X射线荧光分析导论培训资料.ppt

Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,*,Introduction to XRF,LearnXRF,.com,X,射 线 荧 光 分 析,导 论,电子波谱,1,Hz-1kHz,1,kHz-1014Hz,1014,Hz-1015Hz,1015,Hz,-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,Sources,Optics,Filters&Targets,Detectors,Sources,End Window X-Ray Tubes,Side Window X-Ray Tubes,Radioisotopes,Other Sources,Scanning Electron Microscopes,Synchrotrons,Positron 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(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,Isotope,Fe-55,Cm-244,Cd,-109,Am-241,Co-57,Energy(keV),5.9,14.3,18.3,22,88,59.5,122,Elements(K-lines),Al V,Ti-Br,Fe-Mo,Ru,-,Er,Ba,-U,Elements(L-lines),Br-I,I-Pb,Yb,-,Pu,None,none,While isotopes have fallen out of favor they are still useful for many gauging applications.,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.ManySEMs,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.,Source Modifiers,Several 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 Optics,Source Filters,Filters perform one of two functions,Background Reduction,Improved Fluorescence,Detector,X-Ray,Source,Source Filter,Filter Transmission Curve,%,T,R,A,N,S,M,I,T,T,E,D,ENERGY,Low energy x-rays are absorbed,Absorption,Edge,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 transmitted and those that are absorbed,Filter Fluorescence Method,ENERGY(keV),Target peak,With Zn Source filter,Fe,Region,Continuum Radiation,The filter fluorescence method decreases the background and improves the fluorescence yield without requiring huge amounts of extra power.,Filter Absorption Method,ENERGY(keV),Target peak,With Ti Source filter,Fe,Region,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 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 target,The Secondary target fluoresces and excites the sample,The detector detects x-rays from the sample,Secondary Target Method,ENERGY(keV),Tube Target,peak,With Zn Secondary Target,Fe,Region,Continuum Radiation,Secondary Targets produce a more monochromatic source peak with lower background than with filters,Secondary Target Vs Filter,Comparison 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 surface,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.,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.,Collimators,Collimators 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.,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 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 Diode,Silicon Drift Detectors,Proportional Counters,Scintillation Detectors,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 an 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,Dewar,filled with,LN,2,Super-Cooled Cryostat,Cooling:LN,2,or,Peltier,Window:Beryllium or Polymer,Counts Rates:3,000 50,000 cps,Resolution:120-170 eV at Mn K-alpha,FET,Pre-Amplifier,Si,(Li)Cross Section,PIN Diode Detector,Cooling:Thermoelectrically cooled(,Peltier,),Window:Beryllium,Count Rates:3,000 20,000 cps,Resolution:170-240 eV at Mn k-alpha,Silicon Drift Detector-SDD,Packaging:Similar to PIN DetectorCooling:,Peltier,Count Rates;10,000 300,000 cpsResolution:140-180 eV at Mn K-alpha,Proportional Counter,Anode Filament,Fill Gases:Neon,Argon,Xenon,Krypton,Pressure:0.5-2 ATM,Windows:Be or Polymer,Sealed or Gas Flow Versions,Count Rates EDX:10,000-40,000 cps WDX:1,000,000+,Resolution:500-1000+eV,Window,Scintillation Detector,PMT(Photo-multiplier tube),Sodium Iodide Disk,Electronics,Connector,Window:Be or Al,Count Rates:10,000 to 1,000,000+cps,Resolution:1000 eV,Spectral Comparison-Au,Si(Li)Detector,10 vs.14 Karat,Si PIN Diode Detector,10 vs.14 Karat,Polymer Detector Windows,Optional thin polymer windows compared,to a standard beryllium windows,Affords 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,%,T,R,A,N,S,M,I,T,T,E,D,ENERGY,Low energy x-rays are absorbed,EOI is transmitted,Absorption,Edge,X-rays above the absorption edge energy are absorbed,Very high energy,x-rays are transmittedS,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 detection 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 with 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 correlated 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 that 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.,n,l,=2d,sin,q,n=integer,d=crystal lattice or,multilayer,spacing,q,=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 optimized 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,Crystal,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 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 with measurements purges are often used.The two most common purge methods are:,Vacuum -,For use with solids or pressed pellets,Helium-,For use with liquids or powdered materials,Changers and Spinners,Other 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.,Typical PIN Detector Instrument,This configuration is most commonly used in higher end,benchtop,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 produced 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 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 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 Spectra,K&L Spectral Peaks,Rayleigh,Scatter Peaks,Compton Scatter Peaks,Escape Peaks,Sum Peaks,Bremstrahlung,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-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,Scatter,Some of the source X-rays strike the sample and are scattered back at the detector.,Sometimes called,“backscatterSample,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 source 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-“Inelastic”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.,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 appears in the spectra due to deceleration of electrons as they strike the anode of the X-ray tube.,Interferences,Spectral Interferences,Environmental Interferences,Matrix Interferences,Spectral Interferences,Spectral interferences are peaks in the spectrum that overlap the spectral peak(region of interest)of the element to be analyzed.,Examples:,K&L line Overlap-S&Mo,Cl&Rh,As&Pb,Adjacent 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 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 Analyzed 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 used 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,absorbed,Blue,=Ca,enhanced,Source,X-ray,X-Ray Captured by the detector.,Sample,Software