X射线荧光分析导论课件.ppt

1、培训课件X射线荧光分析导论Theory入射入射X射线轰击原子的内层电子，如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（整个原子处于高被驱逐出整个原子（整个原子处于高能态，即激发态）。能态，即激发态）。较高能级的电子跃迁、补充空穴，整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。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射线荧射线荧光光。The HardwareSourcesEnd Window X-Ray

2、 TubesSide Window X-Ray TubesRadioisotopesOther SourcesScanning Electron MicroscopesSynchrotronsPositron and other particle beams End 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(

3、application specific).Side Window X-Ray TubeBe WindowSilicone InsulationGlass EnvelopeFilamentElectron beamTarget(Ti,Ag,Rh,etc.)Copper AnodeHV LeadRadioisotopesIsotopeFe-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-P

4、uNonenone While isotopes have fallen out of favor they are still useful for many gauging applications.Changers and SpinnersSecondary Target MethodImproved FluorescenceEnergy is not lost in collision.Because simple collimation blocks unwanted x-rays it is a highly inefficient method.They are best use

5、d with large area detectors that normally do not require much power.The two most common purge methods are:X-ray TubesSample 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 interf

6、eres with measurements purges are often used.More power=lower detection limitsX-ray radiation ionizes the detector material causing it to become conductive,momentarily.Compton scatter appears as a source peak in spectra,slightly less in energy than Rayleigh Scatter.inside a CapillaryWith Zn Source f

7、ilterPre-AmplifierBrehmstrahlungAKA-“Inelastic”ScatterWindows:Be or PolymerBragg reflectionOther 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

8、 produce x-rays.Many SEMs are equipped with an EDX detector for performing elemental analysisSynchotrons-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 of

9、f x-rays.PIXE is the most common particle beam technique after SEM.Source 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 OpticsSource FiltersFilters perform one of tw

10、o functionsBackground ReductionImproved FluorescenceTarget(Ti,Ag,Soller and similar types of collimators are used to prevent beam divergence.Standards(such as certified reference materials)are required for Quantitative Analysis.Rh X-ray TubePulses are segregated into channels according to energy via

11、 the MCA(Multi-Channel Analyser).Source FiltersDetector FilterK&L Spectral PeaksK-beta lines:M shell e-This is an NDXRF method since detector resolution is not important.Rh X-ray TubeOther SourcesMore power=lower detection limitsResolution:120-170 eV at Mn K-alphaENERGY(keV)入射X射线轰击原子的内层电子，如果能量大于它的吸收

12、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）。The 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.Resolution:500-1000+eVAbsorption-Enhancement AffectsThe Har

13、dwarex-rays are transmittedENERGY(keV)Filter 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 t

14、he source spectrum are transmitted and those that are absorbedFilter 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.Filt

15、er Absorption MethodENERGY(keV)Target peakWith Ti Source filterFeRegionContinuum RadiationThe filter absorption Method decreases the background while maintaining similar excitation efficiency.Secondary TargetsImproved Fluorescence and lower background The characteristic fluorescence of the custom li

16、ne 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.Secondary TargetsSampleX-Ray TubeDetectorSecondary TargetThe x-ray tube excites the secondary targetThe Secondary target fluoresces and

17、excites the sampleThe detector detects x-rays from the sampleSecondary Target MethodENERGY(keV)Tube Target peakWith Zn Secondary TargetFeRegionContinuum RadiationSecondary Targets produce a more monochromatic source peak with lower background than with filtersSecondary Target Vs FilterComparison of

18、optimized direct-filtered excitation with secondary target excitation for minor elements in Ni-200Polarizing Target TheoryX-ray are partially polarized whenever they scatter off a surfaceIf the sample and polarizer are oriented perpendicular to each other and the x-ray tube is not perpendicular to t

19、he target,x-rays from the tube will not reach the detector.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 s

20、econdary targets.Diffractive Targets-They are designed to scatter specific energies more efficiently in order to produce a stronger peak at that energy.CollimatorsCollimators are usually circular or a slit and restrict the size or shape of the source beam for exciting small areas in either EDXRF or

21、uXRF instruments.They may rely on internal Bragg reflection for improved efficiency.SampleTubeCollimator sizes range from 12 microns to several mmFocusing OpticsBecause simple collimation blocks unwanted x-rays it is a highly inefficient method.Focusing optics like polycapillary devices and other Ku

22、makhov 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 CapillaryDetectors Si(Li)PIN Diode Silicon Drift Detectors Proportional Counters Scintillation DetectorsCollim

23、ator sizes range from 12 microns to several mmCollimatorsSample PreparationThis has been historically the most common laboratory grade EDXRF configuration.concentration).Windows:Be or PolymerSample should not stratify during analysis.The Ross method illustrated here for Cl analysis uses intensities

24、through two filters,one transmitting,one absorbing,and the difference is correlated to concentration.Less than 75 um spot sizes are regularly achieved.Quantitative Analysis(Multiple intensity Extraction and Regression methods)The newly freed electrons are accelerated toward the detector anode to pro

25、duce an output pulse.Bragg reflectionn=integerX-ray radiation ionizes the detector material causing it to become conductive,momentarily.A niobium filter absorbs Cl and other higher energy source x-rays while letting S x-rays pass.The transmission curve shows the parts of the source spectrum are tran

26、smitted and those that are absorbed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）。Compton Scatter PeaksThere are three type of Polarization Targets:Multi-Channel AnalyserOptional thin polymer windows comparedFill Gases:Neon,Argon,Xenon,KryptonDetector Principles2Enen=num ber of electron-ho

27、le 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 become conductive,momentarily.The newly freed electrons ar

28、e 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 energySi(Li)DetectorWindowSi(Li)crystalDewarfilled withLN2Super-Cooled CryostatCooling:LN2 or Peltier Window:Beryl

29、lium or PolymerCounts Rates:3,000 50,000 cps Resolution:120-170 eV at Mn K-alphaFETPre-AmplifierSi(Li)Cross Section PIN Diode DetectorCooling:Thermoelectrically cooled(Peltier)Window:BerylliumCount Rates:3,000 20,000 cpsResolution:170-240 eV at Mn k-alphaSilicon Drift Detector-SDDPackaging:Similar t

30、o PIN DetectorCooling:PeltierCount Rates;10,000 300,000 cpsResolution:140-180 eV at Mn K-alphaProportional 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+eVW

31、indowScintillation DetectorPMT(Photo-multiplier tube)Sodium Iodide DiskElectronicsConnectorWindow:Be or AlCount Rates:10,000 to 1,000,000+cpsResolution:1000 eVSpectral Comparison-AuSi(Li)Detector10 vs.14 KaratSi PIN Diode Detector10 vs.14 Karat Optional thin polymer windows compared to a standard be

32、ryllium windows Affords 10 x improvement in the MDL for sodium(Na)Detector FiltersFilters are positioned between the sample and detector in some EDXRF and NDXRF systems to filter out unwanted x-ray peaks.Detector Filter Transmission%TRANSMITTEDENERGYLow energy x-rays are absorbedEOI is transmittedAb

33、sorptionEdgeX-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 Absor

34、ptionFilter Vs.No FilterUnfiltered Tube target,Cl,and Ar Interference 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 muc

35、h power.Detector Filter TransmissionDiffraction DeviceX-ray TubesMost frequent transition,hence most intense peak.Orient surface patterns in same manner so as minimise scatter affects.Multi-Channel AnalyserMatrix Interferences“backscatter”ConcentrationDiffraction DeviceFilter Absorption MethodA dete

36、ctor filter can significantly improve detection limits.The two most common diffraction devices used in WDX instruments are the crystal and multilayer.Vacuum -For use with solids or pressed pelletsAdjacent Element Overlap-Al&Si,S&Cl,K&Ca.The WDX method uses the diffraction device and collimators to o

37、btain good resolution,so The detector does not need to be capable of energy discrimination.The Secondary target fluoresces and excites the sampleMatrix InterferencesSynchotrons-These bright light sources are suitable for research and very sophisticated XRF analysis.Multi-Channel AnalyserSynchrotrons

38、Continuum RadiationRoss Vs.Hull FiltersqThe previous slide was an example of the Hull or simple filter method.qThe Ross method illustrated here for Cl analysis uses intensities through two filters,one transmitting,one absorbing,and the difference is correlated to concentration.This is an NDXRF metho

39、d since detector resolution is not important.Wavelength 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.DetectorX-Ray SourceDi

40、ffraction DeviceCollimatorsDiffractionThe two most common diffraction devices used in WDX instruments are the crystal and multilayer.Both work according to the following formula.nl l=2d sinq qn=integerd=crystal lattice or multilayer spacingq q=The incident anglel wavelengthAtomsMultilayersWhile the

41、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 multilayers are more efficient than crystals and can be optimized for specific elements.Often used for low Z elements.Soll

42、er 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.SampleCrystalCooling and Temperature ControlThe diffraction technique is re

43、latively 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 cont

44、rolled instrument coolersChamber 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 met

45、hods are:Vacuum -For use with solids or pressed pelletsHelium-For use with liquids or powdered materialsBragg reflectionPulses are segregated into channels according to energy via the MCA(Multi-Channel Analyser).Brehmstrahlung(or Continuum)Radiation:German for“breaking radiation”,noise that appears

46、in the spectra due to deceleration of electrons as they strike the anode of the X-ray tube.InterferencesSpectral InterferencesEvacuate air from analysis chamber via a vacuum pump.Anode selection determines optimal source excitation(application specific).Fill Gases:Neon,Argon,Xenon,KryptonCollimators

47、Purge instrument with He(less dense than air=less attenuation).Standards should be of the same matrix as samples to be analyzed.Standards(such as certified reference materials)are required for Quantitative Analysis.Energy Dispersive ElectronicsImproved Fluorescence and lower background The character

48、istic fluorescence of the custom line source is used to excite the sample,with the lowest possible background intensity.140 eV ResolutionRh X-ray TubeWith Zn Source filterMore power=lower detection limits由高能态转化为低能态，释放能量。Detector Filter TransmissionElement TagsX-ray radiation ionizes the detector mat

49、erial causing it to become conductive,momentarily.Changers 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+samplesSample Spinners are used to average ou

50、t surface features and particle size affects possibly over a larger total surface area.This configuration is most commonly used in higher end benchtop EDXRF Instruments.This has been historically the most common laboratory grade EDXRF configuration.Energy Dispersive ElectronicsFluorescence generates