近代分析实验原理：第十课-X射线能谱

1、十、十、X射线能谱射线能谱（X-ray Spectroscopy for Elemental Analysis）近代分析实验原理（Introduction of modern analytical methods）12XRDBraggs law Diffraction patternCrystal structureX-ray Spectroscopycharacteristic X-rays wavelength or energychemical elementsStructure analysisElemental analysisX-ray wavelength dispersive

2、spectroscopy (WDS),X-ray energy dispersive spectroscopy (EDS)X-ray fluorescence spectrometerMicroanalyzerSEMstand-alone equipmentchemical compositions in a microscopic areaoverall chemical compositions in a sample31. Features of Characteristic X-raysExcitation of a characteristic X-ray photon or an

3、Auger electron by a high energy X-ray photon or electron.Moseleys Lawatomic numberB and are constants that depend upon the specific shells俄歇电子41.1 Types of Characteristic X-rays05Allowable electron transitions in K, L and M shells.K series means the characteristic X-rays emitted when an outer shell

4、electron fills an electron vacancy in the K shell. the electron transition from the L1 to the K shell is forbidden 6Primary X-ray radiation generated by the X-ray tube, including continuous and characteristic radiations.the X-ray intensity of .The subscript number also indicates the relative intensi

5、ty, with number 1 as the prises a capital English letter, a lower-case Greek letter anda subscript number.781.2 Comparison of K, L and M SeriesFluorescent yield variation with atomic number and with the K, L and M characteristic lines.1. competition between the characteristic X-ray photon and the Au

6、ger electron2. competitions among generations of K, L and M series of the X-raysFor an atom with an atomic number lower than 4 (Be), the fluorescent yield is zero, and for an atomic number lower than 8 (O), the yield is less than 0.5%.an intrinsic disadvantage for detection of light elements using X

7、-ray spectroscopy20 Ca57 La9Au K1: 66.99 keV, AuL1: 9.713 keV, AuM1: 2.123 keV10Energies of characteristic X-ray photons in the K, L and M lines.heavy elementsL and the M linescomplicatedatomic numbers lower than 20only the K linesL and M lines for low atomic numberstoo low to be detected10410510310

8、2106Atomic number: 79A typical X-ray energy spectrum is in a range from 0.2 to 20 keV.11122. X-ray Fluorescence SpectrometryX-ray fluorescence spectrometry (XRF)Main components and dispersive spectra of: (a) WDS; and (b) EDS.wavelength dispersive spectroscopy (WDS)energy dispersive spectroscopy (EDS

9、)13the X-ray sourcedetection systemdata collection and processing systemXRFFrom X-ray tube, similar to XRD, 0.5-3 kW, 30-50 kVEnsure that the X-rays impinging the specimen exceed the critical potential of exciting characteristic X-rays from the elements in specimens. The optimum ratio of tube voltag

10、e to the critical potential is about 35.single crystal diffractionWDSa photon detectorEDSThe X-ray radiation is produced by electrons striking a metal target in the tube. The target metals include Cr, Rh, W, Ag, Au and Mo.142.1 Wavelength Dispersive Spectroscopyin the early 1950s, earlier than EDSWD

11、S apparatus, which includes the X-ray tube, specimen, primary collimator, analyzing crystal, flow counter, auxiliary collimator and scintillation counter.1. Better resolutionrelative changes in wavelength (/ ) in the range 0.0020.02, corresponding to the energy range 0.010.1 keV, which is about one

12、order of magnitude better than that of EDS.2. Wider range of elemental analysiselements from upward of C (Z=6).3. More complicatedBraggs Lawsimilar to an X-ray diffractometerSequential in nature15Analyzing CrystalIn WDS, the analyzing crystal should be carefully selected because it determines the ra

13、nge of detectable atomic numbers.Normally, the maximum achievable angle in a WDS system is about 73. Thus, the maximum of characteristic X-rays being diffracted is about 1.9d of the analyzing crystal.higher resolution when using a small d-spacing crystal季戊四醇邻苯二甲酸铊reduce the range of wavelength16Wave

14、length Dispersive Spectra1.A WDS spectrum is presented as a diagram in which the characteristic X-ray lines are dispersed in a range of the X-ray wavelengths.2.there is no scale to indicate the real intensities of the X-rays. only relative intensities among the lines, and with respect to the backgro

15、und, are important.3.we need more than one type of analyzing crystal to obtain the whole range of wavelength dispersion from a specimen, because the limitation of wavelength ranges by the crystals d-spacing.4.Braggs Law indicates that the orders of diffraction (n) may generate multiple wavelength pe

16、aks in a spectrum for the same characteristic line. In addition, if a specific wavelength is nearly equal to a multiple of another wavelength, then their peaks will appear superimposed on each other in the spectrum.17WDS spectra of a nickel-based superalloy: (a) spectrum obtained with a LiF analyzin

17、g crystal; and (b) spectrum obtained with a TAP crystal.The two spectra provide a complete chemical analysis of the nickel-based alloy that contains Cr, Co, W, Ta, V, Al and Re.182.2 Energy Dispersive Spectroscopya commercial product in the early 1970s1. More popular2. Structural simple (no moving p

18、arts)3. Faster4. Worse resolutionabout 150200 eV5. Elements from upward of O (Z=8).6. Low costthe detector collects the signals of characteristic X-rays energies from a whole range of elements in a specimen at the same time rather than collecting signals from X-ray wavelength individually.19A Si(Li)

19、 cylinder with annular groove construction: (a) p-type silicon; (b) lithium compensated region; and (c) n-type silicon.DetectorThe Si(Li) is the most commonly used detector in an EDS system.Characteristic X-ray photons can be separated by their energy levels according to the numbers of electronhole

20、pairs they generate.The energy resolution of the detector (R) in eV can be estimated:E is the energy of characteristic X-ray line and F is a constant called the Fano factor, which has a value of 0.12 for Si(Li). noise, the electronic noise factor, plays an important role in the resolution.The averag

21、e energy of photons needed to generate an electronhole pair(ei) is about 3.8 eV in the Si(Li) diode. The higher the photon energy, the more pairs are generated.X-ray photons must pass through a window to reach the Si(Li) diode.Be10 m thickliquid nitrogen (196 C) 20Energy Dispersive SpectraThe intens

22、ity of characteristic X-ray lines across the X-ray energy range.A spectrum in a range from 0.1 to about 1020 keVEDS spectrum of glass which includes Si, O, Ca, Al, Fe and Ba.identification of individual elements from EDS spectra is more straightforward than from WDS spectra21Advances in Energy Dispe

23、rsive Spectroscopydrawbacksthe X-rays could excite more photons than could be counted by the detector, because there is a maximum counting rate of photons for an energy detector.a selected pure element standard with absorption filters is placed in the optical path between the primary X-raysource and

24、 the specimen.the difficulty in examining small samples, the signal is too weakthe total reflection spectrometer (TRXRF)Geometric construction of a total reflection fluorescence spectrometer. The incident angle is less than 0.1 so that the primary X-ray beam is totally reflected from the flat substr

25、ate. (down to 10-12 g)dead time0.5107 secondseliminates primary X-rays radiating substrate materials( 5 mm) 223. Qualitative and Quantitative Analysis3.1 Qualitative AnalysisEDS spectrum of a NiTi alloy obtained in an XRF spectrometer.Modern X-ray spectrometers are equipped with computer software th

26、at is fully capable of identifying the possible elements from a spectrum. We can input the elements that possibly exist in the specimen into the software for qualitative analysis. The computer software will mark peak positions of the input elements in the spectrum. Also, the software will generate s

27、uch peak lines with correct relative intensities in a spectrum, for example, a correct intensityratio of K to K. With computer assistance, the errors in element identification can be reduced to minimum.XRF spectrum of a Pb alloy with primary X-ray radiation from the X-ray tube with a Rhtarget. The R

28、h characteristic lines are not overlapped with those of elements in the alloy specimen.additional peaks of a target material233.2 Quantitative Analysisthe weight fractionThe relative intensities of its peaksthe instrument factorThe matrix factor of specimenconditions of primary sources, the geometri

29、cal arrangement of specimen respect to radiation and detection, and detector characteristics.primary absorptionsecondary absorptionsecondary fluorescencethe radiation that is absorbed on the beams path to reach the atoms to be excited.absorption of fluorescent radiation from atoms that occur along i

30、ts path inside the specimen to the detector.the fluorescent radiation from the atoms which are excited by the fluorescent radiation of atoms with a higher atomic number in the same specimen.reduce the intensity of characteristic X-ray lines in spectrumincreases the intensity of lighter elements24Qua

31、ntitative Analysis by X-ray FluorescenceInternal Standard MethodFor analyzing a single element in a specimen in a known or unknown matrix.Assumption: the instrumental and matrix effects are the same for the standard and the element to be analyzed.Method: adding a standard element of a known concentr

32、ation into the specimen.Fundamental Parameter MethodMethod: With a powerful computer system, the method determines the concentration of an element when its theoretical intensity matches its measured intensity.Estimates the composition of the specimenThe theoretical intensityintensity differenceMay b

33、e differentSimilar atomic number and concentration (10%) good mixing similar particle sizeThe physical parameters include specimen density, thickness, X-ray absorption coefficients and fluorescence yield. The instrumental parameters include excitation voltage of the X-ray tube, optical geometry and

34、detector characteristics.254. Energy Dispersive Spectroscopy in Electron MicroscopesGeometrical arrangement of EDS in a scanning electron microscope (SEM).the primary beam source: the electron beammicroanalysisusing EDS is simply its compactnesselectron probe can be focused on a very small area26Pot

35、ential interference of X-ray detection due to low take-off angle in the SEM.For a low take-off angle, a rough surface may interfere with collection of X-ray photons emitted from a valley area27(a) Comparison of X-ray production regions with specimens of different mass densities (=3 and 10 g cm3 for

36、left and right examples, respectively); and (b) spatial resolution of EDS as a function of acceleration voltage of electrons and specific lines of characteristic X-rays emitted from bulk specimens.acceleration voltage of electronsthe energy required to excite the specific line of characteristic X-ra

37、ysthin specimensthe spatial resolution is of less concern28Operating modes:stationary and scanningThe stationary mode, the probe stays at one location until the collection of X-ray photons is complete.impurities, precipitates and grain boundariesThe scanning mode, the electron probe moves over the s

38、pecimen surface, similar to the way in which the probe moves for obtaining an electron image in the SEM.Co and Cr line scans across an oxidized high-temperature alloy. The top curve is for cobalt, and the bottom curve is for position variations 29Quantitative Analysis in Electron Microscopyseparate

39、standard samples containing the elements in the specimen to be analyzed are necessary.under the same conditionsthe differences in matrix effects between the specimen and standard cannot be ignored.Atomic number effectX-ray absorptionX-ray fluorescencethe specimenstandard30The atomic number factor (Z

40、) counts the difference in characteristic X-rays generation due to atomic number change in the matrix.electron backscattering (R).electron retardation (S).stopping factor (S)An element in a matrix with elements heavier than it will generate a lower intensity than it does in pure form.electron energy

41、 loss due to inelastic scatteringbackscattered and does not generate X-raysS decreases while R increases with increase in atomic distance in the specimenacceleration voltagedensityThe X-ray absorption factor (A) counts the decrease in density of characteristic X-rays from emitting loca

42、tion to detector.The fluorescence factor (F) arises from the excitation of characteristic X-rays of the element to be analyzed by the characteristic X-rays emitted from matrix atoms.ZAF correct valid for medium-weight and heavy elementsInvalid for light elements (Z10)the greatest factor31Standardles

43、s quantitative analysisis desirable when standards are not available.based on prediction of the X-ray intensity that would be obtained from a pure element standard under the same instrumental conditions.This method works well for a specimen consisting of transition elements of which the K lines span the range 510 keV, such as stainless steel.Standardless analysis may generate large err