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ASNTASNT NDTNDT--LEVEL IIILEVEL IIIASME AUTHORIZED INSPECTORASME AUTHORIZED INSPECTOR

Elementary ParticlesElementary Particles The ElectronThe ElectronTheThe electronelectron is an elementary particle that isis an elementary particle that ispresent in all atoms in groupings called shellspresent in all atoms in groupings called shellsaround the nucleusaround the nucleus. When they detach from the. When they detach from thenucleus they are called free electrons. Thenucleus they are called free electrons. Theantiparticleantiparticle of the electron is theof the electron is the positronpositron. An. An

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antiparticleantiparticle of the electron is theof the electron is the positronpositron. An. Anantiparticle is a subatomic particle that has theantiparticle is a subatomic particle that has thesame mass number as another particle and equalsame mass number as another particle and equalbut opposite values of some other property orbut opposite values of some other property orproperties.properties. For example, the antiparticle of theFor example, the antiparticle of theelectron is the positron, which has a positive chargeelectron is the positron, which has a positive chargeequal in magnitude to the electron's negativeequal in magnitude to the electron's negativecharge.charge.

The ProtonThe ProtonThe proton is an elementary particle that isThe proton is an elementary particle that isstable and bears a positive charge equal instable and bears a positive charge equal inmagnitude to that of the electron. The protonmagnitude to that of the electron. The protonoccurs in all atomic nuclei (the hydrogenoccurs in all atomic nuclei (the hydrogenatom contains a single proton).atom contains a single proton).The NeutronThe Neutron


The NeutronThe NeutronThe neutron is a neutral particle that isThe neutron is a neutral particle that isstable in the atomic nucleus but decays intostable in the atomic nucleus but decays intoa proton and electron, and an antineutrinoa proton and electron, and an antineutrinowith a mean life of 12 minutes outside thewith a mean life of 12 minutes outside thenucleus. Neutrons occur in all atomic nucleinucleus. Neutrons occur in all atomic nucleiexcept normal hydrogen.except normal hydrogen.

Atomic StructureAtomic StructureAnAn atomatom is the smallest part of an element that can exist andis the smallest part of an element that can exist and

consists of a small dense nucleus of protons and neutronsconsists of a small dense nucleus of protons and neutronssurrounded by moving electronssurrounded by moving electrons. The number of electrons. The number of electronsequals the number of protons so the overall charge is 0.equals the number of protons so the overall charge is 0.Electrons may be thought of as moving in circular or ellipticalElectrons may be thought of as moving in circular or ellipticalorbits or, more accurately, in regions of space around theorbits or, more accurately, in regions of space around thenucleus.nucleus. Electrons are arranged in shellsElectrons are arranged in shells at various distancesat various distancesfrom the nucleusfrom the nucleus accordingaccording to how muchto how much energy they haveenergy they have..


from the nucleusfrom the nucleus accordingaccording to how muchto how much energy they haveenergy they have..These shells are identified by the letters K, L, M, N, O, P and QThese shells are identified by the letters K, L, M, N, O, P and Qwith K being the closest to the nucleus.with K being the closest to the nucleus. Each shell can holdEach shell can holdonly a certain maximum number of electronsonly a certain maximum number of electrons; the K shell can; the K shell canhold no more than 2, the L shell no more than 8, shell M nohold no more than 2, the L shell no more than 8, shell M nomore than 18, shell N no more than 32, shell O no more thanmore than 18, shell N no more than 32, shell O no more than50, shell P no more than 72 and shell Q no more than 98.50, shell P no more than 72 and shell Q no more than 98.

Atomic NumberAtomic NumberThe atomic number is the number of protons inThe atomic number is the number of protons inthe nucleus of an atom. The atomic number isthe nucleus of an atom. The atomic number isequal to the number of electrons orbiting theequal to the number of electrons orbiting thenucleus in a neutral atom. The symbol fornucleus in a neutral atom. The symbol foratomic number is Z.atomic number is Z.Mass NumberMass NumberThe mass number is the sum of the protons andThe mass number is the sum of the protons and


The mass number is the sum of the protons andThe mass number is the sum of the protons andneutrons in an atom. Although all atoms of anneutrons in an atom. Although all atoms of anelement have the same number of protons, theyelement have the same number of protons, theymay have different numbers of neutrons. Atomsmay have different numbers of neutrons. Atomsthat have the same number of protons butthat have the same number of protons butdifferent numbers of neutrons are calleddifferent numbers of neutrons are calledisotopes.isotopes.

Atomic WeightAtomic WeightThe atomic weight is the weight of an atomThe atomic weight is the weight of an atomexpressed in atomic mass units (expressed in atomic mass units (amuamu).). OneOneatomic mass unit equals 1/12 the weight of anatomic mass unit equals 1/12 the weight of anatom of Catom of C--12.12.IsotopeIsotopeAn isotope is an atom with a specific atomicAn isotope is an atom with a specific atomicnumber and mass number.number and mass number. Each atomic numberEach atomic number


number and mass number.number and mass number. Each atomic numberEach atomic numberelement may exist withelement may exist with different mass numbedifferent mass numberrand these are isotopes For example,and these are isotopes For example, hydrogen (1hydrogen (1proton, no neutrons), deuterium (1 proton, 1proton, no neutrons), deuterium (1 proton, 1neutron), and tritium (1 proton, 2 neutrons) areneutron), and tritium (1 proton, 2 neutrons) areisotopes of hydrogenisotopes of hydrogen. Some isotopes are stable. Some isotopes are stablewhile others are unstable and change state bywhile others are unstable and change state byradioactive decay.radioactive decay.

Electromagnetic RadiationElectromagnetic RadiationThe PhotonThe Photon

Electromagnetic radiation occurs in the formElectromagnetic radiation occurs in the formof individual packets of energy calledof individual packets of energy called photons.photons.When photons travel through space, theyWhen photons travel through space, theyappear as continuous electromagnetic waves.appear as continuous electromagnetic waves.However, when photons of radiation strike aHowever, when photons of radiation strike a


However, when photons of radiation strike aHowever, when photons of radiation strike asubstance, they behave as if they weresubstance, they behave as if they wereseparate particles of energy instead of aseparate particles of energy instead of acontinuous wave.continuous wave. Each photon has a certainEach photon has a certainamount of energy that is proportional to itsamount of energy that is proportional to itsfrequency.frequency.

XX--raysraysXX--rays are produced whenever high energyrays are produced whenever high energy electronselectronssuddenly give up energysuddenly give up energy. This can be done either by. This can be done either byaccelerating electrons to a high speed and then stoppingaccelerating electrons to a high speed and then stoppingthem suddenly, or by these high speed electrons strikingthem suddenly, or by these high speed electrons strikingothers and knocking them out of their normal positions.others and knocking them out of their normal positions.When these dislodged electrons fall back into place, theyWhen these dislodged electrons fall back into place, theygive off Xgive off X--rays. The position of Xrays. The position of X--rays in the electromagneticrays in the electromagnetic


give off Xgive off X--rays. The position of Xrays. The position of X--rays in the electromagneticrays in the electromagneticspectrum is shown in Figurespectrum is shown in Figure 11..Gamma RaysGamma RaysGamma rays are similar to XGamma rays are similar to X--raysrays except that they have aexcept that they have amuch shorter wavelength and differ in their origin.much shorter wavelength and differ in their origin. GammaGammarays are emitted from the nucleus itselfrays are emitted from the nucleus itself during the processduring the processof radioactivity. The position of gamma rays in theof radioactivity. The position of gamma rays in theelectromagnetic spectrum is shown in Figureelectromagnetic spectrum is shown in Figure 11..


Fig.Fig. 11: The Electromagnetic Spectrum: The Electromagnetic Spectrum

Properties of XProperties of X--RaysRaysand Gamma Raysand Gamma Rays

Both XBoth X--rays and gamma rays can berays and gamma rays can becharacterized by frequency, wavelength, andcharacterized by frequency, wavelength, andvelocity. However, they act somewhat like a particlevelocity. However, they act somewhat like a particleat times in that they occur as small "packets" ofat times in that they occur as small "packets" ofenergy and are referred to as "photons."energy and are referred to as "photons." Due toDue to


energy and are referred to as "photons."energy and are referred to as "photons." Due toDue totheir short wavelength they have more energy totheir short wavelength they have more energy topass through matterpass through matter than do the other forms ofthan do the other forms ofenergy in the electromagnetic spectrum.energy in the electromagnetic spectrum. As theyAs theypass through matter, they arepass through matter, they are scatteredscattered andandabsorbedabsorbed and theand the degree of penetration dependsdegree of penetration dependson the kind of matter and the energy of the rays.on the kind of matter and the energy of the rays.

They are not detected by human senses (cannotThey are not detected by human senses (cannotbe seen, heard, felt, etc.).be seen, heard, felt, etc.). They travel inThey travel in straight linesstraight lines at the speed of light.at the speed of light. Their pathsTheir paths cannot be changed by electrical orcannot be changed by electrical ormagnetic fieldsmagnetic fields.. They can be diffracted to a small degree atThey can be diffracted to a small degree atinterfaces between two different materials.interfaces between two different materials.

Properties of X-Rays andGamma Rays


interfaces between two different materials.interfaces between two different materials. TheyThey pass through matterpass through matter until they have auntil they have achance to encounter with an atomic particle.chance to encounter with an atomic particle. TheirTheir degree of penetration depends on theirdegree of penetration depends on theirenergy and the matter they are traveling throughenergy and the matter they are traveling through.. They have enough energy toThey have enough energy to ionize matterionize matter andandcan damage or destroy living cellscan damage or destroy living cells

XX--rays are produced in packets ofrays are produced in packets ofenergy called photons, just like light.energy called photons, just like light.There are two different atomicThere are two different atomicprocesses that can produce Xprocesses that can produce X--rayrayphotons. One is called Bremsstrahlungphotons. One is called Bremsstrahlung


photons. One is called Bremsstrahlungphotons. One is called Bremsstrahlungand is a German term meaningand is a German term meaning"braking radiation." The other is called"braking radiation." The other is calledKK--shell emission.shell emission.

Bremsstrahlung RadiationBremsstrahlung RadiationXX--ray tubes produce xray tubes produce x--ray photons byray photons byaccelerating a stream of electrons to energiesaccelerating a stream of electrons to energiesofof several hundred kilovoltsseveral hundred kilovolts with velocities ofwith velocities ofseveral hundred kilometers per hour andseveral hundred kilometers per hour andcolliding them into a heavy target material. Thecolliding them into a heavy target material. The


colliding them into a heavy target material. Thecolliding them into a heavy target material. Theabrupt acceleration of the charged electronsabrupt acceleration of the charged electronsproduces Bremsstrahlung photons. Xproduces Bremsstrahlung photons. X--rayrayradiation with a continuous spectrum ofradiation with a continuous spectrum ofenergies is produced ranging from a fewenergies is produced ranging from a few keVkeV totoa maximum of energy of the electron beam.a maximum of energy of the electron beam.

KK--shell Emission Radiationshell Emission RadiationThe KThe K--shell is the lowest energy state of an atom.shell is the lowest energy state of an atom.An incoming electron can give a KAn incoming electron can give a K--shell electronshell electronenough energy to knock it out of its energy state.enough energy to knock it out of its energy state.AboutAbout 00..11% of the electrons produce K% of the electrons produce K--shellshellvacancies; most produce heat. Then, a tungstenvacancies; most produce heat. Then, a tungstenelectron of higher energy (from an outer shell) canelectron of higher energy (from an outer shell) canfall into the Kfall into the K--shell. The energy lost by the fallingshell. The energy lost by the falling


fall into the Kfall into the K--shell. The energy lost by the fallingshell. The energy lost by the fallingelectron shows up in an emitted xelectron shows up in an emitted x--ray photon.ray photon.Meanwhile, higher energy electrons fall into theMeanwhile, higher energy electrons fall into thevacated energy state in the outer shell, and so on.vacated energy state in the outer shell, and so on.KK--shell emission produces highershell emission produces higher--intensity xintensity x--raysraysthan Bremsstrahlung, and the xthan Bremsstrahlung, and the x--ray photon comesray photon comesout at a single wavelength. "characteristic xout at a single wavelength. "characteristic x--RayRay

X-Ray Radiation


Fig.Fig.22 A: Bremsstrahlung RadiationA: Bremsstrahlung Radiation Fig.Fig.22 B:B: KK--shell Emission Radiationshell Emission Radiation

Gamma RadiationGamma Radiation

Gamma rays are highGamma rays are high--energy electromagneticenergy electromagneticwaves of relatively short wavelength that are emittedwaves of relatively short wavelength that are emittedduring the radioactive decay of both naturallyduring the radioactive decay of both naturallyoccurring and artificially produced unstableoccurring and artificially produced unstableisotopes.isotopes. In all respects other than their origin,In all respects other than their origin, --raysraysand xand x--rays are identicalrays are identical. Unlike the broad. Unlike the broad--spectrumspectrumradiation produced by an xradiation produced by an x--ray tube,ray tube, ray sourcesray sources


and xand x--rays are identicalrays are identical. Unlike the broad. Unlike the broad--spectrumspectrumradiation produced by an xradiation produced by an x--ray tube,ray tube, --ray sourcesray sourcesemit one or more discrete wavelengths of radiation,emit one or more discrete wavelengths of radiation,each having its own characteristic photon energy.each having its own characteristic photon energy.The two most common radioactive isotopes used inThe two most common radioactive isotopes used inradiography areradiography are iridiumiridium--192192 andand cobaltcobalt--6060..Alpha ParticlesAlpha Particles

Alpha ParticlesAlpha ParticlesCertain radioactive materials of high atomic massCertain radioactive materials of high atomic mass(Ra226, U238, Pu239) decay by the emission of(Ra226, U238, Pu239) decay by the emission ofalpha particles. These alpha particles are tightlyalpha particles. These alpha particles are tightlybound units ofbound units of two neutrons and two protonstwo neutrons and two protons eacheach(He4 nucleus) and have a positive charge.(He4 nucleus) and have a positive charge.Emission of an alpha particle from the nucleusEmission of an alpha particle from the nucleusresults in a decrease of two units of atomicresults in a decrease of two units of atomic


results in a decrease of two units of atomicresults in a decrease of two units of atomicnumber (Z) and four units of mass number (A).number (Z) and four units of mass number (A).Alpha particles are emitted with discrete energiesAlpha particles are emitted with discrete energiescharacteristic of the particular transformationcharacteristic of the particular transformationfrom which they originate. All alpha particles fromfrom which they originate. All alpha particles froma particular radioactive transformation will havea particular radioactive transformation will haveidentical energies.identical energies.

Beta ParticlesBeta ParticlesA nucleus with an unstable ratio of neutronsA nucleus with an unstable ratio of neutronsto protons may decay throughto protons may decay through the emission ofthe emission ofa high speed electron called a beta particlea high speed electron called a beta particle..This results in a net change of one unit ofThis results in a net change of one unit ofatomic number (Z). Beta particles have aatomic number (Z). Beta particles have a


atomic number (Z). Beta particles have aatomic number (Z). Beta particles have anegative charge and the beta particlesnegative charge and the beta particlesemitted by a specific radionuclide will range inemitted by a specific radionuclide will range inenergy from near zero up to a maximumenergy from near zero up to a maximumvalue, which is characteristic of the particularvalue, which is characteristic of the particulartransformation.transformation.

AA nucleusnucleus whichwhich isis inin anan excitedexcited statestate maymay emitemitoneone oror moremore photonsphotons (packets(packets ofofelectromagneticelectromagnetic radiation)radiation) ofof discretediscrete energiesenergies..TheThe emissionemission ofof gammagamma raysrays doesdoes notnot alteralter thethenumbernumber ofof protonsprotons oror neutronsneutrons inin thethe nucleusnucleusbutbut insteadinstead hashas thethe effecteffect ofof movingmoving thethe nucleusnucleus

GammaGamma--raysrays


numbernumber ofof protonsprotons oror neutronsneutrons inin thethe nucleusnucleusbutbut insteadinstead hashas thethe effecteffect ofof movingmoving thethe nucleusnucleusfromfrom aa higherhigher toto aa lowerlower energyenergy statestate (unstable(unstabletoto stable)stable).. GammaGamma rayray emissionemission frequentlyfrequentlyfollowsfollows betabeta decay,decay, alphaalpha decay,decay, andand otherothernuclearnuclear decaydecay processesprocesses..

ActivityActivity is the number ofis the number ofatoms of a radioactiveatoms of a radioactivesubstance that disintegratesubstance that disintegrateper unit time,per unit time, the specific activity is thethe specific activity is theactivity per unit mass of a pureactivity per unit mass of a pureradioisotope.radioisotope.

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radioisotope.radioisotope. The Becquerel (The Becquerel (BqBq), the SI), the SIunit of activity, represents oneunit of activity, represents onespontaneous transition perspontaneous transition persecond.second. ThusThus 11 BqBq == 11 SS--11. The. Theformer unit, the curie (former unit, the curie (CiCi), is), isequal toequal to 33..77xlxl001010Bq.Bq.

Each radioactive substance decays at its ownEach radioactive substance decays at its ownunique rate which cannot be altered by any chemicalunique rate which cannot be altered by any chemicalor physical processor physical process. A useful measure of this rate is. A useful measure of this rate isthe halfthe half--life of the substance.life of the substance. HalfHalf--life is defined aslife is defined asthe time required for the activity of any particularthe time required for the activity of any particular


the time required for the activity of any particularthe time required for the activity of any particularradioactive substance to decrease to oneradioactive substance to decrease to one--half of itshalf of itsinitial value.initial value. HalfHalf--life of two widely used industriallife of two widely used industrialisotopes areisotopes are 74 days for Iridium74 days for Iridium--192, and 5.3 years192, and 5.3 yearsfor Cobaltfor Cobalt--6060. More exacting calculations can be. More exacting calculations can bemade for the halfmade for the half--life of these materials, however,life of these materials, however,these times are commonly used.these times are commonly used.

Inverse Square LawInverse Square Law

Any point source whichAny point source whichspreads its influencespreads its influenceequally in all directionsequally in all directionswithout a limit to itswithout a limit to itsrange will obey therange will obey the


range will obey therange will obey theinverse square law. Thisinverse square law. Thiscomes from strictlycomes from strictlygeometricalgeometricalconsiderationsconsiderations

Attenuation of ElectromagneticAttenuation of ElectromagneticRadiationRadiation

XX--rays andrays and --rays interact with any substance,rays interact with any substance,even gases such as air, as the rays pass througheven gases such as air, as the rays pass throughthe substance.the substance. It is this interaction that enablesIt is this interaction that enablesparts to be inspected by differential attenuationparts to be inspected by differential attenuationof radiation and that enables differences in theof radiation and that enables differences in theintensity of radiation to be detected andintensity of radiation to be detected and


of radiation and that enables differences in theof radiation and that enables differences in theintensity of radiation to be detected andintensity of radiation to be detected andrecordedrecorded. Both these effects are essential to the. Both these effects are essential to theradiographic process. The attenuationradiographic process. The attenuationcharacteristics of materials vary with the type,characteristics of materials vary with the type,intensity, and energy of the radiation and withintensity, and energy of the radiation and withthe density and atomic structure of the material.the density and atomic structure of the material.

The intensity of radiation varies exponentiallyThe intensity of radiation varies exponentiallywith the thickness of homogeneous material throughwith the thickness of homogeneous material throughwhich it passes. This behavior is expressed as:which it passes. This behavior is expressed as:

I=I= IIooexpexp((--tt))wherewhere II is the intensity of the emergent radiation,is the intensity of the emergent radiation, IIoo

is the initial intensity,is the initial intensity, tt is the thickness ofis the thickness ofhomogeneous material, andhomogeneous material, and is a characteristic ofis a characteristic ofthe material known as the linear absorptionthe material known as the linear absorptioncoefficientcoefficient

mass absorption coefficient is (mass absorption coefficient is (//), where), where is theis the


mass absorption coefficient is (mass absorption coefficient is (//), where), where is theis thedensity of the materialdensity of the material

Atomic absorption coefficient (Atomic absorption coefficient (aa) or cross section,) or cross section,is equal to the linear absorption coefficient divided byis equal to the linear absorption coefficient divided bythe number of atoms per unit volume. The crossthe number of atoms per unit volume. The crosssection, usually expressed in barns (section, usually expressed in barns (11 barn =barn = 1010--2424cmcm22), indicates the probability of collision with the), indicates the probability of collision with theatoms of the material.atoms of the material.

Using the transmitted intensity equation above,Using the transmitted intensity equation above,linear attenuation coefficients can be used to make alinear attenuation coefficients can be used to make anumber of calculations. These include:number of calculations. These include: the intensity of the energy transmitted through athe intensity of the energy transmitted through amaterialmaterial when the incident xwhen the incident x--ray intensity, the material andray intensity, the material andthe material thickness are known.the material thickness are known. the intensity of the incident xthe intensity of the incident x--ray energyray energy when thewhen thetransmitted xtransmitted x--ray intensity, material, and materialray intensity, material, and material


transmitted xtransmitted x--ray intensity, material, and materialray intensity, material, and materialthickness are known.thickness are known. the thickness of the materialthe thickness of the material when the incident andwhen the incident andtransmitted intensity, and the material are known.transmitted intensity, and the material are known. the material can be determinedthe material can be determined from the value of from the value of when the incident and transmitted intensity, and thewhen the incident and transmitted intensity, and thematerial thickness are known.material thickness are known.

Atomic Attenuation ProcessesAtomic Attenuation ProcessesSeveral interaction events are usually involvedSeveral interaction events are usually involvedand the total attenuation is the sum of theand the total attenuation is the sum of theattenuation due to different types of interactions.attenuation due to different types of interactions.These interactions include the photoelectricThese interactions include the photoelectriceffect, scattering, and pair production. The figureeffect, scattering, and pair production. The figure


effect, scattering, and pair production. The figureeffect, scattering, and pair production. The figurebelow shows an approximation of the totalbelow shows an approximation of the totalabsorption coefficient, (), in red, for iron plottedabsorption coefficient, (), in red, for iron plottedas a function of radiation energy. The fouras a function of radiation energy. The fourradiation matter interactions that contribute to theradiation matter interactions that contribute to thetotal absorption are shown in black.total absorption are shown in black.

TheThe fourfour typestypes ofof interactionsinteractions areare::photoelectricphotoelectric (PE),(PE), ComptonCompton scatteringscattering (C),(C),pairpair productionproduction (PP),(PP), andand ThomsonThomson ororRayleighRayleigh scatteringscattering (R)(R).. SinceSince mostmostindustrialindustrial radiographyradiography isis donedone inin thethe 00..11 toto


industrialindustrial radiographyradiography isis donedone inin thethe 00..11 toto11..55 MeVMeV range,range, itit cancan bebe seenseen fromfrom thethe plotplotthatthat photoelectricphotoelectric andand ComptonCompton scatteringscatteringaccountaccount forfor thethe majoritymajority ofof attenuationattenuationencounteredencountered..



different mechanisms that cause attenuation of an incident xdifferent mechanisms that cause attenuation of an incident x--ray beamray beam


Photoelectric (PE)Photoelectric (PE) absorption of xabsorption of x--rays occurs when the xrays occurs when the x--ray photonray photonis absorbed, resulting in theis absorbed, resulting in theejection of electrons from the outerejection of electrons from the outershell of the atom, and hence theshell of the atom, and hence theionization of the atom.ionization of the atom.Subsequently, the ionized atomSubsequently, the ionized atom


Subsequently, the ionized atomSubsequently, the ionized atomreturns to the neutral state with thereturns to the neutral state with theemission of an xemission of an x--ray characteristicray characteristicof the atom. This subsequentof the atom. This subsequentemission of lower energy photons isemission of lower energy photons isgenerally absorbed and does notgenerally absorbed and does notcontribute to (or hinder) the imagecontribute to (or hinder) the imagemaking process.making process.


Compton scattering (C)Compton scattering (C) occurs whenoccurs whenthe incident xthe incident x--ray photon is deflectedray photon is deflectedfrom its original path by anfrom its original path by aninteraction with an electron.interaction with an electron. TheTheelectron gains energy and is ejectedelectron gains energy and is ejectedfrom its orbital position.from its orbital position. The xThe x--rayray


from its orbital position.from its orbital position. The xThe x--rayrayphoton looses energy due to thephoton looses energy due to theinteraction but continues to travelinteraction but continues to travelthrough the material along anthrough the material along analtered path.altered path. Since the scattered xSince the scattered x--ray photon has less energy, it,ray photon has less energy, it,therefore, has a longer wavelengththerefore, has a longer wavelengththan the incident photon.than the incident photon.

PairPair productionproduction (PP)(PP) :: cancan occuroccurwhenwhen thethe xx--rayray photonphoton energyenergy isisgreatergreater thanthan 11..0202 MeVMeV,, butbut reallyreallyonlyonly becomesbecomes significantsignificant atat energiesenergiesaroundaround 1010 MeVMeV.. PairPair productionproductionoccursoccurs whenwhen anan electronelectron andandpositronpositron areare createdcreated withwith thetheannihilationannihilation ofof thethe xx--rayray photonphoton..PositronsPositrons areare veryvery shortshort livedlived andand


PositronsPositrons areare veryvery shortshort livedlived andanddisappeardisappear (positron(positron annihilation)annihilation)withwith thethe formationformation ofof twotwo photonsphotons ofof00..5151 MeVMeV energyenergy.. PairPair productionproduction isisofof particularparticular importanceimportance whenwhen highhigh--energyenergy photonsphotons passpass throughthroughmaterialsmaterials ofof aa highhigh atomicatomic numbernumber..

Thomson scattering (R),Thomson scattering (R), ::alsoalsoknown as Rayleigh, coherent, orknown as Rayleigh, coherent, orclassical scattering, occursclassical scattering, occurswhen the xwhen the x--ray photon interactsray photon interactswith the whole atom so that thewith the whole atom so that thephoton is scattered with nophoton is scattered with nochange in internal energy to thechange in internal energy to thescattering atom, nor to the xscattering atom, nor to the x--rayray


scattering atom, nor to the xscattering atom, nor to the x--rayrayphoton.. The scattering occursphoton.. The scattering occurswithout the loss of energy.without the loss of energy.Scattering is mainly in theScattering is mainly in theforward direction.forward direction.

The thickness of any given materialThe thickness of any given materialwherewhere 5050% of the incident energy has% of the incident energy hasbeen attenuated is know as the halfbeen attenuated is know as the half--value layer (HVL). The HVL isvalue layer (HVL). The HVL isexpressed in units of distance (mm orexpressed in units of distance (mm orcm). Like the attenuation coefficient, itcm). Like the attenuation coefficient, it

HalfHalf--Value LayerValue Layer


expressed in units of distance (mm orexpressed in units of distance (mm orcm). Like the attenuation coefficient, itcm). Like the attenuation coefficient, itis photon energy dependant.is photon energy dependant.Increasing the penetrating energy of aIncreasing the penetrating energy of astream of photons will result in anstream of photons will result in anincrease in a material's HVL.increase in a material's HVL.

The HVL is inversely proportional to theThe HVL is inversely proportional to theattenuation coefficient.attenuation coefficient.

I =I = IIooexpexp((--tt))00..55==11 exp(exp(--tt)) If x is theIf x is the HVLHVL then m timesthen m times HVLHVL mustmustequalequal 00..693693 (since the number(since the number 00..693693 isisthe exponent value that gives a value ofthe exponent value that gives a value of


equalequal 00..693693 (since the number(since the number 00..693693 isisthe exponent value that gives a value ofthe exponent value that gives a value of00..55).). Therefore, the HVL andTherefore, the HVL and are relatedare relatedas follows:as follows: HVL=HVL=00..693693/ /

The HVL is often used inradiography simply becauseit is easier to remembervalues and perform simplecalculations. In a shieldingcalculation, such asillustrated to the right, it canbe seen that if the thickness


illustrated to the right, it canbe seen that if the thicknessof one HVL is known, it ispossible to quickly determinehow much material isneeded to reduce theintensity to less than 1%.

ManmadeManmade radioactiveradioactive sourcessources areareproducedproduced byby introducingintroducing anan extraextra neutronneutrontoto atomsatoms ofof thethe sourcesource materialmaterial.. AsAs thethematerialmaterial ridsrids itselfitself ofof thethe neutron,neutron, energyenergy


materialmaterial ridsrids itselfitself ofof thethe neutron,neutron, energyenergyisis releasedreleased inin thethe formform ofof gammagamma raysrays.. TwoTwoofof thethe moremore commoncommon industrialindustrial gammagamma--rayraysourcessources forfor industrialindustrial radiographyradiography areareiridiumiridium--192192 andand cobaltcobalt--6060..

TheseThese isotopesisotopes emitemit radiationradiation inin aa fewfewdiscreetdiscreet wavelengthswavelengths.. CobaltCobalt--6060 willwill emitemit aa11..3333 andand aa 11..1717 MeVMeV gammagamma ray,ray, andand iridiumiridium--192192 willwill emitemit 00..3131,, 00..4747,, andand 00..6060 MeVMeVgammagamma raysrays.. InIn comparisoncomparison toto anan XX--rayray


gammagamma raysrays.. InIn comparisoncomparison toto anan XX--rayraygenerator,generator, cobaltcobalt--6060 producesproduces energiesenergiescomparablecomparable toto aa 11..2525 MeVMeV XX--rayray systemsystem andandiridiumiridium--192192 toto aa 460460 keVkeV XX--rayray systemsystem..

TheseThese highhigh energiesenergies makemake itit possiblepossible totopenetratepenetrate thickthick materialsmaterials withwith aa relativelyrelativelyshortshort exposureexposure timetime.. ThisThis andand thethe factfact thatthatsourcessources areare veryvery portableportable areare thethe mainmainreasonsreasons thatthat gammagamma sourcessources areare widelywidelyusedused forfor fieldfield radiographyradiography.. OfOf course,course, thethe


usedused forfor fieldfield radiographyradiography.. OfOf course,course, thethedisadvantagedisadvantage ofof aa radioactiveradioactive sourcesource isis thatthatitit cancan nevernever bebe turnedturned offoff andand safelysafelymanagingmanaging thethe sourcesource isis aa constantconstantresponsibilityresponsibility..

Physical size of isotope materials variesPhysical size of isotope materials variesbetween manufacturers, but generally anbetween manufacturers, but generally anisotope material is a pellet that measuresisotope material is a pellet that measures11..55 mm xmm x 11..55 mm. Depending on themm. Depending on thelevel of activity desired, a pellet or pelletslevel of activity desired, a pellet or pelletsare loaded into a stainless steel capsuleare loaded into a stainless steel capsuleand sealed by welding. The capsule isand sealed by welding. The capsule isattached to short flexible cable called aattached to short flexible cable called apigtail.pigtail.


The source capsule and the pigtail is housedThe source capsule and the pigtail is housedin a shielding device referred to as an exposure devicein a shielding device referred to as an exposure deviceor camera.or camera. Depleted uranium is often used as a shieldingDepleted uranium is often used as a shieldingmaterial for sources. The exposure device for iridiummaterial for sources. The exposure device for iridium--192192and cobaltand cobalt--6060 sources will containsources will contain 4545 pounds(pounds(2020Kg) andKg) and500500 pounds(pounds(226226Kg) of shielding materials, respectively.Kg) of shielding materials, respectively.Cobalt cameras are often fixed to a trailer andCobalt cameras are often fixed to a trailer andtransported to and from inspection sites. When the sourcetransported to and from inspection sites. When the sourceis not being used to make an exposure, it is locked insideis not being used to make an exposure, it is locked insidethe exposure device.the exposure device.


the exposure device.the exposure device.

ToTo makemake aa radiographicradiographic exposure,exposure, aa crankcrankoutout mechanismmechanism andand aa guideguide tubetube areare attachedattached totooppositeopposite endsends ofof thethe exposureexposure devicedevice.. TheThe guideguidetubetube oftenoften hashas aa collimatorcollimator atat thethe endend toto shieldshieldthethe radiationradiation exceptexcept inin thethe directiondirection necessarynecessary totomakemake thethe exposureexposure.. TheThe endend ofof thethe guideguide tubetube isis


makemake thethe exposureexposure.. TheThe endend ofof thethe guideguide tubetube isissecuredsecured inin thethe locationlocation wherewhere thethe radiationradiationsourcesource needsneeds toto produceproduce thethe radiographradiograph.. TheThecrankcrank outout cablecable isis stretchedstretched asas farfar asas possiblepossible totoputput asas muchmuch distancedistance asas possiblepossible betweenbetween thetheexposureexposure devicedevice andand thethe radiographerradiographer..

ToTo makemake thethe exposure,exposure, thethe radiographerradiographer quicklyquickly crankscranksthethe sourcesource outout ofof thethe exposureexposure devicedevice andand intointo positionpositioninin thethe collimatorcollimator atat thethe endend ofof thethe guideguide tubetube.. AtAt thethe endendofof thethe exposureexposure time,time, thethe sourcesource isis crankedcranked backback intointo thetheexposureexposure devicedevice.. ThereThere isis aa seriesseries ofof safetysafety procedures,procedures,whichwhich includeinclude severalseveral radiationradiation surveys,surveys, thatthat mustmust bebeaccomplishedaccomplished whenwhen makingmaking anan exposureexposure withwith aa gammagammasourcesource


sourcesource

Production of XProduction of X--RaysRaysXX--rays are produced when electrons,rays are produced when electrons,traveling at high speed, collide with mattertraveling at high speed, collide with matteror change directionor change direction. In the usual type of x. In the usual type of x--ray tube, an incandescent filament suppliesray tube, an incandescent filament suppliesthe electrons and thus forms the cathode,the electrons and thus forms the cathode,or negative electrode, of the tube. A highor negative electrode, of the tube. A high


the electrons and thus forms the cathode,the electrons and thus forms the cathode,or negative electrode, of the tube. A highor negative electrode, of the tube. A highvoltage applied to the tube drives thevoltage applied to the tube drives theelectrons to the anode, or target. Theelectrons to the anode, or target. Thesudden stopping of these rapidly movingsudden stopping of these rapidly movingelectrons in the surface of the targetelectrons in the surface of the targetresults in the generation of xresults in the generation of x--radiation.radiation.

XX--Ray TubesRay TubesXX--ray tubes are electronic devices that convertray tubes are electronic devices that convertelectrical energy into xelectrical energy into x--rays.rays. Typically, an xTypically, an x--rayraytube consists of a cathode structure containing atube consists of a cathode structure containing afilament and an anode structure containing afilament and an anode structure containing atarget all within an evacuated chamber ortarget all within an evacuated chamber orenvelope .A lowenvelope .A low--voltage power supply, usuallyvoltage power supply, usuallycontrolled by a rheostat, generates the electriccontrolled by a rheostat, generates the electric


controlled by a rheostat, generates the electriccontrolled by a rheostat, generates the electriccurrent that heats, the filament to incandescence.current that heats, the filament to incandescence.This incandescence of the filament produces anThis incandescence of the filament produces anelectron cloud, which is directed to the anode by aelectron cloud, which is directed to the anode by afocusing system and accelerated to the anode byfocusing system and accelerated to the anode bythe high voltage applied between the cathode andthe high voltage applied between the cathode andthe anode.the anode.

WhenWhen thethe acceleratedaccelerated electronselectronsimpingeimpinge onon thethe targettarget immediatelyimmediatelybeneathbeneath thethe focalfocal spot,spot, thethe electronselectronsareare slowedslowed andand absorbed,absorbed, andand bothbothbremsstrahlungbremsstrahlung andand characteristiccharacteristic xx--raysrays areare producedproduced.. MostMost ofof thethe energyenergyinin thethe impingingimpinging electronelectron beambeam isistransformedtransformed intointo heat,heat, whichwhich mustmust bebedissipateddissipated.. SevereSevere restrictionsrestrictions areare


transformedtransformed intointo heat,heat, whichwhich mustmust bebedissipateddissipated.. SevereSevere restrictionsrestrictions areareimposedimposed onon thethe designdesign andand selectionselectionofof materialsmaterials forfor thethe anodeanode andand targettargettoto ensureensure thatthat structuralstructural damagedamagefromfrom overheatingoverheating doesdoes notnotprematurelyprematurely destroydestroy thethe targettarget..

AnodeAnode heatingheating alsoalso limitslimits thethesizesize ofof thethe focalfocal spotspot.. BecauseBecausesmallersmaller focalfocal spotsspots produceproducesharpersharper radiographicradiographic images,images, thethedesigndesign ofof thethe anodeanode andand targettargetrepresentsrepresents aa compromisecompromise betweenbetweenmaximummaximum radiographicradiographic definitiondefinitionandand maximummaximum targettarget lifelife.. InIn manymanyxx--rayray tubes,tubes, aa long,long, narrow,narrow, actualactual


xx--rayray tubes,tubes, aa long,long, narrow,narrow, actualactualfocalfocal spotspot isis projectedprojected asas aa roughlyroughlysquaresquare effectiveeffective focalfocal spotspot bybyinclininginclining thethe anodeanode faceface atat aa smallsmallangleangle (usually(usually aboutabout 2020)) toto thethecenterlinecenterline ofof thethe xx--rayray beambeam..

Tube Design and MaterialsTube Design and MaterialsThe cathode structure in a conventional xThe cathode structure in a conventional x--rayraytube incorporates a filament and a focusingtube incorporates a filament and a focusingcup, which surrounds the filament. Thecup, which surrounds the filament. Thefocusing cup,focusing cup, usually made of pure iron orusually made of pure iron orpure nickel, functions as an electrostatic lenspure nickel, functions as an electrostatic lenswhose purpose is to direct the electron beamwhose purpose is to direct the electron beam


whose purpose is to direct the electron beamwhose purpose is to direct the electron beamtoward the anode.toward the anode. The filament,The filament, usually a coilusually a coilof tungsten wire, is heated to incandescenceof tungsten wire, is heated to incandescenceby an electric current produced by a relativelyby an electric current produced by a relativelylow voltage, similar to the operation of anlow voltage, similar to the operation of anordinary incandescent light bulb.ordinary incandescent light bulb.

At incandescence,At incandescence, the filament emitsthe filament emitselectrons, which are accelerated across theelectrons, which are accelerated across theevacuated space between the cathode andevacuated space between the cathode andthe anode. The driving force for accelerationthe anode. The driving force for accelerationis a high electrical potential (voltage)is a high electrical potential (voltage)between anode and cathode, which isbetween anode and cathode, which is


between anode and cathode, which isbetween anode and cathode, which isapplied during exposure.applied during exposure. The anodeThe anode usually consists of a button ofusually consists of a button ofthe target material embedded in a mass ofthe target material embedded in a mass ofcopper that absorbs much of the heatcopper that absorbs much of the heatgenerated by electron collisions with thegenerated by electron collisions with thetarget.target.

TungstenTungsten is the preferred material for traditionalis the preferred material for traditionalxx--ray tubes used in radiography because its highray tubes used in radiography because its highatomic number makes it an efficient emitter of xatomic number makes it an efficient emitter of x--rays and because its high melting point enables it torays and because its high melting point enables it towithstand the high temperatures of operation.withstand the high temperatures of operation. GoldGoldand platinumand platinum are also used in xare also used in x--ray tubes forray tubes forradiography, but targets made of these metals mustradiography, but targets made of these metals must


radiography, but targets made of these metals mustradiography, but targets made of these metals mustbe more effectively cooler than targets made ofbe more effectively cooler than targets made oftungsten. Other materials are used, particularly attungsten. Other materials are used, particularly atlow energies, to take advantage of theirlow energies, to take advantage of theircharacteristic radiation. Most highcharacteristic radiation. Most high--intensity xintensity x--rayraytubes have forced liquid cooling to dissipate thetubes have forced liquid cooling to dissipate thelarge amounts of anode heat generated duringlarge amounts of anode heat generated duringoperation.operation.

Tube envelopesTube envelopes are constructed ofare constructed of glass,glass,ceramic materials or metals, orceramic materials or metals, orcombinations of these materials.combinations of these materials. TubeTubeenvelopes must have good structuralenvelopes must have good structuralstrength at high temperatures to withstandstrength at high temperatures to withstand


strength at high temperatures to withstandstrength at high temperatures to withstandthe combined effect of forces imposed bythe combined effect of forces imposed byatmospheric pressure on the evacuatedatmospheric pressure on the evacuatedchamber and radiated heat from the anode.chamber and radiated heat from the anode.

The shape of the envelope varies withThe shape of the envelope varies withthe cathodethe cathode--anode arrangement and withanode arrangement and withthe maximum rated voltage of the tube.the maximum rated voltage of the tube.Electrical connections for the anode andElectrical connections for the anode andcathode are fused into the walls of thecathode are fused into the walls of the


cathode are fused into the walls of thecathode are fused into the walls of theenvelope. Generally, these are made ofenvelope. Generally, these are made ofmetals or alloys having thermal expansionmetals or alloys having thermal expansionproperties that match those of theproperties that match those of theenvelope material.envelope material.

XX--ray tubes are inserted into metallic housingsray tubes are inserted into metallic housingsthat contain an insulating mediumthat contain an insulating medium such assuch astransformer oil or an insulating gas. The maintransformer oil or an insulating gas. The mainpurpose of the insulated housing ispurpose of the insulated housing is to provideto provideprotection from highprotection from high--voltage electrical shockvoltage electrical shock..Housings usually contain quick disconnects forHousings usually contain quick disconnects for

Cairo Inspection Company (CIC) Www. CICCairo Inspection Company (CIC) Www. CIC--Egypt.com Eng. Ibrahim EldesokyEgypt.com Eng. Ibrahim Eldesoky

Housings usually contain quick disconnects forHousings usually contain quick disconnects forelectrical cables from the highelectrical cables from the high--voltage powervoltage powersupply or transformer. On selfsupply or transformer. On self--contained units,contained units,most of which are portable, both the xmost of which are portable, both the x--ray tuberay tubeand the highand the high--voltage transformer are contained involtage transformer are contained ina single housing, and no higha single housing, and no high--voltage cables arevoltage cables areused.used.

There are three important electricalThere are three important electricalcharacteristics of xcharacteristics of x--ray tubes:ray tubes: The filament currentThe filament current, which controls, which controlsthe filament temperature and in turn thethe filament temperature and in turn thequantity of electrons that are emittedquantity of electrons that are emitted


quantity of electrons that are emittedquantity of electrons that are emitted The tube voltageThe tube voltage, or anode, or anode--toto--cathodecathodepotential, which controls the energy ofpotential, which controls the energy ofimpinging electrons and therefore theimpinging electrons and therefore theenergy or penetrating power, of the xenergy or penetrating power, of the x--rayraybeambeam

The tube current, which is directlyThe tube current, which is directlyrelated to filament temperature and isrelated to filament temperature and isusually referred to as theusually referred to as the milliamperagemilliamperage ofofthe tubethe tubeThe strength, or radiation output, of theThe strength, or radiation output, of thebeam is approximately proportional tobeam is approximately proportional tomilliamperagemilliamperage,, which is used as one of thewhich is used as one of the


milliamperagemilliamperage,, which is used as one of thewhich is used as one of thevariables in exposure calculations. Thisvariables in exposure calculations. Thisradiation output, orradiation output, or RR--output,output, is usuallyis usuallyexpressed inexpressed in roentgens per minute (orroentgens per minute (orhour) athour) at 11 mm

The RThe R--outputoutputThe RThe R--output of an xoutput of an x--ray tube varies withray tube varies withtube voltage (accelerating potential), tubetube voltage (accelerating potential), tubecurrent (number of electrons impinging oncurrent (number of electrons impinging onthe target per unit time),the target per unit time), and physicaland physicalfeatures of the individual equipment.features of the individual equipment.


features of the individual equipment.features of the individual equipment.Because of the last factor, the RBecause of the last factor, the R--output of anoutput of anindividual source also varies with position inindividual source also varies with position inthe radiation beam, position usually beingthe radiation beam, position usually beingexpressed as the angle relative to theexpressed as the angle relative to thecentral axis of the beam.central axis of the beam.

EffectEffect ofof TubeTube VoltageVoltage..TheThe effecteffect ofof tubetube voltagevoltage onon thethevariationvariation ofof intensityintensity (R(R--output)output) isisshownshown inin FigFig.. TheThe overalloverall RR--outputoutputvariesvaries approximatelyapproximately asas thethesquaresquare rootroot ofof tubetube voltagevoltage.. TheThecombinedcombined effecteffect ofof greatergreaterphotonphoton energyenergy andand increasedincreased RR--


photonphoton energyenergy andand increasedincreased RR--outputoutput produces,produces, forfor filmfilmradiography,radiography, aa decreasedecrease ininexposureexposure timetime ofof aboutabout 5050%% forfor aa1010%% increaseincrease inin tubetube voltagevoltage.. TheTheeffecteffect isis similarsimilar withwith otherotherpermanentpermanent imageimage recordingrecordingmedia,media, asas inin paperpaper radiographyradiographyandand xeroradiographyxeroradiography..

EffectEffect ofof TubeTube CurrentCurrent..onlyonly thethe RR--outputoutput (intensity)(intensity)variesvaries.. BecauseBecause tubetube currentcurrentisis aa directdirect measuremeasure ofof thethenumbernumber ofof electronselectronsimpingingimpinging onon thethe targettarget perper


impingingimpinging onon thethe targettarget perperunitunit ofof time,time, andand thereforethereforethethe numbernumber ofof photonsphotonsemittedemitted perper unitunit ofof timetime atateacheach valuevalue ofof photonphoton energy,energy,RR--outputoutput variesvaries directlydirectly withwithtubetube currentcurrent..

Heel Effect.Heel Effect.XX--ray tubes exhibit aray tubes exhibit adetrimental feature knowndetrimental feature knownas the heel effect. When theas the heel effect. When thedirection in which xdirection in which x--rays arerays areemitted from the targetemitted from the targetapproaches the anode heelapproaches the anode heelplane, the intensity ofplane, the intensity of


approaches the anode heelapproaches the anode heelplane, the intensity ofplane, the intensity ofradiation at a given distanceradiation at a given distancefrom the focal spot is lessfrom the focal spot is lessthan the intensity of thethan the intensity of thecentral beam because ofcentral beam because ofselfself--absorption by the target.absorption by the target.

RadiographsRadiographs ofof largelarge--areaarea testtest piecespieces thatthat arearemademade atat relativelyrelatively shortshortsourcesource--toto--detectordetectordistancesdistances willwill exhibitexhibit lesslessphotographicphotographic densitydensity(film)(film) oror lessless brightnessbrightness(real(real--time)time) inin thethe regionregionwherewhere thethe incidentincident


wherewhere thethe incidentincidentradiationradiation isis lessless intenseintensebecausebecause ofof thethe heelheeleffecteffect.. ThisThis cancan leadlead totoerrorserrors inin interpretationinterpretationunlessunless thethe heelheel effecteffect isisrecognizedrecognized

Inherent Filtration.Inherent Filtration.In the radiography (film or realIn the radiography (film or real--time)time)of thin or lightweight materials,of thin or lightweight materials,which requires lowwhich requires low--energy radiation,energy radiation,filtration by the glass walls of the xfiltration by the glass walls of the x--ray tube becomes a problem. Ninetyray tube becomes a problem. Ninety--five percent of afive percent of a 3030--kV xkV x--ray beam isray beam isabsorbed by the glass walls of anabsorbed by the glass walls of anordinary xordinary x--ray tube. Consequently, inray tube. Consequently, ina tube used to radiograph thin ora tube used to radiograph thin orlightweight materials, alightweight materials, a berylliumberylliumwindowwindow is fused into the glass wall inis fused into the glass wall in


windowwindow is fused into the glass wall inis fused into the glass wall inthe path of the xthe path of the x--ray beam.ray beam. BerylliumBerylliumis one of the lightest of metals and isis one of the lightest of metals and ismore transparent to xmore transparent to x--rays than anyrays than anyother metal. The beryllium windowother metal. The beryllium windowtube has a minimum of inherenttube has a minimum of inherentfiltration and allows most of the veryfiltration and allows most of the verylow energy xlow energy x--rays to escape from therays to escape from thetubetube

HighHigh--Energy XEnergy X--Ray SourcesRay SourcesAbove aboutAbove about 400400 kV, the conventional design ofkV, the conventional design ofan xan x--ray tube and its highray tube and its high--voltage ironvoltage iron--core transformercore transformerbecomes cumbersome and large. Although xbecomes cumbersome and large. Although x--ray machinesray machineswith ironwith iron--core transformers have been built forcore transformers have been built for 600600 kVkV(maximum), there are no commercial versions operating(maximum), there are no commercial versions operatingaboveabove 500500 kV. For higherkV. For higher--energy xenergy x--rays, other designs arerays, other designs areused. Some of the machine designs for the production ofused. Some of the machine designs for the production ofhighhigh--energy xenergy x--rays include:rays include:

Linear acceleratorsLinear accelerators BetatronsBetatrons


BetatronsBetatrons Van deVan de GraaffGraaff generatorsgenerators XX--ray tubes with aray tubes with a resonant transformerresonant transformer

AA radiographradiograph isis aa photographicphotographicrecordrecord producedproduced byby thethe passagepassageofof xx--raysrays oror gammagamma raysrays throughthroughanan objectobject ontoonto aa filmfilm.. SeeSee thethefigurefigure.. WhenWhen filmfilm isis exposedexposed toto xx--rays,rays, gammagamma rays,rays, oror light,light, ananinvisibleinvisible changechange calledcalled aa latentlatentimageimage isis producedproduced inin thethe filmfilm


imageimage isis producedproduced inin thethe filmfilmemulsionemulsion.. TheThe areasareas soso exposedexposedbecomebecome darkdark whenwhen thethe filmfilm isisimmersedimmersed inin aa developingdevelopingsolution,solution, thethe degreedegree ofof darkeningdarkeningdependingdepending onon thethe amountamount ofofexposureexposure..

AfterAfter development,development, thethe filmfilm isis rinsed,rinsed, preferablypreferablyinin aa specialspecial bath,bath, toto stopstop developmentdevelopment.. TheThe filmfilmisis nextnext putput intointo aa fixingfixing bath,bath, whichwhich dissolvesdissolvesthethe undarkenedundarkened portionsportions ofof thethe sensitivesensitive saltsalt.. ItItisis thenthen washedwashed toto removeremove thethe fixerfixer andand drieddried soso


isis thenthen washedwashed toto removeremove thethe fixerfixer andand drieddried sosothatthat itit maymay bebe handled,handled, interpreted,interpreted, andand filedfiled..TheThe developing,developing, fixing,fixing, andand washingwashing ofof thetheexposedexposed filmfilm maymay bebe donedone eithereither manuallymanually oror ininautomatedautomated processingprocessing equipmentequipment..

A radiograph is a shadow picture of an objectA radiograph is a shadow picture of an objectthat has been placed in the path of an xthat has been placed in the path of an x--ray orray orgammagamma--ray beamray beam, between the tube anode and, between the tube anode andthe film or between the source of gammathe film or between the source of gammaradiation and the film. It naturally follows,radiation and the film. It naturally follows,therefore, that the appearance of an image thustherefore, that the appearance of an image thusrecorded is materially influenced by the relativerecorded is materially influenced by the relative


therefore, that the appearance of an image thustherefore, that the appearance of an image thusrecorded is materially influenced by the relativerecorded is materially influenced by the relativepositions of the object and the film and by thepositions of the object and the film and by thedirection of the beam. For these reasons,direction of the beam. For these reasons,familiarity with the elementary principles offamiliarity with the elementary principles ofshadow formation is important to those makingshadow formation is important to those makingand interpreting radiographs.and interpreting radiographs.

xx--rays and gamma rays obey the commonrays and gamma rays obey the commonlaws of light, their shadow formation may belaws of light, their shadow formation may beexplained in a simple manner in terms of light. Itexplained in a simple manner in terms of light. Itshould be borne in mind that the analogyshould be borne in mind that the analogybetween light and these radiations is not perfectbetween light and these radiations is not perfectsince all objects are, to a greater or lessersince all objects are, to a greater or lesser


since all objects are, to a greater or lessersince all objects are, to a greater or lesserdegree, transparent to xdegree, transparent to x--rays and gamma raysrays and gamma raysand since scattering presents greater problemsand since scattering presents greater problemsin radiography than in optics. However, thein radiography than in optics. However, thesame geometric laws of shadow formation holdsame geometric laws of shadow formation holdfor both light and penetrating radiation.for both light and penetrating radiation.

Suppose, as in Figure below, thatSuppose, as in Figure below, thatthere is light from a point L falling onthere is light from a point L falling ona white card C, and that an opaquea white card C, and that an opaqueobject O is interposed between theobject O is interposed between thelight source and the card. A shadowlight source and the card. A shadow


light source and the card. A shadowlight source and the card. A shadowof the object will be formed on theof the object will be formed on thesurface of the card.surface of the card.

This shadow cast by the object will naturallyThis shadow cast by the object will naturallyshow some enlargement because the object isshow some enlargement because the object isnot in contact with the card; the degree ofnot in contact with the card; the degree ofenlargement will vary according to the relativeenlargement will vary according to the relativedistances of the object from the card and fromdistances of the object from the card and fromthe light source. The law governing the size of thethe light source. The law governing the size of theshadow may be stated: The diameter of the objectshadow may be stated: The diameter of the objectis to the diameter of the shadow as the distanceis to the diameter of the shadow as the distance


is to the diameter of the shadow as the distanceis to the diameter of the shadow as the distanceof the light from the object is to the distance ofof the light from the object is to the distance ofthe light from the card. Mathematically, thethe light from the card. Mathematically, thedegree of enlargement may be calculated by usedegree of enlargement may be calculated by useof the following equations:of the following equations:

wherewhere SSOO isis thethe sizesize ofof thethe objectobject;; SSii isis thethe sizesize ofofthethe shadowshadow (or(or thethe radiographicradiographic image)image);; DDOO thethedistancedistance fromfrom sourcesource ofof radiationradiation toto objectobject;; andandDD thethe distancedistance fromfrom thethe sourcesource ofof radiationradiation toto


DDii thethe distancedistance fromfrom thethe sourcesource ofof radiationradiation totothethe recordingrecording surfacesurface (or(or radiographicradiographic film)film)..FigureFigure AA toto FF showsshows thethe effecteffect ofof changingchanging thethe sizesize ofof thethesourcesource andand ofof changingchanging thethe relativerelative positionspositions ofof source,source,object,object, andand cardcard..

From an examination of these drawings, it will beFrom an examination of these drawings, it will beseen that the following conditions must be fulfilled toseen that the following conditions must be fulfilled toproduce the sharpest, truest shadow of the objectproduce the sharpest, truest shadow of the object::

11.. The source of light should be small, that is, as nearly aThe source of light should be small, that is, as nearly apoint as can be obtained. Compare Figurepoint as can be obtained. Compare Figure 1111, A and C., A and C.22.. The source of light should be as far from the object asThe source of light should be as far from the object aspractical. Compare Figurepractical. Compare Figure 1111, B and C., B and C.


practical. Compare Figurepractical. Compare Figure 1111, B and C., B and C.33.. The recording surface should be as close to the objectThe recording surface should be as close to the objectas possible. Compare Figureas possible. Compare Figure 1111,B and D.,B and D.44.. The light rays should be directed perpendicularly to theThe light rays should be directed perpendicularly to therecording surface. See Figurerecording surface. See Figure 1111,A and E.,A and E.55.. The plane of the object and the plane of the recordingThe plane of the object and the plane of the recordingsurface should be parallel. Compare Figuresurface should be parallel. Compare Figure 1111, A and F., A and F.

GeometricGeometric unsharpnessunsharpness refersrefersto the loss of definition that is theto the loss of definition that is theresult of geometric factors of theresult of geometric factors of theradiographic equipment andradiographic equipment andsetup. It occurs because thesetup. It occurs because the


setup. It occurs because thesetup. It occurs because theradiation does not originate from aradiation does not originate from asingle point but rather over ansingle point but rather over anarea.area.

Codes and standards used in industrialCodes and standards used in industrialradiography require that geometricradiography require that geometric unsharpnessunsharpnessbe limited. In general, the allowable amount isbe limited. In general, the allowable amount is11//100100 of the material thickness up to a maximumof the material thickness up to a maximumofof 00..040040 inch. These values refer to the degree ofinch. These values refer to the degree ofpenumbrapenumbra shadow in a radiographic image. Sinceshadow in a radiographic image. Sincethe penumbra is not nearly as well defined in thethe penumbra is not nearly as well defined in the


the penumbra is not nearly as well defined in thethe penumbra is not nearly as well defined in theimage, it is difficult to measure it in a radiograph.image, it is difficult to measure it in a radiograph.Therefore it is typically calculated. The source sizeTherefore it is typically calculated. The source sizemust be obtained from the equipmentmust be obtained from the equipmentmanufacturer or measured. Then themanufacturer or measured. Then theunsharpnessunsharpness can be calculated usingcan be calculated usingmeasurements made of the setup.measurements made of the setup.

UgUg = f* b/a= f* b/aUgUg= Geometric= Geometric unsharpnessunsharpnessff = source focal= source focal--spot size.spot size.aa = distance from x= distance from x--ray source toray source to

front surface offront surface ofmaterial/objectmaterial/objectbb = distance from the front= distance from the front


bb = distance from the front= distance from the frontsurfacesurface ofof the object to thethe object to thedetectordetector

DefinitionDefinitionRadiographicRadiographic definitiondefinition isis thetheabruptnessabruptness ofof changechange inin goinggoing fromfromoneone areaarea ofof aa givengiven radiographicradiographicdensitydensity toto anotheranother.. GeometricGeometricfactorsfactors ofof thethe equipmentequipment andand thetheradiographicradiographic setup,setup, andand filmfilm andandscreenscreen factorsfactors bothboth havehave anan effecteffectonon definitiondefinition.. GeometricGeometric factorsfactorsincludeinclude thethe sizesize ofof thethe areaarea ofof originorigin


includeinclude thethe sizesize ofof thethe areaarea ofof originoriginofof thethe radiation,radiation, thethe sourcesource--toto--detectordetector (film)(film) distance,distance, thethespecimenspecimen--toto--detectordetector (film)(film) distance,distance,movementmovement ofof thethe source,source, specimenspecimenoror detectordetector duringduring exposure,exposure, thethe angleanglebetweenbetween thethe sourcesource andand somesome featurefeatureandand thethe abruptnessabruptness ofof changechange ininspecimenspecimen thicknessthickness oror densitydensity..

It can be seen that the details,It can be seen that the details,particularly the small circle, areparticularly the small circle, aremuch easier to see in the highmuch easier to see in the highdefinition radiograph. It can bedefinition radiograph. It can besaid that the detail portrayed insaid that the detail portrayed in


said that the detail portrayed insaid that the detail portrayed inthe radiograph is equivalent tothe radiograph is equivalent tothe physical change present inthe physical change present inthe step wedgethe step wedge

Geometric FactorsGeometric Factorsto produce the highest level of definition, the focalto produce the highest level of definition, the focal--spot orspot orsource size should be as close to a point source assource size should be as close to a point source aspossible, the sourcepossible, the source--toto--detector distance should be a greatdetector distance should be a greatas practical, and the specimenas practical, and the specimen--toto--detector distance shoulddetector distance shouldbe a small as practical.be a small as practical. This is shown graphically in theThis is shown graphically in theimages below.images below.

DefinitionDefinition


The angle between theThe angle between theradiation and some features willradiation and some features willalso have an effect onalso have an effect ondefinition.definition. If the radiation isIf the radiation isparallel to an edge or linearparallel to an edge or lineardiscontinuity, a sharp distinctdiscontinuity, a sharp distinctboundary will be seen in theboundary will be seen in the



boundary will be seen in theboundary will be seen in theimage.image. However, if the radiationHowever, if the radiationis not parallel with theis not parallel with thediscontinuity, the feature willdiscontinuity, the feature willappear distorted, out of positionappear distorted, out of positionand less defined in the image.and less defined in the image.

Abrupt changes in thickness and/orAbrupt changes in thickness and/ordensitydensity will appear more defined in awill appear more defined in aradiograph than will areas of gradualradiograph than will areas of gradualchange.change.Lastly, anyLastly, any movement of the specimenmovement of the specimen,,source or detector during the exposuresource or detector during the exposurewill reduce definition. Similar towill reduce definition. Similar to


will reduce definition. Similar towill reduce definition. Similar tophotography, any movement will result inphotography, any movement will result inblurring of the image. Vibration fromblurring of the image. Vibration fromnearby equipment may be an issue innearby equipment may be an issue insome inspection situations.some inspection situations.


Film and Screen FactorsFilm and Screen FactorsThe last set of factors concernThe last set of factors concern the filmthe film and theand theuse ofuse of fluorescent screens.fluorescent screens.A fine grain filmA fine grain film is capable of producing anis capable of producing animage with a higher level of definition than is aimage with a higher level of definition than is acoarse grain film.coarse grain film. Wavelength of the radiationWavelength of the radiation


coarse grain film.coarse grain film. Wavelength of the radiationWavelength of the radiationwill influence apparentwill influence apparent graininessgraininess. As the. As thewavelength shortens and penetration increases,wavelength shortens and penetration increases,the apparent graininess of the film will increase.the apparent graininess of the film will increase.Also, increased development of the film willAlso, increased development of the film willincrease the apparent graininess of theincrease the apparent graininess of theradiograph.radiograph.

DefinitionDefinition The use of fluorescent screensThe use of fluorescent screens also results inalso results inlower definition. This occurs for a couple oflower definition. This occurs for a couple ofdifferent reasons. The reason that fluorescentdifferent reasons. The reason that fluorescentscreens are sometimes used is because incidentscreens are sometimes used is because incidentradiation causes them to give off light that helpsradiation causes them to give off light that helpsto expose the film. However, the light theyto expose the film. However, the light theyproduce spreads in all directions, exposing theproduce spreads in all directions, exposing the


produce spreads in all directions, exposing theproduce spreads in all directions, exposing thefilm in adjacent areas, as well as in the areasfilm in adjacent areas, as well as in the areaswhich are in direct contact with the incidentwhich are in direct contact with the incidentradiation. Fluorescent screens also produceradiation. Fluorescent screens also producescreenscreen mottlemottle on radiographs. Screen mottle ison radiographs. Screen mottle isassociated with the statistical variation in theassociated with the statistical variation in thenumbers of photons that interact with the screennumbers of photons that interact with the screenfrom one area to the next.from one area to the next.

Radiographic contrast is theRadiographic contrast is thedegree of density differencedegree of density differencebetween two areas on abetween two areas on aradiograph.radiograph. Contrast makes itContrast makes iteasier to distinguish features ofeasier to distinguish features ofinterest, such as defects, frominterest, such as defects, fromthe surrounding area. Thethe surrounding area. Thecontrast between different partscontrast between different partsof the image is what forms theof the image is what forms the


contrast between different partscontrast between different partsof the image is what forms theof the image is what forms theimage and the greater theimage and the greater thecontrast, the more visiblecontrast, the more visiblefeatures become. Radiographicfeatures become. Radiographiccontrast has two maincontrast has two maincontributors:contributors: subject contrast andsubject contrast anddetector (film) contrast.detector (film) contrast.

The image to the right showsThe image to the right showstwo radiographs of the sametwo radiographs of the samestep wedge. The upperstep wedge. The upperradiograph has a high level ofradiograph has a high level ofcontrast and the lowercontrast and the lower


contrast and the lowercontrast and the lowerradiograph has a lower level ofradiograph has a lower level ofcontrastcontrast

Subject ContrastSubject ContrastSubject contrast is the ratio ofSubject contrast is the ratio ofradiation intensities transmittedradiation intensities transmittedthrough different areas of thethrough different areas of thecomponent being evaluated.component being evaluated. It isIt isdependant on the absorptiondependant on the absorption


dependant on the absorptiondependant on the absorptiondifferences in the component, thedifferences in the component, thewavelength of the primary radiation,wavelength of the primary radiation,and intensity and distribution ofand intensity and distribution ofsecondary radiation due to scattering.secondary radiation due to scattering.

It should be no surprise that absorptionIt should be no surprise that absorptiondifferences within the subject will affect thedifferences within the subject will affect thelevel of contrast in a radiograph.level of contrast in a radiograph. The largerThe largerthe difference in thickness or densitythe difference in thickness or densitybetween two areas of the subject, the largerbetween two areas of the subject, the largerthe difference in radiographic de

cic presentation rt rev.0

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