X射线荧光分析指导课件.pdf

Introduction to XRFLearnXRF.comX X X X 射射射射 线线线线 荧荧荧荧 光光光光 分分分分 析析析析导导导导论论论论Introduction to XRFLearnXRF.com电子波谱1Hz-1kHz1kHz-1014Hz1014Hz-1015Hz1015Hz-1021Hz超低频率电磁波无线电波微波超低频率电磁波无线电波微波红外线红外线可见光可见光伽马射线伽马射线紫外线紫外线Low energyHigh energyX射线射线Introduction to XRFLearnXRF.comTheory?入射入射X射线轰击原子的内层电子，如果能量大于它的吸收边，该内层电子被驱逐出整个原子（整个原子处于高能态，即激发态）。射线轰击原子的内层电子，如果能量大于它的吸收边，该内层电子被驱逐出整个原子（整个原子处于高能态，即激发态）。?较高能级的电子跃迁、补充空穴，整个原子处于低能态，即基态。较高能级的电子跃迁、补充空穴，整个原子处于低能态，即基态。?由高能态转化为低能态，释放能量。由高能态转化为低能态，释放能量。E=Eh-El.能量将以能量将以X射线的释放，产生射线的释放，产生X射线荧光射线荧光。Introduction to XRFLearnXRF.comThe Hardware SourcesSources OpticsOptics Filters&TargetsFilters&Targets DetectorsDetectorsIntroduction to XRFLearnXRF.comSourcesEnd Window X-Ray TubesSide Window X-Ray TubesRadioisotopesOther SourcesScanning Electron MicroscopesSynchrotronsPositron and other particle beams Introduction to XRFLearnXRF.comEnd Window X-Ray Tube X-ray Tubes Voltage determines which elements can be excited.More power=lower detection limits Anode selection determines optimal source excitation(application specific).Introduction to XRFLearnXRF.comSide Window X-Ray TubeBe WindowSilicone InsulationGlass EnvelopeFilamentElectron beamTarget(Ti,Ag,Rh,etc.)Copper AnodeHV LeadIntroduction to XRFLearnXRF.comRadioisotopesIsotopeFe-55Cm-244Cd-109Am-241Co-57Energy(keV)5.914.3,18.322,8859.5122Elements(K-lines)Al V Ti-BrFe-MoRu-ErBa-UElements(L-lines)Br-II-PbYb-PuNonenone?While isotopes have fallen out of favor they are still useful for many gauging applications.Introduction to XRFLearnXRF.comOther SourcesSeveral other radiation sources are capable of exciting material to produce x-ray fluorescence suitable for material analysis.?Scanning Electron Microscopes(SEM)Electron beams excite the sample and produce x-rays.Many SEMs are equipped with an EDX detector for performing elemental analysis?Synchotrons-These bright light sources are suitable for research and very sophisticated XRF analysis.?Positrons and other Particle Beams All high energy particles beams ionize materials such that they give off x-rays.PIXE is the most common particle beam technique after SEM.Introduction to XRFLearnXRF.comSource ModifiersSeveral Devices are used to modify the shape or intensity of the source spectrum or the beam shape?Source Filters?Secondary Targets?Polarizing Targets?Collimators?Focusing OpticsIntroduction to XRFLearnXRF.comSource FiltersFilters perform one of two functionsBackground ReductionImproved FluorescenceDetectorDetectorX X-Ray Ray SourceSourceSource FilterSource FilterIntroduction to XRFLearnXRF.comFilter Transmission Curve%TRANSMITTEDENERGYLow energy x-rays are absorbedAbsorptionEdgeX-rays above the absorption edge energy are absorbedVery high energy x-rays are transmitted Ti CrTitanium Filter transmission curve The transmission curve shows the parts of the source spectrum are transmitted and those that are absorbedIntroduction to XRFLearnXRF.comFilter Fluorescence MethodENERGY(keV)Target peakWith Zn Source filterFeRegionContinuum RadiationThe filter fluorescence method decreases the background and improves the fluorescence yield without requiring huge amounts of extra power.Introduction to XRFLearnXRF.comFilter Absorption MethodENERGY(keV)Target peakWith Ti Source filterFeRegionContinuum RadiationThe filter absorption Method decreases the background while maintaining similar excitation efficiency.Introduction to XRFLearnXRF.comSecondary TargetsImproved Fluorescence and lower backgroundThe characteristic fluorescence of the custom line source is used to excite the sample,with the lowest possible background intensity.It requires almost 100 x the flux of filter methods but gives superior results.Introduction to XRFLearnXRF.comSecondary TargetsSampleX-Ray TubeDetectorSecondary TargetA.The x-ray tube excites the secondary targetB.The Secondary target fluoresces and excites the sampleC.The detector detects x-rays from the sampleIntroduction to XRFLearnXRF.comSecondary Target MethodENERGY(keV)Tube Target peakWith Zn Secondary TargetFeRegionContinuum RadiationSecondary Targets produce a more monochromatic source peak with lower background than with filtersIntroduction to XRFLearnXRF.comSecondary Target Vs FilterComparison of optimized direct-filtered excitation with secondary target excitation for minor elements in Ni-200Introduction to XRFLearnXRF.comPolarizing Target Theorya)X-ray are partially polarized whenever they scatter off a surfaceb)If the sample and polarizer are oriented perpendicular to each other and the x-ray tube is not perpendicular to the target,x-rays from the tube will not reach the detector.c)There are three type of Polarization Targets:Barkla Scattering Targets-They scatter all source energies to reduce background at the detector.Secondary Targets-They fluoresce while scattering the source x-rays and perform similarly to other secondary targets.Diffractive Targets-They are designed to scatter specific energies more efficiently in order to produce a stronger peak at that energy.Introduction to XRFLearnXRF.comCollimatorsCollimators are usually circular or a slit and restrict the size or shape of the source beam for exciting small areas in either EDXRF or uXRF instruments.They may rely on internal Bragg reflection for improved efficiency.SampleTubeCollimator sizes range from 12 microns to several mmIntroduction to XRFLearnXRF.comFocusing OpticsBecause simple collimation blocks unwanted x-rays it is a highly inefficient method.Focusing optics like polycapillary devices and other Kumakhov lens devices were developed so that the beam could be redirected and focused on a small spot.Less than 75 um spot sizes are regularly achieved.SourceDetectorBragg reflection inside a CapillaryIntroduction to XRFLearnXRF.comDetectors Si(Li)PIN Diode Silicon Drift Detectors Proportional Counters Scintillation DetectorsIntroduction to XRFLearnXRF.comDetector Principles2Enen=num ber of electron-hole pairs producedE=X-ray photon energye=3.8ev for Si at LN tem perw here:atures=?A detector is composed of a non-conducting or semi-conducting material between two charged electrodes.?X-ray radiation ionizes the detector material causing it to becomeconductive,momentarily.?The newly freed electrons are accelerated toward the detector anode to produce an output pulse.?In ionized semiconductor produces electron-hole pairs,the number of pairs produced is proportional to the X-ray photon energyIntroduction to XRFLearnXRF.comSi(Li)DetectorWindowSi(Li)crystalDewarfilled withLN2Super-Cooled CryostatCooling:LN2or PeltierWindow:Beryllium or PolymerCounts Rates:3,000 50,000 cps Resolution:120-170 eV at Mn K-alphaFETPre-AmplifierIntroduction to XRFLearnXRF.comSi(Li)Cross Section Introduction to XRFLearnXRF.comPIN Diode DetectorCooling:Thermoelectrically cooled(Peltier)Window:BerylliumCount Rates:3,000 20,000 cpsResolution:170-240 eV at Mn k-alphaIntroduction to XRFLearnXRF.comSilicon Drift Detector-SDDPackaging:Similar to PIN DetectorCooling:PeltierCount Rates;10,000 300,000 cpsResolution:140-180 eV at Mn K-alphaIntroduction to XRFLearnXRF.comProportional CounterAnode FilamentFill Gases:Neon,Argon,Xenon,KryptonPressure:0.5-2 ATMWindows:Be or PolymerSealed or Gas Flow VersionsCount Rates EDX:10,000-40,000 cps WDX:1,000,000+Resolution:500-1000+eVWindowIntroduction to XRFLearnXRF.comScintillation DetectorPMT(Photo-multiplier tube)Sodium Iodide DiskElectronicsConnectorWindow:Be or AlCount Rates:10,000 to 1,000,000+cpsResolution:1000 eVIntroduction to XRFLearnXRF.comSpectral Comparison-AuSi(Li)Detector10 vs.14 KaratSi PIN Diode Detector10 vs.14 KaratIntroduction to XRFLearnXRF.comPolymer Detector WindowsPolymer Detector Windows Optional thin polymer windows compared to a standard beryllium windows Affords 10 x improvement in the MDL for sodium(Na)Introduction to XRFLearnXRF.comDetector FiltersFilters are positioned between the sample and detector in some EDXRF and NDXRF systems to filter out unwanted x-ray peaks.SampleSampleDetectorDetectorX X-Ray Ray SourceSourceDetector FilterDetector FilterIntroduction to XRFLearnXRF.comDetector Filter Transmission%TRANSMITTEDENERGYLow energy x-rays are absorbedEOI is transmittedAbsorptionEdgeX-rays above the absorption edge energy are absorbedVery high energy x-rays are transmitted S ClA niobium filter absorbs Cl and other higher energy source x-rays while letting S x-rays pass.A detector filter can significantly improve detection limits.Niobium Filter Transmission and AbsorptionIntroduction to XRFLearnXRF.comFilter Vs.No FilterUnfiltered Tube target,Cl,and ArInterference PeakDetector filters can dramatically improve the element of interest intensity,while decreasing the background,but requires 4-10 times more source flux.They are best used with large area detectors that normally do not require much power.Introduction to XRFLearnXRF.comRoss Vs.Hull Filters?The previous slide was an example of the Hull or simple filter method.?The Ross method illustrated here for Clanalysis uses intensities through two filters,one transmitting,one absorbing,and the difference is correlated to concentration.This is an NDXRF method since detector resolution is not important.Introduction to XRFLearnXRF.comWavelength Dispersive XRFWavelength Dispersive XRF relies on a diffractive device such as crystal or multilayer to isolate a peak,since the diffracted wavelength is much more intense than other wavelengths that scatter of the device.SampleSampleDetectorX-Ray SourceDiffraction DeviceCollimatorsIntroduction to XRFLearnXRF.comDiffractionThe two most common diffraction devices used in WDX instruments are the crystal and multilayer.Both work according to the following formula.n =2d sin n=integerd=crystal lattice or multilayer spacing =The incident angle=wavelengthAtomsIntroduction to XRFLearnXRF.comMultilayersWhile the crystal spacing is based on the natural atomic spacing at a given orientation the multilayer uses a series of thin film layers of dissimilar elements to do the same thing.Modern multilayersare more efficient than crystals and can be optimized for specific elements.Often used for low Z elements.Introduction to XRFLearnXRF.comSoller CollimatorsSoller and similar types of collimators are used to prevent beam divergence.The are used in WDXRF to restrict the angles that are allowed to strike the diffraction device,thus improving the effective resolution.SampleCrystalIntroduction to XRFLearnXRF.comCooling and Temperature ControlThe diffraction technique is relatively inefficient and WDX detectors can operate at much higher count rates,so WDX Instruments are typically operated at much higher power than direct excitation EDXRF systems.Diffraction devices are also temperature sensitive.Many WDXRF Instruments use:X-Ray Tube Coolers,and Thermostatically controlled instrument coolersIntroduction to XRFLearnXRF.comChamber AtmosphereSample and hardware chambers of any XRF instrument may be filled with air,but because air absorbs low energy x-rays from elements particularly below Ca,Z=20,and Argon sometimes interferes with measurements purges are often used.The two most common purge methods are:Vacuum -For use with solids or pressed pelletsHelium-For use with liquids or powdered materialsIntroduction to XRFLearnXRF.comChangers and SpinnersOther commonly available sample handling features are sample changers or spinners.?Automatic sample changers are usually of the circular or XYZ stage variety and may have hold 6 to 100+samples?Sample Spinners are used to average out surface features and particle size affects possibly over a larger total surface area.Introduction to XRFLearnXRF.comTypical PIN Detector InstrumentTypical PIN Detector InstrumentThis configuration is most commonly used in higher end benchtop EDXRF Instruments.Introduction to XRFLearnXRF.comTypical Si(Li)Detector Instrument Typical Si(Li)Detector Instrument This has been historically the most common laboratory grade EDXRF configuration.Introduction to XRFLearnXRF.comEnergy Dispersive ElectronicsFluorescence generates a current in the detector.In a detectorintended for energy dispersive XRF,the height of the pulse produced is proportional to the energy of the respective incoming X-ray.DETECTORSignal to ElectronicsElement AElement CElement BElement DIntroduction to XRFLearnXRF.comMulti-Channel AnalyserDetector current pulses are translated into counts(counts per second,“CPS”).Pulses are segregated into channels according to energy via the MCA(Multi-Channel Analyser).Signal from DetectorChannels,EnergyIntensity(#of CPS per Channel)Introduction to XRFLearnXRF.comWDXRF Pulse Processing?The WDX method uses the diffraction device and collimators to obtain good resolution,so The detector does not need to be capable of energy discrimination.This simplifies the pulse processing.?It also means that spectral processing is simplified since intensity subtraction is fundamentally an exercise in background subtraction.Note:Some energy discrimination is useful since it allows for rejection of low energy noise and pulses from unwanted higher energy x-rays.Introduction to XRFLearnXRF.comEvaluating Spectra K&L Spectral Peaks Rayleigh Scatter Peaks Compton Scatter Peaks Escape Peaks Sum Peaks BremstrahlungIn addition to elemental peaks,other peaks appear in the spectra:Introduction to XRFLearnXRF.comK&L Spectral Lines?K-alpha lines:L shell e-transition to fill vacancy in K shell.Most frequent transition,hence most intense peak.?K-beta lines:M shell e-transitions to fill vacancy in K shell.L ShellK Shell?L-alpha lines:M shell e-transition to fill vacancy in L shell.?L-beta lines:N shell e-transition to fill vacancy in L shell.K alphaK betaM ShellL alphaN ShellL betaIntroduction to XRFLearnXRF.comK&L Spectral PeaksRh X-ray TubeL-linesK-LinesIntroduction to XRFLearnXRF.comScatterSome of the source X-rays strike the sample and are scattered back at the detector.Sometimes called“backscatter”SampleSourceDetectorIntroduction to XRFLearnXRF.comRayleigh ScatterX-rays from the X-ray tube or target strike atom without promoting fluorescence.Energy is not lost in collision.(EI=EO)They appear as a source peak in spectra.AKA-“Elastic”ScatterEIEORh X-ray TubeIntroduction to XRFLearnXRF.comCompton ScatterX-rays from the X-ray tube or target strike atom without promoting fluorescence.Energy is lost in collision.(EI EO)Compton scatter appears as a source peak in spectra,slightly less in energy than RayleighScatter.AKA-“Inelastic”ScatterEIEORh X-ray TubeIntroduction to XRFLearnXRF.comSum Peaks?2 photons strike the detector at the same time.?The fluorescence is captured by the detector,recognized as 1 photon twice its normal energy.?A peak appears in spectra,at:2 X(Element keV).Introduction to XRFLearnXRF.comEscape PeaksX-rays strike the sample and promote elemental fluorescence.Some Si fluorescence at the surface of the detector escapes,and is not collected by the detector.The result is a peak that appears in spectrum,at:Element keV-Si keV(1.74 keV).Rh X-ray Tube1.74 keVIntroduction to XRFLearnXRF.comBrehmst