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1、X 射 线 荧 光 分 析导 论,电子波谱,1Hz-1kHz,1kHz-1014Hz,1014Hz-1015Hz,1015Hz-1021Hz,超低频率电磁波,无线电波,微波,红外线可见光,伽马射线,紫外线,Low energy,High energy,X射线,Theory,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）。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。E=Eh-El.能量将以X射线的释放，产生X射线荧光。,The Hardware,SourcesOpticsFilters
2、&TargetsDetectors,Sources,End Window X-Ray TubesSide Window X-Ray TubesRadioisotopesOther SourcesScanning Electron MicroscopesSynchrotronsPositron and other particle beams,End Window X-Ray Tube,X-ray TubesVoltage determines which elements can be excited.More power=lower detection limitsAnode selecti
3、on determines optimal source excitation(application specific).,Side Window X-Ray Tube,Be Window,Silicone Insulation,Glass Envelope,Filament,Electron beam,Target(Ti,Ag,Rh,etc.),Copper Anode,HV Lead,Radioisotopes,While isotopes have fallen out of favor they are still useful for many gauging applicatio
4、ns.,Other Sources,Several 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 a
5、nalysisSynchotrons-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.,Source Modifiers,S
6、everal Devices are used to modify the shape or intensity of the source spectrum or the beam shape,Source FiltersSecondary TargetsPolarizing TargetsCollimatorsFocusing Optics,Source Filters,Filters perform one of two functionsBackground ReductionImproved Fluorescence,Detector,X-Ray Source,Source Filt
7、er,Filter Transmission Curve,%TRANSMITTED,ENERGY,Low energy x-rays are absorbed,AbsorptionEdge,X-rays above the absorption edge energy are absorbed,Very high energy x-rays are transmitted,Ti Cr,Titanium Filter transmission curve,The transmission curve shows the parts of the source spectrum are trans
8、mitted and those that are absorbed,Filter Fluorescence Method,ENERGY(keV),Target peak,With Zn Source filter,FeRegion,Continuum Radiation,The filter fluorescence method decreases the background and improves the fluorescence yield without requiring huge amounts of extra power.,Filter Absorption Method
9、,ENERGY(keV),Target peak,With Ti Source filter,FeRegion,Continuum Radiation,The filter absorption Method decreases the background while maintaining similar excitation efficiency.,Secondary Targets,Improved Fluorescence and lower background The characteristic fluorescence of the custom line source is
10、 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 Targets,Sample,X-Ray Tube,Detector,Secondary Target,The x-ray tube excites the secondary targetThe Secondary target fluoresces and excite
11、s the sampleThe detector detects x-rays from the sample,Secondary Target Method,ENERGY(keV),Tube Target peak,With Zn Secondary Target,FeRegion,Continuum Radiation,Secondary Targets produce a more monochromatic source peak with lower background than with filters,Secondary Target Vs Filter,Comparison
12、of optimized direct-filtered excitation with secondary target excitation for minor elements in Ni-200,Polarizing Target Theory,X-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
13、 to the 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 ot
14、her secondary targets.Diffractive Targets-They are designed to scatter specific energies more efficiently in order to produce a stronger peak at that energy.,Collimators,Collimators are usually circular or a slit and restrict the size or shape of the source beam for exciting small areas in either ED
15、XRF or uXRF instruments.They may rely on internal Bragg reflection for improved efficiency.,Sample,Tube,Collimator sizes range from 12 microns to several mm,Focusing Optics,Because simple collimation blocks unwanted x-rays it is a highly inefficient method.Focusing optics like polycapillary devices
16、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.,Source,Detector,Bragg reflection inside a Capillary,Detectors,Si(Li)PIN DiodeSilicon Drift DetectorsProportional CountersScintillation De
17、tectors,Detector Principles,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 are accelerated toward the detector anode to produce a
18、n output pulse.In ionized semiconductor produces electron-hole pairs,the number of pairs produced is proportional to the X-ray photon energy,Si(Li)Detector,Window,Si(Li)crystal,Dewarfilled withLN2,Super-Cooled Cryostat,Cooling:LN2 or Peltier Window:Beryllium or PolymerCounts Rates:3,000 50,000 cps R
19、esolution:120-170 eV at Mn K-alpha,FET,Pre-Amplifier,Si(Li)Cross Section,PIN Diode Detector,Cooling:Thermoelectrically cooled(Peltier)Window:BerylliumCount Rates:3,000 20,000 cpsResolution:170-240 eV at Mn k-alpha,Silicon Drift Detector-SDD,Packaging:Similar to PIN DetectorCooling:PeltierCount Rates
20、;10,000 300,000 cpsResolution:140-180 eV at Mn K-alpha,Proportional Counter,Anode Filament,Fill 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+eV,Window,Scintillation Detector,PMT(Pho
21、to-multiplier tube),Sodium Iodide Disk,Electronics,Connector,Window:Be or AlCount Rates:10,000 to 1,000,000+cpsResolution:1000 eV,Spectral Comparison-Au,Si(Li)Detector10 vs.14 Karat,Si PIN Diode Detector10 vs.14 Karat,Polymer Detector Windows,Optional thin polymer windows compared to a standard bery
22、llium windowsAffords 10 x improvement in the MDL for sodium(Na),Detector Filters,Filters are positioned between the sample and detector in some EDXRF and NDXRF systems to filter out unwanted x-ray peaks.,Sample,Detector,X-Ray Source,Detector Filter,Detector Filter Transmission,%TRANSMITTED,ENERGY,Lo
23、w energy x-rays are absorbed,EOI is transmitted,AbsorptionEdge,X-rays above the absorption edge energy are absorbed,Very high energy x-rays are transmitted,S Cl,A niobium filter absorbs Cl and other higher energy source x-rays while letting S x-rays pass.A detector filter can significantly improve d
24、etection limits.,Niobium Filter Transmission and Absorption,Filter Vs.No Filter,Unfiltered Tube target,Cl,and Ar Interference Peak,Detector 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 w
25、ith large area detectors that normally do not require much power.,Ross Vs.Hull Filters,The previous slide was an example of the Hull or simple filter method.The Ross method illustrated here for Cl analysis uses intensities through two filters,one transmitting,one absorbing,and the difference is corr
26、elated to concentration.This is an NDXRF method since detector resolution is not important.,Wavelength Dispersive XRF,Wavelength 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 t
27、hat scatter of the device.,Sample,Detector,X-Ray Source,Diffraction Device,Collimators,Diffraction,The two most common diffraction devices used in WDX instruments are the crystal and multilayer.Both work according to the following formula.,nl=2d sinq,n=integerd=crystal lattice or multilayer spacingq
28、=The incident angle=wavelength,Atoms,Multilayers,While 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 multilayers are more efficient than crystals and can be optimi
29、zed for specific elements.Often used for low Z elements.,Soller Collimators,Soller 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.,Sample,Cryst
30、al,Cooling and Temperature Control,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.Diffraction devices are also temperature sensitive.,Many
31、 WDXRF Instruments use:X-Ray Tube Coolers,and Thermostatically controlled instrument coolers,Chamber Atmosphere,Sample 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
32、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 materials,Changers and Spinners,Other commonly available sample handling features are sample changers or spinners.Automatic sample change
33、rs are usually of the circular or XYZ stage variety and may have hold 6 to 100+samplesSample Spinners are used to average out surface features and particle size affects possibly over a larger total surface area.,Typical PIN Detector Instrument,This configuration is most commonly used in higher end b
34、enchtop EDXRF Instruments.,Typical Si(Li)Detector Instrument,This has been historically the most common laboratory grade EDXRF configuration.,Energy Dispersive Electronics,Fluorescence generates a current in the detector.In a detector intended for energy dispersive XRF,the height of the pulse produc
35、ed is proportional to the energy of the respective incoming X-ray.,DETECTOR,Signal to Electronics,Element A,Element C,Element B,Element D,Multi-Channel Analyser,Detector current pulses are translated into counts(counts per second,“CPS”).Pulses are segregated into channels according to energy via the
36、 MCA(Multi-Channel Analyser).,Signal from Detector,Channels,Energy,Intensity(#of CPS per Channel),WDXRF 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
37、 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.,Evaluating Sp
38、ectra,K&L Spectral PeaksRayleigh Scatter PeaksCompton Scatter PeaksEscape PeaksSum PeaksBremstrahlung,In addition to elemental peaks,other peaks appear in the spectra:,K&L Spectral Lines,K-alpha lines:L shell e-transition to fill vacancy in K shell.Most frequent transition,hence most intense peak.,K
39、-beta lines:M shell e-transitions to fill vacancy in K shell.,L Shell,K 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 alpha,K beta,M Shell,L alpha,N Shell,L beta,K&L Spectral Peaks,Rh X-ray Tube,L-lines,K-Lines,Sc
40、atter,Some of the source X-rays strike the sample and are scattered back at the detector.Sometimes called“backscatter”,Sample,Source,Detector,Rayleigh Scatter,X-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 sourc
41、e peak in spectra.AKA-“Elastic”Scatter,EI,EO,Rh X-ray Tube,Compton Scatter,X-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 Rayleigh Scatter.AKA-“Inelast
42、ic”Scatter,EI,EO,Rh X-ray Tube,Sum 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).,Escape Peaks,X-rays strike the sample and promote elemental fluorescence.
43、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 Tube,1.74 keV,Brehmstrahlung,Brehmstrahlung(or Continuum)Radiation:German for“breaking radiation”,noise that appear
44、s in the spectra due to deceleration of electrons as they strike the anode of the X-ray tube.,Interferences,Spectral InterferencesEnvironmental InterferencesMatrix Interferences,Spectral Interferences,Spectral interferences are peaks in the spectrum that overlap the spectral peak(region of interest)
45、of the element to be analyzed.Examples:K&L line Overlap-S&Mo,Cl&Rh,As&PbAdjacent Element Overlap-Al&Si,S&Cl,K&Ca.Resolution of detector determines extent of overlap.,220 eV Resolution,140 eV Resolution,Adjacent Element Overlap,Environmental Interferences,Light elements(Na-Cl)emit weak X-rays,easily
46、attenuated by air.Solution:Purge instrument with He(less dense than air=less attenuation).Evacuate air from analysis chamber via a vacuum pump.Either of these solutions also eliminate interference from Ar(spectral overlap to Cl).Argon(Ar)is a component of air.,Air Environment,He Environment,Al Analy
47、zed with Si Target,Matrix Interferences,Absorption:Any element can absorb or scatter the fluorescence of the element of interest.Enhancement:Characteristic x-rays of one element excite another element in the sample,enhancing its signal.,Influence Coefficients,sometimes called alpha corrections are u
48、sed to mathematically correct for Matrix Interferences,Absorption/Enhancement Effects,Absorption-Enhancement Affects,Incoming source X-ray fluoresces Fe.Fe fluorescence is sufficient in energy to fluoresce Ca.Ca is detected,Fe is not.Response is proportional to concentrations of each element.,Red=Fe
49、,absorbedBlue=Ca,enhanced,Source X-ray,X-Ray Captured by the detector.,Sample,Software,Qualitative AnalysisSemi-Quantitative Analysis(SLFP,NBS-GSC.)Quantitative Analysis(Multiple intensity Extraction and Regression methods),Qualitative Scan Peak ID,This spectrum also contrasts the resolution of a PI
50、N diode detector with a proportional counter to illustrate the importance of detector resolution with regard to qualitative analysis.,Automated Peak identification programs are a useful qualitative examination tool,Element Tags,Semi-Quantitative Analysis,The algorithm computes both the intensity to
