1

XX--ray Generationray Generation

Chapter 1Chapter 1

X-RaysX-Rays

•• XX--raysrays areare electromagneticelectromagnetic radiationradiation ofofwavelengthwavelength aboutabout 11 ÅÅ ((1010--1010 m),m), (about(about thethesamesame sizesize asas anan atom)atom)..

•• TheyThey occuroccur inin thatthat portionportion ofof thetheelectromagneticelectromagnetic spectrumspectrum betweenbetween γγ--raysrays andandthethe ultravioletultraviolet..

•• XX--raysrays werewere discovereddiscovered inin 18951895 byby RontgenRontgen

andand areare usedused forfor diffractiondiffraction purposespurposes toto studystudythethe crystalcrystal structurestructure atat thethe atomicatomic levellevel..

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Electromagnetic SpectrumElectromagnetic Spectrum

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X-rays are electromagnetic radiation of exactly the same nature as light but very much shorter wavelength.

X-ray wavelength : 0.5-2.5Å , visible light wavelength: ~ 6000 Å

X-rays are electromagnetic radiation of exactly the same nature as light but very much shorter wavelength.

X-ray wavelength : 0.5-2.5Å , visible light wavelength: ~ 6000 Å

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Production of X-RaysProduction of X-Rays

•• XX--raysrays areare producedproduced whenwhen rapidlyrapidly movingmoving electronselectrons strikestrike aasolidsolid targettarget andand theirtheir kinetickinetic energyenergy isis convertedconverted intointo radiationradiation..TheThe wavelengthwavelength ofof thethe emittedemitted radiationradiation dependsdepends onon thethe energyenergyofof thethe electronselectrons

•• ToTo generategenerate XX--rays,rays, threethree basicbasic requirementsrequirements mustmust bebe availableavailable::

� Source of electrons (cathode)

� Means of directing and accelerating the electrons to highvelocities (high voltage)

� Target to receive the impact of the electrons (Anode)

•• XX--raysrays tubestubes areare generallygenerally usedused toto produceproduce XX--raysrays

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X-Ray TubeX-Ray Tube

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Production of X-RaysProduction of X-Rays

•• AA veryvery highhigh voltagevoltage isis placedplaced acrossacross thethe electrodeselectrodes (anode(anode andandcathode)cathode) inin thethe twotwo endsends ofof thethe tubetube..

•• BothBoth electrodeselectrodes shouldshould bebe containedcontained withinwithin anan insulating,insulating,vacuumvacuum tighttight tubetube mademade ofof glass,glass, aa metalmetal oror aa combinationcombination ofofmaterialsmaterials..

•• TheThe currentcurrent flowsflows betweenbetween thethe twotwo electrodeselectrodes andand thethe electronselectronsareare drawndrawn towardtoward thethe anodeanode target,target, andand becausebecause ofof thethe highhighvoltagevoltage thethe electronselectrons strikestrike thethe targettarget withwith veryvery highhigh velocityvelocity..ThisThis causescauses thethe emissionemission ofof XX--raysrays..

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Production of X-RaysProduction of X-Rays

•• LessLess thanthan 11%% ofof thethe kinetickinetic energyenergy ofof thethe electronselectrons whichwhich strikestrikethethe targettarget isis convertedconverted intointo XX--raysrays.. TheThe restrest isis convertedconverted intointoheatheat..

•• IfIf thisthis heatheat isis notnot dissipateddissipated inin somesome way,way, thethe targettarget materialmaterialcouldcould meltmelt oror crackscracks andand thenthen becomesbecomes uselessuseless..

•• ForFor thisthis purpose,purpose, thethe targettarget metalmetal isis usuallyusually embeddedembedded inin aacoppercopper blockblock whichwhich isis waterwater--cooledcooled soso thatthat heatheat generatedgenerated cancanbebe dissipateddissipated..

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5

Production of X-RaysProduction of X-Rays

•• TheThe XX--rayray tubetube mustmust alsoalso possesspossess goodgood structuralstructural strengthstrength atathighhigh temperaturestemperatures inin orderorder toto withstandwithstand thethe combinedcombined effectseffects ofofthethe extremeextreme heatheat generatedgenerated atat thethe anodeanode andand thethe forcesforces exertedexertedbyby thethe atmosphericatmospheric pressurepressure onon thethe highhigh vacuumvacuum interiorinterior..

•• TheThe envelopeenvelope mustmust containcontain aa windowwindow oppositeopposite thethe anodeanode targettargettoto permitpermit thethe XX--rayray beambeam toto leaveleave thethe tubetube..

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Production of X-RaysProduction of X-Rays

•• TheThe windowwindow isis generallygenerally mademade ofof aa lowlow atomicatomic numbernumber elementelementtoto minimiseminimise XX--rayray absorptionabsorption.. UsuallyUsually aa 33 -- 44 mmmm thicknessthickness ofofberylliumberyllium (Be)(Be) isis usedused asas thethe windowwindow..

•• 33.. INTENSITYINTENSITY vsvs WAVELENGTHWAVELENGTH OFOF XX--RAYSRAYS

•• IfIf oneone plotsplots aa graphgraph ofof thethe wavelengthwavelength ofof thethe XX--raysrays emittedemittedagainstagainst theirtheir intensityintensity forfor varyingvarying appliedapplied acceleratingaccelerating voltagesvoltagesoneone obtainsobtains resultsresults suchsuch thosethose shownshown inin FigureFigure 33 forfor aa targettargetmaterialmaterial mademade ofof molybdenummolybdenum (Mo)(Mo)

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Production of X-RaysProduction of X-Rays

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Figure 3: X-ray spectrum of molybdenum at different potentials

•• TheThe curvescurves whichwhich areare producedproduced withwith acceleratingaccelerating voltagesvoltagesofof ≤≤ 2020 kVkV areare smoothsmooth whereaswhereas thosethose producedproduced byby aavoltagevoltage ofof 2525 kVkV areare notnot

•• TheThe radiationradiation generatedgenerated byby aa particularparticular XX--rayray tubetube isis notnotofof oneone singlesingle wavelengthwavelength butbut coverscovers aa rangerange ofofwavelengthswavelengths..

•• TwoTwo typestypes ofof interactionsinteractions occuroccur andand thethe XX--raysrays areareproducedproduced byby twotwo typestypes ofof processesprocesses::

�� CONTINUOUSCONTINUOUS && CHARACTERISTICCHARACTERISTIC XX-- raysrays

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Production of X-Rays

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Continuous X-ray RadiationContinuous X-ray Radiation

•• Continuous radiation is represented by the smooth curves Continuous radiation is represented by the smooth curves

in Figure 3. It is also called white radiation because it is in Figure 3. It is also called white radiation because it is

made up of rays of many wavelengths (like white light )made up of rays of many wavelengths (like white light )

•• ContinuousContinuous radiationradiation isis producedproduced atat lowlow voltagesvoltages.. TheirTheir

intensityintensity isis zerozero upup toto aa certaincertain wavelength,wavelength, calledcalled thethe

““shortshort wavelengthwavelength limitlimit”” ((λλSWLSWL))

•• WhatWhat isis interestinginteresting (in(in FigureFigure 33)) isis thethe veryvery sharpsharp cutcut--offoff

shortshort wavelengthwavelength ((λλSWLSWL)).. ThisThis correspondscorresponds toto thethe maximummaximum

efficiencyefficiency ofof conversionconversion ofof thethe kinetickinetic energyenergy toto

electromagneticelectromagnetic radiationradiation..

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Continuous X-ray RadiationContinuous X-ray Radiation

•• ContinuousContinuous radiationradiation isis producedproduced

becausebecause anyany acceleratedaccelerated electronelectron

emitsemits energyenergy (Figure(Figure 44))

•• SomeSome electrons,electrons, whenwhen theythey hithit thethe

targettarget areare stoppedstopped inin oneone impactimpact andand

thereforetherefore loselose allall theirtheir energyenergy atat onceonce

((λλSWLSWL))..

•• OtherOther electronselectrons areare forcedforced toto changechange

directiondirection aa numbernumber ofof timestimes byby thethe

atomsatoms inin thethe targettarget.. TheThe radiationradiation

emittedemitted hashas lowlow energyenergy..

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Figure 4: illustration of the origin of continuous

radiation.

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Characteristic X-ray RadiationCharacteristic X-ray Radiation

•• WhenWhen thethe voltagevoltage isis increasedincreased aboveabove aa certaincertain criticalcritical valuevalue(characteristic(characteristic ofof thethe targettarget metal),metal), sharpsharp intensityintensity maximamaximaappearappear atat certaincertain wavelengthwavelength

•• TheseThese maximamaxima areare veryvery narrownarrow andand theirtheir wavelengthswavelengths arearecharacteristiccharacteristic ofof thethe targettarget metalmetal usedused.. TheyThey areare calledcalledcharacteristiccharacteristic lineslines oror radiationradiation..

•• QQ:: HowHow isis characteristiccharacteristic radiationradiation produced?produced?

•• AnswerAnswer:: These characteristic lines are caused by electrons being knockedout of the K shell of an atom and then the electrons from the L shell move intothe vacancies left in this K shell (Figure 5).

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Characteristic X-ray RadiationCharacteristic X-ray Radiation

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Characteristic RadiationCharacteristic Radiation

Continuous Continuous SpectrumSpectrum

Figure 5: Process of the emission of characteristic X-rays.

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Characteristic X-ray RadiationCharacteristic X-ray Radiation

•• TheThe energyenergy emittedemitted inin thisthis processprocesscorrespondscorresponds toto thethe KKαα (when(when thetheelectronelectron comescomes fromfrom thethe LL shell)shell)andand KKββ (when(when thethe electronelectron comescomesfromfrom thethe MM shell)shell) lineslines..

•• TheThe aboveabove theorytheory explainexplain whywhy aacriticalcritical voltagevoltage existsexists forforcharacteristiccharacteristic radiationradiation..

•• KK radiationradiation cannotcannot bebe excitedexcitedunlessunless thethe acceleratingaccelerating voltagevoltage iisshighhigh enoughenough toto givegive thethebombardingbombarding electronselectrons enoughenoughenergyenergy toto knockknock anan electronelectron outout ofofthethe KK shellshell ofof thethe targettarget atomatom

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Characteristic X-ray RadiationCharacteristic X-ray Radiation

•• If the bombarding electrons knock an electron from the L shell, If the bombarding electrons knock an electron from the L shell, L radiation is emitted. L radiation is emitted.

•• However,However, onlyonly thethe KK lineslines areare usefuluseful forfor diffractiondiffraction experimentsexperimentswhichwhich areare carriedcarried outout inin crystallographiccrystallographic workwork becausebecause longerlongerwavelengthswavelengths areare absorbedabsorbed tootoo easilyeasily..

•• ThereThere areare severalseveral lineslines inin thethe KK setset butbut onlyonly thethe 33 strongeststrongest areareusedused inin normalnormal xx--rayray diffractiondiffraction experimentsexperiments.. TheyThey areare KKαα11,, KKαα22

andand KKββ

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Characteristic X-ray RadiationCharacteristic X-ray Radiation

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Resolved Kαααα doublets is shown on an expanded wavelength scale at right

Spectrum of Mo at 35 kV.

Characteristic X-ray RadiationCharacteristic X-ray Radiation

•• ForFor MoMo theirtheir wavelengthswavelengths areare == 00..709709,, 00..714714 && 00..632632 ÅÅrespectivelyrespectively..

•• KKαα11,, KKαα22 havehave veryvery closeclose wavelengthswavelengths whichwhich theythey areare impossibleimpossibletoto resolveresolve.. Thus,Thus, theythey areare calledcalled KKαα

•• IfIf severalseveral metalsmetals areare presentpresent inin thethe target,target, eacheach willwill emitemit itsitscharacteristiccharacteristic radiationradiation independentlyindependently..

•• ThisThis propertyproperty cancan bebe usedused toto determinedetermine qualitativelyqualitatively whichwhichelementselements areare presentpresent inin anan alloyalloy byby makingmaking itit thethe targettarget inin anan xx--rayray tubetube andand scanningscanning allall wavelengthswavelengths emittedemitted byby thethe targettarget..

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0

WN

WM

WL

WK

N

K����

N

M

K����

L����

M����

Neutral atom

Valence electron removed

N state (N electron removed)

M state (M electron removed)

L state (L electron removed)

K state (K electron removed)

En

erg

y o

f Ato

m

K e

xcita

tion

L e

xcita

tion

Atomic energy levels. Excitation and emission indicated by arrows (Cullity & Stock)

Characteristic X-ray RadiationCharacteristic X-ray Radiation

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Absorption of X-ray RadiationAbsorption of X-ray Radiation

• X-rays are partly transmitted and partly absorbed by solids.

• The absorption phenomenon can be described by a very simpleequation:

• Where:� µ is a linear absorption coefficient and x is the path length (thickness)

through which the x-rays pass.

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xx eII

µ−= 0

Absorption of X-ray RadiationAbsorption of X-ray Radiation

� I0: intensity of the incident beam

� Ix: intensity of the transmitted beam after passing through thicknessx of the substance involved.

�� TheThe linearlinear absorptionabsorption coefficientcoefficient isis proportionalproportional toto thethedensity,density, ρρ,, soso thethe quantityquantity µµ// ρρ isis aa constantconstant forfor thethe substancesubstance..

�� µµ// ρρ isis calledcalled thethe massmass absorptionabsorption coefficientcoefficient.. TheThe aboveaboveequationequation thenthen becomesbecomes::

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x

x eIIρ

ρµ

−

= 0

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Absorption of X-ray RadiationAbsorption of X-ray Radiation

•• TheThe massmass absorptionabsorption coefficientcoefficient ofof aa substancesubstance whichwhich containscontainsmoremore thanthan oneone elementelement isis givengiven byby::

•• WhereWhere:: ww11,, ww22 ……areare thethe weightweight fractionsfractions ofof elementelement 11,, 22 …… ininthethe substancesubstance..

•• TheThe absorptionabsorption coefficientcoefficient increasesincreases asas thethe atomicatomic numbernumber (Z)(Z) ofofthethe elementelement concernedconcerned increasesincreases..

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........2

21

1 +

+

=

ρµ

ρµ

ρµ

ww

Absorption of X-ray RadiationAbsorption of X-ray Radiation

•• TheThe absorptionabsorption coefficientcoefficient alsoalso variesvaries withwith thethe wavelengthwavelength ofofthethe incidentincident xx--raysrays..

•• FigureFigure 66 showsshows anan exampleexample forfor NiNi.. TheThe sharpsharp discontinuitydiscontinuity isiscalledcalled anan absorptionabsorption edgeedge λλKK..

•• TheThe totaltotal absorptionabsorption givengiven byby µµ// ρρ arisesarises fromfrom twotwo processesprocesses::

�� ScatteringScattering

�� TrueTrue absorptionabsorption

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Absorption of X-ray RadiationAbsorption of X-ray Radiation

Scattering

•• ScatteringScattering occursoccurs inin allall directionsdirections andand absorptionabsorption isis saidsaid toto occuroccurbecausebecause thethe scatteredscattered raysrays dodo notnot appearappear inin thethe transmittedtransmitted beambeam..

•• ThisThis typetype ofof absorptionabsorption accountsaccounts forfor onlyonly aa smallsmall fractionfraction ofof thethetotaltotal absorptionabsorption..

True Absorption

•• ThisThis isis causedcaused byby electronicelectronic transitiontransition withinwithin thethe atomatom..

•• WeWe havehave seenseen howhow characteristiccharacteristic radiationradiation isis producedproduced whenwhen highhighenergyenergy electronelectron knockknock aa KK electronelectron outout ofof anan atomatom..

•• AnAn incidentincident beambeam ofof XX--raysrays cancan dodo thethe samesame ifif itit hashas sufficientsufficientenergyenergy..

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X-Ray Fluorescence (XRF)X-Ray Fluorescence (XRF)

� In this case, the ejected electron is called a photoelectron and theemitted characteristic radiation is called fluorescent radiation.

• In Figure 6, the absorption decreases as the wavelength decreasesbecause the X-rays pass through the solid substance more easily ifthey have higher energy.

• When the wavelength is reduced just below the critical value λK,the absorption increases very significantly.

• A large fraction of the incident x-ray beam is absorbed andconverted into K-fluorescent radiation.

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X-Ray Fluorescence (XRF)X-Ray Fluorescence (XRF)

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Figure 6: Variation of mass absorption coefficient with wavelength of X-ray radiation

Filtering of X-RaysFiltering of X-Rays

•• An XAn X--ray beam which contains the Kray beam which contains the Kαα line also contains line also contains the weaker Kthe weaker Kββ line and the continuous radiation. line and the continuous radiation.

•• XX--rayray diffractiondiffraction experimentsexperiments requirerequire thatthat thethe beambeam isis asasnearnear toto monochromaticmonochromatic (with(with aa singlesingle wavelength)wavelength) asaspossiblepossible..

•• ThisThis cancan bebe obtainedobtained byby usingusing aa FILTERFILTER

•• ThisThis isis achievedachieved byby removingremoving thethe KKββ lineline,, thethe ratioratio ofofcharacteristiccharacteristic toto continuouscontinuous intensityintensity willwill bebe suchsuch thatthat thethecontinuouscontinuous spectrumspectrum cancan bebe ignoredignored (Figure(Figure 77))

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Filtering of X-RaysFiltering of X-Rays

•• TheThe KKββ lineline cancan bebe removedremoved fromfrom thethe xx--rayray radiationradiation byby

placingplacing immediatelyimmediately outsideoutside thethe windowwindow aa thinthin foilfoil ofof anan

elementelement withwith anan absorptionabsorption edgeedge ofof wavelengthwavelength thatthat lieslies

betweenbetween thethe KKαα andand KKββ wavelengthswavelengths ofof thethe targettarget metalmetal..

•• ForFor metalsmetals withwith anan atomicatomic numbernumber Z,Z, thethe filterfilter materialmaterial willwill

havehave anan atomicatomic numbernumber ZZ -- 11..

•• WithWith anan increaseincrease inin filterfilter thickness,thickness, thethe ratioratio ofof KKαα // KKββ inin thethe

transmittedtransmitted beambeam decreasesdecreases..

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Filtering of X-RaysFiltering of X-Rays

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Figure 7: Comparison of the spectra of Cu radiation. (a) Before and (b) after

passage through a Ni filter. The dashed line is the mass absorption

coefficient of Ni.

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Filtering of X-RaysFiltering of X-Rays

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