBase fog level shall not be increased.1. Keep away from radiation sources

2. Keep films cool3. Keep dry4. Keep away from chemicals and fumes5. Store on edge to prevent pressure marks

FILM PACKAGING1. Boxed films-2. Envelope Packed with/without lead screens3. Roll Pack (Ready Pack) with/without lead screens

DARKROOMS AND FILM PROCESSINGThe size, layout and equipment of a darkroom will depend on its use, the number of operators working at any one time, availabspace.

Each room must be away from radiation sources and lightproof with no stray light from around or under doors.

Drainage and a hot and cold water supply are essential as well as heating for the processing unit. Ease of cleaning is importanSafe lighting should be provided.The processing unit should be as far away as possible from the loading or dry bench.Access to darkrooms should be possible without interrupting work by opening doors.

DEVELOPMENTActions and chemicals-The primary purpose of the developer is to amplify the effect of exposure by converting the latent image into black metallic silveThere are four constituents of a developer bath.

Developing Agent-Converts the exposed silver halides into black metallic silver. Metol-hydroquinone or phenidone-hydroquinone.

Accelerator-A buffer which keeps the solution alkaline, which is the condition required if the developing agent is to continue working. Sodiumcarbonate.

Restrainer-Controls the activity of the developing agent and ensures only the exposed silver halides are converted to black metallic silver.Potassium bromide

Preservative-Prevents the developing agent from being oxidized by air and thus rendering it useless. Sodium sulphite

Developers must always be mixed with warm water and dilution rates must be in accordance with manufacturers instructions.

Replenisher contains the original mix of chemicals augmented by additional chemicals to prolong the life of the developer.

Method of Development-

3 to 5 minutes at 20C. Higher the temperature, the more rapid its action. Above 25C, the emulsion becomes soft and swollen.Below 14C, time and temperature cease to have a relationship. Brisk agitation for 10-15 seconds every minute so that there areno air bubbles trapped on the surface. During agitation, the by-products released will not leave flow marks. Agitation should be the up and down movement. Another method of agitation is by using frequent bursts of inert gas, usually nitrogen. Compressedair should not be used, as it will increase oxidation.Drain the excess developer.Replenish the tank as per manufacturers recommendation.

RINSERinsing is done in running water to remove the developer from the film.

STOP BATHAn alternative to rinsing is to use a stop bath. This is typically a 3% solution of glacial acetic acid. The acid neutralizes thealkalinity of the developer. Litmus papers can be used to check the state of the stop bath. Do not top up the acid to avoidstaining.


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There is a commercially available stop bath solution that contains a dye that is amber while the solution is acid, but as thesolution nears neutral or alkaline, the colour turns purple.

FIXERUsed to remove the unexposed, undeveloped silver halides so producing a stable image. Sodium thiosulphate or ammoniumthiosulphate is used as a silver halide solvent. Acetic acid is also included to toughen the emulsion against abrasion and reducthe volume of water absorbed by the emulsion. Generally, the hardeners used are aluminium potassium sulphate (potash alumor potassium chromium sulphate (chrome alum). The fixing time shall be twice the clearing time; general fixing time is 8 to 10minutes.

Prolonged fixing in excess of 30 minutes will result in the thiosulphates dissolving the black silver image causing the bleaching the image. Bleaching occurs very rapidly in the presence of air.

WASHINGAfter fixation, the film emulsion is saturated with fixer solution. The thiosulphate complexes in solution are unstable and wouldcause discoloration and bleaching. It is essential, therefore, that all films are thoroughly washed before drying and storage. Themost common method of washing is by using running water. Cascading. Typical washing times are in the order of 20 to 30minutes.

WETTING AGENTSUsed to reduce the surface tension of the water and so to reduce the thickness of water layer on the film. This will reduce theprobability of drying marks and give a slight decrease in drying time. Immersion time 10-15 seconds.

DRYINGExcessive drying can cause the emulsion to become brittle. Temperatures of 30-40C and a relative humidity of less than 60%

are sufficient for practical purposes.

TESTING FOR FIXER REMOVALARCHIVAL WASHING

Hypo test solution

Water - 750 mL28% Acetic Acid - 125 mLSilver Nitrate (Crystals) - 7.5 gramsWater to make - 1 litre

(To make approximately 28% acetic acid from glacial acetic acid, dilute 3 parts of glacial acetic acid with 8 parts of water.

Store the solution in a screw cap or glass-stoppered brown bottle away from strong light. Avoid contact of test solution with thehands, clothing, negatives, prints, or undeveloped photographic materials; otherwise black stains may result.

The KODAK Hypo Estimator consists of four colour patches reproduced on a strip of transparent plastic. It is used in conjunctiowith KODAK Hypo Test Solution. For use in the test, an unexposed piece of film of the same type is processed with theradiographs whose fixer content is to be determined. After the test film is dried, one drop of the solution is placed on it andallowed to stand for 2 minutes. The excess test solution is then blotted off, and the stain on the film compared with the colorpatches of the Hypo Estimator. The comparison should be made on a conventional X-ray illuminator. Direct sunlight should beavoided since it will cause the spot to darken rapidly.

For commercial use, the test spot should be no darker than two thicknesses of Patch 4 of the Hypo Estimator. Two thicknessescan be obtained by folding the estimator along the center of the patch.

RECIPROCITY LAW FAILURE The Bunsen-Roscoe reciprocity law states that the resultant of a photochemical reaction is dependent only on the product (I x t

of the radiation intensity (I) and the duration of the exposure (t), and is independent of the absolute values of either quantity.Applied to radiography, this means that the developed density in a film depends only on the product of x- or gamma-ray intensireaching the film and the time of exposure.

The reciprocity law is valid for direct X-ray or gamma-ray exposures, or those made with lead foil screens, over a range ofradiation intensities and exposure times much greater than those normally used in practice. It fails, however, for exposures tolight and, therefore, for exposures using fluorescent intensifying screens.

The logarithms of the exposures (I x t) that produce a given density are plotted against the logarithms of the individualintensities. It can be seen that for a particular intensity, the exposure required to produce the given density is a minimum. It is fthis intensity of light that the film is most efficient in its response. For light intensities higher and lower than this, the exposurerequired to produce the given density is greater than (I x t). Phrased differently, there is a certain intensity of light for which aparticular film is most efficient in its response. In radiography with fluorescent screens, the failure of the reciprocity lawsometimes gives results that appear to be a failure of the inverse square law.


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In most industrial radiography, the brightness of fluorescent intensifying screens is very low, and the exposures used lie on theleft-hand branch of the curve.

METAL INTENSIFYING SCREENS

RADIATION SCREEN FRONT SCREEN BACK SCREENMATERIAL THICKNESS (mm) THICKNESS (mm)

(Minimum)

120 kV Lead None 0.2120-250 kV Lead 0.025-0.05 0.1250-400 kV Lead 0.05-0.15 0.11 MV X-rays Lead 1.5-2.0 1.05-10 MV X-rays Copper 1.5-2.0 1.5-2.015-31MV X-rays Tantalum 1.0-1.5 NoneIr-192 Lead 0.05-0.15 0.15Cs-137 Lead 0.05-0.15 0.15Co-60 Copper 0.5-2.0 0.25-1.0

Steel

FLUOROMETALLIC SCREENS SPEED FACTORS

MATERIAL LEAD SCREEN FLUOROMETALLIC SPEED

THICKNESS EXPOSURE TIME SCREENS EXPOSURE FACTOR(mm) TIME

25 50s 5s 10:137 260s 38s 7:150 15 mins 3 mins 5:160 24 mins 8 mins 3:1

METAL SCREENSIntensification is due to the photoelectric, and Compton scattering processes; electrons are emitted by the screen. Lead, being high atomic number, releases high quantity of electrons. The lead is alloyed with antimony or bismuth to harden and improveabrasion resistance. The intensification is about double thus halving the exposure time. For RT with Co-60, copper or steelscreens produce a better image, but the exposure time is doubled. Additionally, scatter is absorbed. In addition to theintensifying effect, metal screens have an important part in the reduction of scatter reaching the film by absorbing the low energscattered radiation.

SALT SCREENSFluorescent salts, usually calcium tungstate, emit visible light when exposed to radiation. This offers higher intensification at thecost of definition. Scatter is not reduced with salt screens. The light emitted is not directly proportional to the intensity of radiatioand so the reciprocity law fails.

FLUOROMETALLIC SCREENSA thin layer, typically 100-200 microns thick, of calcium tungstate is coated onto a lead screen. The film receives light andelectrons from the screens. Only a few films are suitable for use with these screens. Reciprocity law fails. These screensrespond better at low temperatures.

Salt screens if contaminated with dust or fibres will reveal them in the radiograph. Scratches or cracks will show indication on thradiograph.

CONTROL OF SCATTERThis is the secondary radiation emitted from any source other than that giving the primary desired rectilinear propagation. Theeffect of scatter is poorer contrast and definition and spurious indications; also creates difficulties with radiation protection.Scatter is produced by low energy radiation.

Scatter is produced as an effect of Compton and photoelectric effects. After a few collisions of the photon with the electrons inthe medium, the direction of the scattered radiation may be in a direction that is opposite to the original direction of the primarybeam. Scatter from the side as well as from the backside (back scatter) will cause fuzzy image. It is the electrons created in thelead intensifying screen that help in the formation of the image in the film. Hence the thin front screens of 0.005 inch thickness general are useful in producing image forming electrons while cutting off the scattered radiation. A screen thickness of 0.005inch in the front and 0.010 inch thick in the rear may also be useful in the 1 to 2 Mev X-rays. Thicker screens will produce highenergy electrons in the multimillion volt X-ray radiography that can cause further secondary radiation. However, thicker screensprohibit intimate contact with the film thus affecting definition and this may warrant the use of vacuum cassettes.


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Scatter originating from matter outside the specimen is more serious. Here, filters made of lead or copper are useful in reducingthe diverging beam. Filters help in reducing excessive subject contrast by absorbing soft components of the radiation. Howeveit must be recognized that filtering will increase the exposure time. Further filters near the tube do not help in the million voltradiography in improving the quality of the image. When a high quality is expected in multimillion volt radiography, acomparatively thin front screen (about 0.005 inch) is used, and the back screen is eliminated. This necessitates a considerableincrease in exposure time.

Collimation, beam filtration (0.5 to 1 mm thick lead), blocking edges to reduce undercutting of radiation, use of grids on betweeobject and film (moved sideways during exposure), increased beam energy (scatter is in the forward direction) are a few of themethods to reduce scatter.

Scatter hitting the film under thick portion of an object through the thin portion is called undercut.

SPURIOUS INDICATIONS (or ARTIFACTS) These are caused by incorrect processing or careless handling of the film. Occasionally they may be due to manufacturingfaults. May give rise to misinterpretation.

ARTEFACTS ARISING BEFORE EXPOSURE1. Film mottle: due to use of old film2. Radiation fogging: this occurs when the film is stored too close to a source of radiation, or when a film is inadvertently left inthe exposure bay during the exposure of another film.3. Light fog: caused by storage of film in a faulty storage box or bin; leaving the lid off the box; exposure to white light in a faultydark room; the use of the wrong type of safelight; too strong a bulb in the safelight; or to the use of a faulty film holder. It isusually local, but may be an overall fog.

4. Pressure markings: due to clumsy handling of the film when loading or unloading the cassette or film holder. They are often the shape of dark or light crescents. If dark, they are caused by local pressure on the film after exposure; if light they are causeby local pressure on the film before exposure.5. Scratch marks: usually caused by a fingernail or abrasive material.6. Static: this has the appearance of dark branched and jagged fine lines; it is due to electric discharges on the surface of theemulsion when the film is removed rapidly from a tight wrapper; it is very rare.7. High or low density finger marks: caused when handling the film with greasy or chemically stained fingers.8. Low density patches or smears: due to splashes of water or fixing solution on the film.9. Dark patches or smears: due to splashes of developer on the film.10. Radioactive spotting: this occurs as very intense black spots, often with a halo round them; caused by radioactivecontamination of the wrapping paper; now very rare.11. Light spots: caused by dust particles between the film and intensifying screens.

ARTEFACTS CAUSED DURING EXPOSURE2,3 and 4 above; and:12. Screen marks: due to contamination of the intensifying screens with chemicals, or to defects in the screens such as crackinor buckling, or to the presence of dust.

ARTEFACTS CAUSED DURING PROCESSING3,4,7 and 10 above; and:13. Air bells: these are shown as discs of lower density, caused by air trapped on the surface of the emulsion, usually during thearly stages of development, due to insufficient initial agitation.14. Patchiness or streaks: due to inefficient agitation during development, or failure to agitate in the rinsing bath; quite common15. Oxidation (aerial) fog: caused by excessive exposure of the film to air during development.16. Neighbourhood effects: these are shown as streaks from part of the image and are due to insufficient agitation duringdevelopment, especially when the image contains a sharp boundary separating areas of high and low density.17. Reticulation: this has the appearance of leather grain and is due to rupture of the emulsion caused by great differences intemperature between successive processing solutions; now very rare.

18. Drying marks: these are due to drops or streaks of water remaining on the surface of the film after it has been partially driedand they often occur when attempting to dry films rapidly at a high temperature in a drying cabinet; quite common.

DIFFRACTION (MOTTLE)This gives rise to images on film of a dark line accompanied by a light line and in welds can give a herringbone pattern. Achange in beam angle or kilovoltage alters the pattern and therefore allows diffraction mottling to be identified as a spuriousindication.


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Defects Defini tion Radiographic AppearanceClusterporosity

Rounded/elongated cavities due toentrapped gas in clusters

Rounded/slightly elongated dark spots occur incluster

Excesspenetration Excess weld metal at root of weld

Lighter density in centre of weld image occureither extended along weld/in isolated circulardrops

External

Undercut

Groove/channel in surface of plate

along the edge

Irregular dark density line following the edge of

weld image

Internalundercut

Groove in main object stretched alongthe edge at bottom/inner surface ofweld

Irregular dark density near centre of weld andalong edge of root pass image

Lack ofpenetration

Lack of fusion in root of weld/a gapleft by failure of weld metal to fill theroot Dark continuous/intermittent line in centre of weld

TungstenInclusion

Small pieces of tungsten entrappedduring welding

Irregular lower density spots randomly located inweld image

Slag lineElongated cavities contain slag/otherlower density foreign matter

Darker density lines irregular in width and parallelto edge of weld

Lack offusion

Elongated voids between weld andbase metal

Elongated parallel/single darker density lines

sometimes darker density spots dispersed alongLOF lines which are very straight in lengthwisedirection & not winding like elongated slag line

Scatterporosity Cavities due to entrapped gas Sharply defined dark shadows of rounded contour

Mismatch Nonalignment of plates to be weldedAbrupt change in film density across width of weldimage

Wagontracks

Elongated cavities containslag/foreign matter in a line on bothside of root

Elongated parallel dark lines irregular in width;occurs in SAW welding

Weldspatter Spilled weld metal near weld White spots near weld

Longitudinalcrack Discontinuity produced by fracture inweld metal longitudinal to weld Dark continuous/intermittent line along length ofweld

Transversecrack

Discontinuity produced by fracture inweld metal across weld

Fine crack line straight/wandering in directionrunning across width of weld image

Burnthrough

Severe depression/crater type hole atbottom of weld

Localized darker density with fuzzy edges incentre of weld image; circular or slightly oval

RADIATION MONITORING INSTRUMENTSIonisation ChamberThe passage of radiation through a gas causes ionization. If this ionization occurs in a tube in which a moderate voltage isapplied across two electrodes, then the negative ions will flow to the positive electrode, and the positive ions flow to the negativelectrode and an electric current is set up in the system.

Geiger-Muller Counter

The detector tube is filled with gas at low pressure and a higher voltage is used. When the radiation causes ionization, a singleion can create further ionization by an avalanche effect. The ions are collected and recorded as pulses per unit time. It is not

equally responsive to all energies, therefore correction charts are required to convert counts per second into Gy, mGy or Gyper hour. Most instruments are compensated Geiger-Muller tubes, that is the tube has a metal filter to even out the differences energy.

Scintillation CounterCertain crystals, e.g. sodium iodide, emit flashes of light, scintillations, when the crystal absorbs photons of radiation. Whencoupled to a photo multiplier tube these scintillations are converted into electrical pulses which are then amplified and displayed

Film BadgeThis relies on the photographic effect of radiation. The density of the processed film is measured and by using a density/dosecurve the dose can be assessed. By using a holder with various filters, information on the type and energy of radiation can bededuced.


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Thermoluminescence dosemeter (TLD)Lithium fluoride has the ability to absorb radiation and store it. When heated to around 200C, the radiation is emitted in the formof visible light, the amount of light is proportional to the dose of radiation received. In practice the light is measured in a computoperated system and dose records are kept on the computer store. After further heat treatment, the dosemeter is sent back tothe wearer for further use.

Quartz Fibre Electroscope (dose meter)This is a pocket dosemeter about the size of a large pen and gives a visual indication of accumulated dose. It consists of a smaionizing chamber in which a quartz fibre is attached to a charging pin. When the pin is charged, the quartz fibre deflected. On

exposure to radiation, ionization of the gas within the chamber allows the fibre to discharge and its deflection is reduced. A lenssystem allows the position of the fibre on a calibrated scale to be read and this gives the dose received.

Audible MonitorsThese form another type of pocket dosemeter. They are electronic and a bleeper is triggered when a predetermined dose levelis exceeded. Some of these types have a LED display of dose level and/or accumulated dose.

EXPOSURE DEVICE


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RADIOGRAPHIC TECHNIQUES

SWSI (Single wall Single Image)

Single wall exposure Single wall viewing

Radiation passes through only wall.

IQI based on single wall thickness plus reinforcement

Source side IQI.

Multiple exposures required.

SWSI (Single wall Single Image)

Single wall exposure Single wall viewingRadiation passes through only wall.

IQI based on single wall thickness plus reinforcement

Source side IQI.

Multiple exposures required.


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DWSI (Double Wall Single Image) OR

DWE / SWV (Double Wall Exposure Single Wall

Viewing)

IQI on film side.

IQI based on single wall thickness.

Will require 3 or more exposures based on t, D & SFD.

Weld nearest the film interpreted.

DWSI (Double Wall Single Image) OR

DWE / SWV (Double Wall Exposure Single Wall

Viewing)

IQI on film side.

IQI based on single wall thickness.Will require 3 or more exposures based on t, D & SFD.

Source must be offset particularly when placed away from the

nearest wall.

Weld nearest the film interpreted.

DWDI (Double Wall Double Image) OR

DWE / DWV (Double Wall Exposure / Double Wall Viewing)

Elliptical image.

IQI based on single wall thickness as per ASME.

IQI Source side as per ASME.

Will require 2 exposures depending upon pipe diameter and wa

thickness.

Applicable for narrow cap width (low wall thickness).


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DWDI (Double Wall Double Image) OR

DWE/DWV (Double Wall Exposure Double Wall Viewing)

Image of both walls superimposed.

IQI based on single wall thickness as per ASME.

IQI Source side as per ASME.

Will require 3 exposures.

Applicable for large cap width (thicker pipes)

Both walls interpreted.

PANORAMIC EXPOURE (SWSI)

Source at the center of the pipe.

Complete weld is radiographed in one exposure.

Source side IQI required.

3 IQI equispaced.


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Radiograph Interpretation - Welds

In addition to producing high quality radiographs, the radiographer must also be skilled in radiographic

interpretation. Interpretation of radiographs takes place in three basic steps: (1) detection, (2) interpretation, and (3evaluation. All of these steps make use of the radiographer's visual acuity. Visual acuity is the ability to resolve aspatial pattern in an image. The ability of an individual to detect discontinuities in radiography is also affected by thlighting condition in the place of viewing, and the experience level for recognizing various features in the image. Thfollowing material was developed to help students develop an understanding of the types of defects found inweldments and how they appear in a radiograph.

Discontinuities

Discontinuities are interruptions in the typical structure of a material. These interruptions may occur in the basemetal, weld material or "heat affected" zones. Discontinuities, which do not meet the requirements of the codes orspecifications used to invoke and control an inspection, are referred to as defects.

General Welding Discontinuities

The following discontinuities are typical of all types of welding.

Cold lap is a condition where the weld filler metal does not properly fuse with the base metal or the previous weldpass material (interpass cold lap). The arc does not melt the base metal sufficiently and causes the slightly moltenpuddle to flow into the base material without bonding.

Porosity is the result of gas entrapment in the solidifying metal. Porosity can take many shapes on a radiographbut often appears as dark round or irregular spots or specks appearing singularly, in clusters, or in rows.Sometimes, porosity is elongated and may appear to have a tail. This is the result of gas attempting to escape whithe metal is still in a liquid state and is called wormhole porosity. All porosity is a void in the material and it will hava higher radiographic density than the surrounding area.


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.

Cluster porosity is caused when flux coated electrodes are contaminated with moisture. The moisture turns into agas when heated and becomes trapped in the weld during the welding process. Cluster porosity appear just likeregular porosity in the radiograph but the indications will be grouped close together.

Slag inclusions are nonmetallic solid material entrapped in weld metal or between weld and base metal. In aradiograph, dark, jagged asymmetrical shapes within the weld or along the weld joint areas are indicative of slag

inclusions.

Incomplete penetration (IP) or lack of penetration (LOP) occurs when the weld metal fails to penetrate the joint.It is one of the most objectionable weld discontinuities. Lack of penetration allows a natural stress riser from whichcrack may propagate. The appearance on a radiograph is a dark area with well-defined, straight edges that followsthe land or root face down the center of the weldment.


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Incomplete fusion is a condition where the weld filler metal does not properly fuse with the base metal.Appearance on radiograph: usually appears as a dark line or lines oriented in the direction of the weld seam alongthe weld preparation or joining area.

Internal concavity or suck back is a condition where the weld metal has contracted as it cools and has been

drawn up into the root of the weld. On a radiograph it looks similar to a lack of penetration but the line has irregulaedges and it is often quite wide in the center of the weld image.

Internal or root undercut is an erosion of the base metal next to the root of the weld. In the radiographic image itappears as a dark irregular line offset from the centerline of the weldment. Undercutting is not as straight edged asLOP because it does not follow a ground edge.


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External or crown undercut is an erosion of the base metal next to the crown of the weld. In the radiograph, itappears as a dark irregular line along the outside edge of the weld area.

Offset or mismatch are terms associated with a condition where two pieces being welded together are not

properly aligned. The radiographic image shows a noticeable difference in density between the two pieces. Thedifference in density is caused by the difference in material thickness. The dark, straight line is caused by the failuof the weld metal to fuse with the land area.

Inadequate weld reinforcement is an area of a weld where the thickness of weld metal deposited is less than thethickness of the base material. It is very easy to determine by radiograph if the weld has inadequate reinforcementbecause the image density in the area of suspected inadequacy will be higher (darker) than the image density ofthe surrounding base material.


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Excess weld reinforcement is an area of a weld that has weld metal added in excess of that specified byengineering drawings and codes. The appearance on a radiograph is a localized, lighter area in the weld. A visualinspection will easily determine if the weld reinforcement is in excess of that specified by the engineeringrequirements.

Cracks can be detected in a radiograph only when they are propagating in a direction that produces a change in

thickness that is parallel to the x-ray beam. Cracks will appear as jagged and often very faint irregular lines. Crackcan sometimes appear as "tails" on inclusions or porosity.

Discontinuities in TIG welds

The following discontinuities are unique to the TIG welding process. These discontinuities occur in most metalswelded by the process, including aluminum and stainless steels. The TIG method of welding produces a cleanhomogeneous weld which when radiographed is easily interpreted.


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Tungsten inclusions. Tungsten is a brittle and inherently dense material used in the electrode in tungsten inertgas welding. If improper welding procedures are used, tungsten may be entrapped in the weld. Radiographically,tungsten is more dense than aluminum or steel, therefore it shows up as a lighter area with a distinct outline on theradiograph.

Oxide inclusions are usually visible on the surface of material being welded (especially aluminum). Oxideinclusions are less dense than the surrounding material and, therefore, appear as dark irregularly shaped

discontinuities in the radiograph.

Discontinuities in Gas Metal Arc Welds (GMAW)

The following discontinuities are most commonly found in GMAW welds.

Whiskers are short lengths of weld electrode wire, visible on the top or bottom surface of the weld or containedwithin the weld. On a radiograph they appear as light, "wire like" indications.

Burn-Through results when too much heat causes excessive weld metal to penetrate the weld zone. Often lumpsof metal sag through the weld, creating a thick globular condition on the back of the weld. These globs of metal arereferred to as icicles. On a radiograph, burn-through appears as dark spots, which are often surrounded by lightglobular areas (icicles).


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Radiograph Interpretation - Castings

The major objective of radiographic testing of castings is the disclosure of defects that adversely affect the strengtof the product. Castings are a product form that often receive radiographic inspection since many of the defectsproduced by the casting process are volumetric in nature, and are thus relatively easy to detect with this method.These discontinuities of course, are related to casting process deficiencies, which, if properly understood, can leadto accurate accept-reject decisions as well as to suitable corrective measures. Since different types and sizes ofdefects have different effects of the performance of the casting, it is important that the radiographer is able toidentify the type and size of the defects. ASTM E155, Standard for Radiographs of castings has been produced tohelp the radiographer make a better assessment of the defects found in components. The castings used to produc

the standard radiographs have been destructively analyzed to confirm the size and type of discontinuities present.The following is a brief description of the most common discontinuity types included in existing reference radiograpdocuments (in graded types or as single illustrations).

RADIOGRAPHIC INDICATIONS FOR CASTINGS

Gas porosit y or blow ho les are caused by accumulated gas or airwhich is trapped by the metal. These discontinuities are usuallysmooth-walled rounded cavities of a spherical, elongated or flattenedshape. If the sprue is not high enough to provide the necessary heattransfer needed to force the gas or air out of the mold, the gas or airwill be trapped as the molten metal begins to solidify. Blows can alsobe caused by sand that is too fine, too wet, or by sand that has a lowpermeability so that gas cannot escape. Too high a moisture content


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in the sand makes it difficult to carry the excessive volumes of water vapor away from the casting. Another cause oblows can be attributed to using green ladles, rusty or damp chills and chaplets.

Sand inclus ions and dross are nonmetallic oxides, which appearon the radiograph as irregular, dark blotches. These come fromdisintegrated portions of mold or core walls and/or from oxides(formed in the melt) which have not been skimmed off prior to theintroduction of the metal into the mold gates. Careful control of themelt, proper holding time in the ladle and skimming of the melt during

pouring will minimize or obviate this source of trouble.

Shrinkageis a form of discontinuity that appears as dark spots onthe radiograph. Shrinkage assumes various forms, but in all cases itoccurs because molten metal shrinks as it solidifies, in all portions ofthe final casting. Shrinkage is avoided by making sure that thevolume of the casting is adequately fed by risers which sacrificiallyretain the shrinkage. Shrinkage in its various forms can berecognized by a number of characteristics on radiographs. There are at least four types of shrinkage: (1) cavity; (2dendritic; (3) filamentary; and (4) sponge types. Some documents designate these types by numbers, without actunames, to avoid possible misunderstanding.

Cavity shr inkage appears as areas with distinct jagged boundaries.It may be produced when metal solidifies between two original

streams of melt coming from opposite directions to join a commonfront. Cavity shrinkage usually occurs at a time when the melt hasalmost reached solidification temperature and there is no source ofsupplementary liquid to feed possible cavities.

Dendritic shrinkage is a distribution ofvery fine lines or smallelongated cavities

that may vary indensity and are usually unconnected.

Filamentary shrinkage usually occurs as a continuousstructure of connected lines or branches of variable length, width andensity, or occasionally as a network.

Sponge shrinkageshows itself as areas of lacy texture with diffuse outlines, generallytoward the mid-thickness of heavier casting sections. Spongeshrinkage may be dendritic or filamentary shrinkage. Filamentarysponge shrinkage appears more blurred because it is projected

through the relativelythick coating betweenthe discontinuities andthe film surface.

Cracks are thin(straight or jagged)linearly disposed discontinuities that occur after the melt hassolidified. They generally appear singly and originate at


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casting surfaces.

Cold shuts generally appear on or near a surface of cast metal as a result of two streams of liquid meeting andfailing to unite. They may appear on a radiograph as cracks or seams with smooth or rounded edges.

Inclusions are nonmetallic materials in an otherwise solid metallicmatrix. They may be less or more dense than the matrix alloy andwill appear on the radiograph, respectively, as darker or lighterindications. The latter type is more common in light metal castings.

Core shift showsitself as a variation insection thickness,usually on radiographic views representing diametricallyopposite portions of cylindrical casting portions.

Hot tears are linearly disposed indications that represent fracturesformed in a metal during solidification because of hinderedcontraction. The latter may occur due to overly hard (completelyunyielding) mold or core walls. The effect of hot tears as a stressconcentration is similar to that of an ordinary crack, and hot tears are usually systematic flaws. If flaws are identifieas hot tears in larger runs of a casting type, explicit improvements in the casting technique will be required.

Misruns appear on the radiograph as prominent dense areas of variable dimensions with a definite smooth outlineThey are mostly random in occurrence and not readily eliminated by specific remedial actions in the process.

Mottling is a radiographic indication that appears as an indistinct area of more or less dense images. The conditionis a diffraction effect that occurs on relatively vague, thin-section radiographs, most often with austenitic stainlesssteel. Mottling is caused by interaction of the object's grain boundary material with low-energy X-rays (300 kV orlower). Inexperienced interpreters may incorrectly consider mottling as indications of unacceptable casting flaws.

Even experienced interpreters often have to check the condition by re-radiography from slightly different source-filangles. Shifts in mottling are then very pronounced, while true casting discontinuities change only slightly inappearance.

Radiographic Indications for Casting Repair Welds

Most common alloy castings require welding either in upgrading from defective conditions or in joining to othersystem parts. It is mainly for reasons of casting repair that these descriptions of the more common weld defects arprovided here. The terms appear as indication types in ASTM E390. For additional information, see theNondestructive Testing Handbook, Volume 3, Section 9 on the "Radiographic Control of Welds."


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Slag is nonmetallic solid material entrapped in weld metal or between weld material and base metal.Radiographically, slag may appear in various shapes, from long narrow indications to short wide indications, and ivarious densities, from gray to very dark.

Porosity is a series of rounded gas pockets or voids in the weld metal, and is generally cylindrical or elliptical inshape.

Undercut is a groove melted in the base metal at the edge of a weld and left unfilled by weld metal. It represents astress concentration that often must be corrected, and appears as a dark indication at the toe of a weld.

Incomplete penetration, as the name implies, is a lack of weld penetration through the thickness of the joint (orpenetration which is less than specified). It is located at the center of a weld and is a wide, linear indication.

Incomplete fusion is lack of complete fusion of some portions of the metal in a weld joint with adjacent metal(either base or previously deposited weld metal). On a radiograph, this appears as a long, sharp linear indication,occurring at the centerline of the weld joint or at the fusion line.

Melt-throughis a convex or concave irregularity (on the surface of backing ring, strip, fused root or adjacent basemetal) resulting from the complete melting of a localized region but without the development of a void or open holeOn a radiograph, melt-through generally appears as a round or elliptical indication.

Burn-through is a void or open hole in a backing ring, strip, fused root or adjacent base metal.

Arc str ike is an indication from a localized heat-affected zone or a change in surface contour of a finished weld oradjacent base metal. Arc strikes are caused by the heat generated when electrical energy passes between thesurfaces of the finished weld or base metal and the current source.

Weld spatteroccurs in arc or gas welding as metal particles which are expelled during welding. These particles donot form part of the actual weld. Weld spatter appears as many small, light cylindrical indications on a radiograph.

Tungsten inclusion is usually denser than base-metal particles. Tungsten inclusions appear very light radiographimages. Accept/reject decisions for this defect are generally based on the slag criteria.

Oxidation is the condition of a surface which is heated during welding, resulting in oxide formation on the surface,due to partial or complete lack of purge of the weld atmosphere. The condition is also called sugaring.

Root edge condition shows the penetration of weld metal into the backing ring or into the clearance between the

backing ring or strip and the base metal. It appears in radiographs as a sharply defined film density transition.

Root undercut appears as an intermittent or continuous groove in the internal surface of the base metal, backingring or strip along the edge of the weld root.

UW 51 RADIOGRAPHIC AND RADIOSCOPIC EXAMINATION OF WELDED JOINTSThe below mentioned imperfections shall be repaired.(1) Any indication characterized as a crack or a zone of incomplete fusion or penetration;(2) Any other elongated indication on the radiograph which has a length greater than:(a) in. for t up to in.;(b) t/3 for t from in. to 2-1/4 in.;(c) in. for t over 2-1/4 inwhere t is the thickness of the weld excluding any allowable reinforcement. For a butt weld having different

thicknesses at the weld, t is the thinner of these two thicknesses. If a full penetration weld includes a fillet, then thethickness of the throat of the fillet shall be included in t.(3) Any group of aligned indications that have an aggregate length greater than t in a length of 12t, except when thdistance between the successive imperfections exceeds 6L where L is the length of the longest imperfection in thegroup;(4) Rounded indications in excess by that specified in the acceptance standards given in Appendix 4.

UW 52 SPOT EXAMINATION OF WELDED JOINTS(1) Welds in which indications are characterized as crack, or zones of incomplete fusion, or penetration shall beunacceptable.(2) Welds in which indications are characterized as slag inclusions, or cavities shall be unacceptable if the length oany such indication is greater than 2/3 t where t is the thickness of the weld excluding any allowable reinforcementFor a butt weld joining two members having different thicknesses at the weld, t is the thinner of these two


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thicknesses. If a full penetration weld includes a fillet, then the thickness of the throat of the fillet shall be included t. If several indications within the above limitations exist in a line, the weld shall be judged acceptable if the sum ofthe longest dimensions of all such indications is not more than t in a length of 6t (or proportionately for radiographsshorter than 6t) and if the longest indications considered are separated by 3L of acceptable weld metal where L isthe length of the longest indication. The maximum length of acceptable indications shall be in. Any suchindication shorter than in. shall be acceptable for any plate thickness.(3) Rounded indications are not a factor in the acceptability of welds not required to be fully radiographed.

ASME SECTION VIII DIVISION 1


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ASME SECTION IX

QW-191.2 Radiographic Acceptance CriteriaQW-191.2.1 Terminology(a) Linear Indications. Cracks, incomplete fusion, inadequate penetration, and slag are represented on theradiograph as linear indications in which the length is more than three times the width.(b) Rounded Indications. Porosity and inclusions such as slag or tungsten are represented on the radiograph asrounded indications with a length three times the width or less. These indications may be circular, elliptical, orirregular in shape; may have tails; and may vary in density.QW-191.2.2 Acceptance Standards. Welder and welding operator performance tests by radiography of welds intest assemblies shall be judged unacceptable when the radiograph exhibits any imperfections in excess of the limispecified below(a) Linear Indications(1) any type of crack or zone of incomplete fusion or penetration(2) any elongated slag inclusion which has a length greater than(a) 18 in. (3 mm) for t up to 38 in. (10 mm), inclusive(b) 13t for t over 38 in. (10 mm) to 214 in. (57 mm), inclusive(c) 34 in. (19 mm) for t over 214 in. (57 mm)(3) any group of slag inclusions in line that have an aggregate length greater than t in a length of 12t, exceptwhen the distance between the successive imperfections exceeds 6L where L is the length of the longestimperfectionin the group (b) Rounded Indications

(1) The maximum permissible dimension for rounded indications shall be 20% of t or 18 in. (3 mm), whichever issmaller.(2) For welds in material less than 18 in. (3 mm) in thickness, the maximum number of acceptable roundedindications shall not exceed 12 in a 6 in. (150 mm) length of weld. A proportionately fewer number of roundedindications shall be permitted in welds less than 6 in. (150 mm) in length.(3) For welds in material 18 in. (3 mm) or greater in thickness, the charts in Appendix I represent the maximumacceptable types of rounded indications illustrated in typically clustered, assorted, and randomly dispersedconfigurations. Rounded indications less than 132 in. (0.8 mm) in maximum diameter shall not be considered in theradiographic acceptance tests of welders and welding operators in these ranges of material thicknesses.


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RADIATION PROTECTION

PRINCIPLESJustification-Use of radiation cannot be justified unless there is significant benefit to the recipient like cancer cure; and there is no othermethod to carry out the specific work.

Optimisation-All doses shall be kept as low as reasonably achievable.

Limitation-Dose limit shall not be exceeded.

RadioisotopesThese carry an excess of neutrons in the nucleus and hence become unstable. The extra neutron is supplied in a nuclearreactor to a stable element. The element absorbs the neutron, becomes unstable due to an acquired higher energy level andreleases the excess energy in the form of radiation. The radiation is in the form of penetrating electromagnetic radiation (calledX-rays or gamma-rays) in most of the cases. When all the excess energy is released, the isotope becomes stable.

Half-lifeThe release of energy by the isotope is exponential. That is, moment after moment, the activity of the isotope gets reduced. This averaged in the form of half-life. The time period in which the activity of the isotope becomes half is known as half life. Thehalf-life of Iridium-192 is 75 days. The activity of the isotope is expressed in Curies(Ci) or Bequerel(Bq).1 Bequerel = 1 disintegration per second1 Curie = 37 Billion Bequerels = 37 Billion disintegrations per second

A 100 Ci isotope of Iridium-192 will become 50 Ci after a period of 75 days and it will become 25 Ci after another 75 days and on.

Property of Radiation The property that concerns us is ionization. The radiation is capable of knocking off an electron from the atom it encountersmaking it an ion. While this property is used in medical and industrial applications, they are capable of killing the cells in ourbody. The nature has a mechanism of replacing the dead cells (every day, thousands of cells die in our body and these arereplaced) and the health is maintained normally. Even in the case of an excessive loss of cells over a period of time as in thecase of a radiation worker, so long as this excess is within the limits of nature, the dead cells are replaced over a period of timeA radiation worker is made to change his occupation different from radiation work until he recovers.

When too many cells are destroyed in a short period of time, an equal replacement is not made in reasonable time. This is whethe health is affected. This occurs when the body is exposed to a heavy dose of radiation in a short time. As in the case of

Hiroshima/Nagasaki/Chernobyl. Such doses are very high; many times the dose permissible for a radiation worker.

UNITS OF DOSEGray (Gy)-SI unit for Radiation Absorbed Dose (RAD) and is the amount of radiation that will deposit one joule of energy per kilogram ofabsorber. It is the equivalent of 100 RADS.

Quality Factor (Q)-The degree of damage for a given absorbed dose is not the same for all types of radiation. Alpha particulate radiation is moredamaging than Beta particulate radiation and photon (X-ray and gamma-ray) radiation. The Q factor for Beta particles andphoton is 1.Sievert (Sv)-This is the unit for radiobiological effectiveness (also called equivalent dose); this is obtained byGrays x Q factor = Sieverts.

For X-ray and gamma-ray, Gray = Sievert

OTHER UNITS OF RADIATIONRoentgen This is the old unit for dose rate or the radiation output of the source.REM This is the equivalent dose received by humans.Roentgen and REM are numerically equal for gamma radiation and X-ray radiation.1 Roentgen = 1 REM = 1000 milli R = 1000 milli REM1 Sv = 1000 milli Sv = 1000 000 micro Sv = 100 R = 100 REM1 milli Sv = 0.1 R = 100 milli R1 micro Sv = 0.1 milli R7.5 micro Sv = 0.75 milli R1 milli R = 10 micro Sv = 0.01 milli Sv


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DOSE RATE OF ISOTOPES (SOURCES)For gamma sources the dose rates at one metre are:

milli Gy/hr/Ci R/hr/Ci micro Gy/hr/GBq

Co-60 - 13 1.3 357Ir-192 - 4.8 0.48 130Yb-169 - 1.25 0.125 33.8Se 75 - 2 0.2 55Th-170 - 0.025 0.0025 0.676

Curie (Ci) is the activity of the source represented by 37 billion disintegrations of the atoms per second. This is the decay rate.The SI unit for activity is Bequerel (Bq), which is one disintegration per second.1 Ci = 37 Billion Bq

For radiation from X-ray machines, such specific dose rates are not available since they vary with machines and these have todetermined experimentally with radiation survey meters.

Permissible accumulated dose

the equivalent dose limit for global exposure for radiation workers (classified workers) is 100mSv (0.1 Sv) in any fiveconsecutive calendar years, with the additional condition that the limit of 50 mSv (0.05 Sv) is not exceeded in one calendayear;

the equivalent dose limit for lens of the eye is 150 mSv (0.15 Sv) in one calendar year;

the equivalent dose limit for the skin is 500 mSv (0.5 Sv) in one calendar year; this limit is applied to the average equivalendose on any 1 cm

2surface;

the equivalent dose limit for hands, forearms, feet and ankles is 500 mSv (0.5 Sv) in a calendar year

the equivalent dose limit for global exposure for general public is 1 mSv (0.001 Sv) in a calendar year

IMMEDIATE EFFECTS OF EXPOSURE TO RADIATIONDose(Sv) Organ Immediate Effect 0.25 Reproductive Changes noticeable in chromosomes and sperm count2.5 Reproductive Damage to chromosomes, Temporary sterility for 1-2

years4.0 Skin Erythema (temporary)4.0 Reproductive Among older women permanent sterility and ovulation

stops6.0 Reproductive Permanent sterility for men7.0 Skin Erythema of high intensity- permanent

8.0 Reproductive Permanent sterility among young women and ovulationstops

EFFECTS OF GLOBAL EXPOSURE (whole body)Dose (Sv) Effect0.25 Affects the cells and chromosomes0.5 - Nausea, loss of appetite, reduction in WBC1.0 - Acute sickness

- Initial phase Nausea, vomiting- Subsequent phase loss of appetite, reduction in WBC, fever, haemorrage, diarrhea,

prone for infection2.0 - Hospitalization acute sickness possible accidental death

- Initial phase shock, nausea, vomiting, loss of appetite- Subsequent phase loss of appetite, fever, depression, tachichardia, anemia,

haemorrage, diarrhea, reduction in WBC, disposition to infection

4.0 - Mortality 50% in 30 to 60 days; debility, haemorrage, anemia, cracks in mouth6.0 - Mortality 100% in 30 days10-30 - Mortality certain much sooner than 30 days


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WHOLE BODY DOSE

Gy REM

CLINICAL AND LABORATORY FINDINGS

0.05 - 0.25 5 25 Asymptomatic, conventional blood studies normal, a very small number of chromosome aberrationdetectable above 0.1 Gy.

0.25 - 1 25 100 Asymptomatic, minor depressions of white cells and platelets detectable in a few persons on day 36.

1 - 3 100 300 Anorexia, nausea, vomiting, fatigue in about 10 to 20 per cent of persons within 2 days. Depressioof white cells (lymphocytes) and platelets on day 3 to 6. Progression in second and third week andchance of infection and bleeding. Above 3 Gy hair loss on day 9. Recovery in week 4 to 6.

4.5 450 Serious, disabling illness in most persons with about 50% mortality.

Over 6 Over 600 Accelerated version of acute radiation syndrome with gastrointestinal complications, bleeding,infections and death in most exposed persons within 2 weeks.

Over 50 Over 5000 Fulminating course with gastro intestinal, central nervous and vascular complications resulting indeath within 24 to 72 hours.

For gamma radiation 1 Gy = 100 REM = 100,000 mR

RADIATION DETECTIONThe property of ionization of matter by radiation makes it useful in detection. The amount of ionization current is measured andthis current is calibrated to read the dose rate or accumulated dose.

Ionization Chamber type Survey MeterThis is filled with suitable gas that is ionized. The current is measured and read as mR/hour or mSv/hour. This type of meterdoes not indicate the accumulated dose.

Quartz fibre type pocket dosemeterA quartz fibre when charged indicates zero reading. The radiation passing through will cause the charge to reduce which is themeasured as mR. This type of dosemeter indicates accumulated dose instantly.

Thermoluminescence Dosemeter (TLD)This uses a material called lithium fluoride that has the ability to absorb radiation and store it. When heated, visible is emittedproportional to the dose of radiation received. After further heat treatment, the dosemeter is reused.

Audible MonitorsThis is another type of pocket dosemeter. This is an electronic type and bleeper is triggered when a predetermined dose level iexceeded. Some of these types have a LED display of dose level and/or accumulated dose.

Film BadgeThis uses a photographic film. Different types of filters are used to absorb radiation of different type or different wavelengths. T

density of the processed film is measured indicates the dose of radiation received.

INVERSE SQUARE LAWThe intensity of radiation is governed by the inverse square law. That is, the dose rate varies inversely with the square of thedistance.

D1= Original distanceD2= Required distanceR1= Original dose rateR2= Required dose rate

R1 1/D 12

R2 1/D 22

R2/R1= D 12/ D2

2

Considering the activity of the Ir-192 source is 20 Ci the safe distance for safety barrier (7.5 micro Sv/hr) will be:

D1= 1 metreD2= Required distanceR1= 4.8 x 1000 x 20 micro Sv/hrR2= 7.5 micro Sv/hr

D2 = [(1m)2x 4.8 x 1000 x 20 /7.5]

1/2

= 113.137 metres

We can also calculate the dose rate at a given distance. In the above instance, if the distance is 50 metres at which the doserate is measured, then


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D1= 1 metreD2= 50 metresR1= 4.8 x 1000 x 20 micro Sv/hrR2= Required dose rate

R2 = (1m)2x 4.8 x 1000 x 20 /50

2

= 38.4 micro Sv/hr

SAFE WORKINGControlled Area-

Any area in which the dose rates are likely to exceed three tenths of the annual dose limit for radiation workers (aged 18 yearsand over) is designated a controlled area. This is an instantaneous dose rate of 7.5 micro Sv/hr. The area shall be demarcatedby barriers and access shall be restricted. Only classified workers can enter this area.

However, areas in which the instantaneous dose rate exceeds 7.5 micro Sv/hr, but does not exceed 2 milli Sv/hr need not bedesignated as controlled area if:

(1) the time average dose rate does not exceed 240 micro Sv/hr; and(2) no one person remains for more than one hour in any period of 8 hours; and(3) no person receives a dose exceeding 60 micro Sv in any working period of 8 hours.

Supervised Area-Any area in which the dose rate is likely to exceed one third of the dose rate of a controlled area, that is, the dose rates exceed2.5 micro Sv/hr.

Areas designated as supervised areas because of the presence of sealed radioactive sources need to be identified in local rul

and people should be encouraged not to remain in supervised areas longer than necessary.

Site Work-This applies to situations when it is neither practicable nor feasible to carry out radiography inside a walled enclosure. Barriersmust be placed at a distance from the radiation source where the instantaneous dose rate does not exceed 7.5 micro Sv/hr andall persons must be excluded. If it is not feasible for the control point to be outside the controlled area then only classifiedpersons can enter and are subject to a maximum dose rate of 2milliSv/hr. Use may also be made of local shielding andcollimation is also desirable.

Setting up Barriers-For gamma sources, the safe distance should be calculated and barriers should be set up at this distance. It is VITALLYIMPORTANT, HOWEVER, THAT THIS DISTANCE SHOULD NOT BE TREATED AS ACCURATE AND FINAL. The radiationlevel at the barrier should be checked with a survey meter immediately as the source is exposed, and the position of thebarrier should be adjusted as appropriate. Outside the barrier, the dose rate shall not be in excess of 7.5 micro Sv/hr.

Shielding Material Radiation is absorbed in any material in an exponential manner. The absorption property of a material for a particular type ofradiation is expressed in half value thickness (HVT). HVT is the thickness of the material that reduces the intensity of theincident radiation to half its value. The source enclosure is constructed with one or a combination of these absorbing materials.The HVT of a few materials is given below.

MATERIAL Half Value Thickness in mm

Ir-192 Co-60 Se-75

Lead 5.5 11 1.1-1.5

Cement 43 63

Steel 12 20

Working Hours for a Classified Worker-Let us presume that whole body dose permissible for a classified worker is 50 milli Sv for a calendar year. Except when

operating the exposure device, the worker waits outside the Controlled Area. During this waiting period, he receives a dose ratof 7.5 micro Sv/hr which will accumulate a dose of three tenths of the maximum permissible annual dose.The accumulated dose in this area = 50 x 3/10 milli Sv = 15 milli Sv.The number of hours of exposure to receive this dose @ 7.5 micro Sv/hr = 15 x 1000/7.5 = 2000. This is equal to 250 days of 8hours of exposure per day. Actually, the worker does not receive the full 8 hours of exposure per day, but may be an hour ortwo.

In this example, we have not considered the full permissible dose of 50 milli Sv; instead, three tenths of the full dose has beenconsidered. The rest of the seven tenths is reserved for operating the exposure device (when a higher dose may be received) ofor any emergency.

Non radiation Worker-Let us assume that a non-radiation worker such as a welder is exposed to a radiation dose rate of 7.5 micro Sv/hr in thesupervised area. We have considered a figure of 7.5 although the dose rate in this area will be much lower. If the exposure is ftwo hours in a week, the total dose received in a year will be 52 x 7.5 x 2 = 780 micro Sv = 0.78 milli Sv.


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Assuming the total permissible dose of non-radiation worker as 1 milli Sv per year, the safe dose per week works out to 1000/5= 20 micro Sv. Such doses are not experienced in our Saipem Base since the quantum of radiography work is less. The dosereceived during a medical X-ray examination would be higher than this.

It is known that damage is caused by sudden exposure to a large quantity of radiation whereby the cells in the human body aredestroyed beyond recovery. In the case of small cumulative doses, damage does occur; however, the biological system replacethe destroyed cells with new cells in quick time.

Biological Effects

The occurrence of particular health effects from exposure to ionizing radiation is a complicated function

of numerous factors including:

Type of radiation involved.All kinds of ionizing radiation can produce health effects. The maindifference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects

is the amount of energy they have. Their energy determines how far they can penetrate into tissueand how much energy they are able to transmit directly or indirectly to tissues.

Size of dose received. The higher the dose of radiation received, the higher the likelihood of

health effects.

Rate the dose is received. Tissue can receive larger dosages over a period of time. If the dosage

occurs over a number of days or weeks the results are often not as serious if a similar dose was

received in a matter of minutes.

Part of the body exposed.Extremities such as the hands or feet are able to receive a greateramount of radiation with less resulting damage than blood forming organs housed in the torso.

See radiosensitivity page for more information.

The age of the individual. As a person ages, cell division slows and the body is less sensitive tothe effects of ionizing radiation. Once cell division has slowed the effects of radiation are

somewhat less damaging than when cells were rapidly dividing.

Biological differences. Some individuals are more sensitive to the e

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