WDS and EDS for EM

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WDS and EDS for EM

  1. 1. X-ray Microanalysis An inelastic collision between a primary beam electron and an inner orbital electron results in the emission of that electron from the atom. The energy released from an electron replacement event produces a photon with an energy exactly equal to the drop in energy.
  2. 2. X-rays can have an energy nearly equal to that of the primary beam electron and thus can escape from very deep within the specimen
  3. 3. Energy Dispersive Spectroscopy (EDS or EDX)
  4. 4. When an electron from a K-shell is replaced by one from the next closest shell (L), it is designated as a Kα event
  5. 5. When an electron from a K-shell is replaced by one from the second closest shell (M), it is designated as a Kβ event Kα Kβ
  6. 6. Lα - When an electron from a L-shell is replaced by one from the next closest shell (M). The K shell will never donate its electron as this would require an increase in energy, not a drop.
  7. 7. Certain events such as Mα, Lβ, and Kγ are only possible in atoms of sufficient atomic weight
  8. 8. There are a wide variety of subsets of X-rays since each electron shell has multiple orbitals
  9. 9. An X-ray spectrum for a sample is composed of allAn X-ray spectrum for a sample is composed of all the possible signals for that given set of elements.the possible signals for that given set of elements. These will differ in terms of energies (KeV) andThese will differ in terms of energies (KeV) and probabilities (likelihood) scored as number of suchprobabilities (likelihood) scored as number of such signals collected over a given period of time.signals collected over a given period of time. #Counts X-ray Energy in KeV
  10. 10. Each element has a family of characteristic X-raysEach element has a family of characteristic X-rays associated with itassociated with it
  11. 11. Positive identification of an element is best done byPositive identification of an element is best done by evaluating the entire family of peaks for a givenevaluating the entire family of peaks for a given element.element.
  12. 12. "Bremsstrahlung" means "braking radiation" and comes from"Bremsstrahlung" means "braking radiation" and comes from the original German to describe the radiation which is emittedthe original German to describe the radiation which is emitted when electrons are decelerated or "braked" when they interactwhen electrons are decelerated or "braked" when they interact with the specimen.with the specimen. Although they contribute to the total X-ray signal they containAlthough they contribute to the total X-ray signal they contain no useful information because their energies are nonspecificno useful information because their energies are nonspecific and therefore are considered as part of the background .and therefore are considered as part of the background .
  13. 13. Bremsstrahlung X-rays are the major part of theBremsstrahlung X-rays are the major part of the continuum X-ray signal that can escape from thecontinuum X-ray signal that can escape from the deepest portion of the interaction region.deepest portion of the interaction region.
  14. 14. Chrysotile Asbestos FibersChrysotile Asbestos Fibers
  15. 15. Bullet fragments (blue) can be identified on clothBullet fragments (blue) can be identified on cloth fibers and distinguished from other metal pieces byfibers and distinguished from other metal pieces by their elemental compositiontheir elemental composition
  16. 16. Gunshot Residue (GSR) AnalysisGunshot Residue (GSR) Analysis
  17. 17. Gunshot Residue (GSR) AnalysisGunshot Residue (GSR) Analysis •Particles are veryParticles are very characteristic, thereforecharacteristic, therefore presence of these particlespresence of these particles forms evidence of firing aforms evidence of firing a gun.gun. •Particles normally consistParticles normally consist of Pb (lead), Sb (antimony)of Pb (lead), Sb (antimony) and Ba (barium).and Ba (barium). •New ammunition:New ammunition: environmentally friendly (noenvironmentally friendly (no Sb).Sb).
  18. 18. The proportion ofThe proportion of elements present inelements present in GSR differ slightlyGSR differ slightly and databases ofand databases of GSR from differentGSR from different manufacturers canmanufacturers can be used to identifybe used to identify what ammunitionwhat ammunition was used in a crime.was used in a crime. GSR is often foundGSR is often found on criminals and alsoon criminals and also on victims if shot aton victims if shot at close range.close range.
  19. 19. X-ray MappingX-ray Mapping
  20. 20. X-ray analysis of paint fragmentsX-ray analysis of paint fragments The combinedThe combined (a) backscatter image(a) backscatter image and X-ray maps ofand X-ray maps of (b) Au,(b) Au, (c) Ba(c) Ba (d) Ca(d) Ca Different layers ofDifferent layers of paint can be identifiedpaint can be identified
  21. 21. EDS = EnergyEDS = Energy DispersiveDispersive SpectroscopySpectroscopy WDS = WavelengthWDS = Wavelength DispersiveDispersive SpectroscopySpectroscopy X-ray DetectionX-ray Detection
  22. 22. EDS WDS
  23. 23. Pulse Processor Measures the electronic signals to determine the energy of each X-ray detected X-ray Detector Detects and converts X-rays into electronic signals Analyzer Displays and interprets the X-ray data
  24. 24. Cut-away diagram showingCut-away diagram showing the construction of a typicalthe construction of a typical EDS detector.EDS detector. FET Crystal Window Collimator
  25. 25. Lithium doped Silicon (SiLi) crystal detectorLithium doped Silicon (SiLi) crystal detector acts as a semiconductor that carries current in a rateacts as a semiconductor that carries current in a rate proportional to the number of ionization events andproportional to the number of ionization events and acts as an indirect measurement of the energyacts as an indirect measurement of the energy contained in the X-raycontained in the X-ray..
  26. 26. Absorbed X-rays create an ionization eventAbsorbed X-rays create an ionization event similar to that of a scintillatorsimilar to that of a scintillator
  27. 27. Each ionized atom of silicon absorbs 3.8 eV ofEach ionized atom of silicon absorbs 3.8 eV of energy, so an X-ray of 3.8 KeV will ionizeenergy, so an X-ray of 3.8 KeV will ionize approximately 1000 silicon atomsapproximately 1000 silicon atoms.
  28. 28. CollimatorCollimator to limit BSE and stray X-raysto limit BSE and stray X-rays WindowWindow usually made of beryllium (limited to sodium, atomicusually made of beryllium (limited to sodium, atomic number 11) or thin plastic to detect down to boron (Atomicnumber 11) or thin plastic to detect down to boron (Atomic number 5) protects cooled crystal from air.number 5) protects cooled crystal from air. FET Crystal Window Collimator
  29. 29. DetectorDetector : crystal silicon wafer with lithium added in. For each: crystal silicon wafer with lithium added in. For each 3.8 eV from an X-ray, produce an electron and hole. This3.8 eV from an X-ray, produce an electron and hole. This produces a pulse of current, the voltage of which isproduces a pulse of current, the voltage of which is proportional to the X-ray energy. Must keep the crystal at LNproportional to the X-ray energy. Must keep the crystal at LN temperature to keep noise to a minimum.temperature to keep noise to a minimum. FETFET :: The field effect transistor is positioned just behind theThe field effect transistor is positioned just behind the detecting crystal. It is the first stage of thedetecting crystal. It is the first stage of the amplification process that measures the charge liberated inamplification process that measures the charge liberated in the crystal by an incident X-ray and converts it to a voltagethe crystal by an incident X-ray and converts it to a voltage output.output. FET Crystal Window Collimator
  30. 30. Multichannel Analyzer (MCA)Multichannel Analyzer (MCA) The changes in conductivity of the SiLi crystal can beThe changes in conductivity of the SiLi crystal can be counted for a given time and displayed as a histogramcounted for a given time and displayed as a histogram using a multichannel analyzer.using a multichannel analyzer.
  31. 31. Multichannel Analyzer (MCA)Multichannel Analyzer (MCA) MCA consists of an analog to digital converter which “scores”MCA consists of an analog to digital converter which “scores” the analog signal coming from the field effect transistor (FET).the analog signal coming from the field effect transistor (FET). Newer systems employ a digital pulse processor whichNewer systems employ a digital pulse processor which converts the signal on the flyconverts the signal on the fly ThenThen NowNow
  32. 32. Factors affecting signal collectionFactors affecting signal collection Distance between detector and X-ray source Angle at which detector is struck Volume of signal collected.
  33. 33. For a given angle of electron incidence, the length ofFor a given angle of electron incidence, the length of the absorption path is directly proportional to thethe absorption path is directly proportional to the cosecant of the take-off angle,cosecant of the take-off angle, φφ Take-off AngleTake-off Angle
  34. 34. Solid AngleSolid Angle The solid angleThe solid angle ΩΩ of a detector is defined as angleof a detector is defined as angle of theof the conecone of signal entering the detector. Theof signal entering the detector. The greater the size of the detector surface area thegreater the size of the detector surface area the greater will be the solid angle.greater will be the solid angle.
  35. 35. Larger SiLi crystals will be able to sample a largerLarger SiLi crystals will be able to sample a larger volume of signal (bettervolume of signal (better ΩΩ)) but because ofbut because of imperfections in the crystal they have slightly greaterimperfections in the crystal they have slightly greater noise and thus slightly lower resolution.noise and thus slightly lower resolution.
  36. 36. One can also increase the solid angle by placingOne can also increase the solid angle by placing the detector closer to the source.the detector closer to the source. One then tries to maximize both the solid angleOne then tries to maximize both the solid angle and the take-off angle.and the take-off angle.
  37. 37. One reason that the final lens of an SEM is conical inOne reason that the final lens of an SEM is conical in shape is so that the EDS detector can be positioned atshape is so that the EDS detector can be positioned at a high take-off angle and inserted close to thea high take-off angle and inserted close to the specimen for a high solid angle.specimen for a high solid angle.
  38. 38. William Henry BraggWilliam Henry Bragg 1862 – 19421862 – 1942 Nobel Prize in PhysicsNobel Prize in Physics 19151915 X-ray diffraction in a crystal.X-ray diffraction in a crystal. Like an electron beam an X-rayLike an electron beam an X-ray has its own wavelength which ishas its own wavelength which is proportional to its energyproportional to its energy
  39. 39. Crystal:Crystal: A solid formed byA solid formed by the solidification of athe solidification of a chemical and having achemical and having a highly regular atomichighly regular atomic structure. May bestructure. May be composed of a singlecomposed of a single element (C = diamond) orelement (C = diamond) or multiple elements.multiple elements.
  40. 40. CubicCubic HexagonalHexagonal
  41. 41. If a wavelength enters a crystal at the appropriate angle it willIf a wavelength enters a crystal at the appropriate angle it will be diffracted rather than being absorbed or scattered by thebe diffracted rather than being absorbed or scattered by the crystalcrystal
  42. 42. For a given wavelengthFor a given wavelength λλ there is a specific anglethere is a specific angle θθ (Bragg’s angle) at which diffraction will occur.(Bragg’s angle) at which diffraction will occur. Bragg’s angle is determined by the d-spacingBragg’s angle is determined by the d-spacing (interplanar spacing) of the crystal and the order of(interplanar spacing) of the crystal and the order of diffraction (n = 1, 2, 3….).diffraction (n = 1, 2, 3….).
  43. 43. A WDS detector takes advantage of the fact that an X-ray of aA WDS detector takes advantage of the fact that an X-ray of a given wavelength can be focused by a crystal if it encountersgiven wavelength can be focused by a crystal if it encounters the crystal at the proper Bragg’s angle.the crystal at the proper Bragg’s angle. To better accomplish this crystals are bent and ground to formTo better accomplish this crystals are bent and ground to form a curved surface which will bring all the diffracted X-raya curved surface which will bring all the diffracted X-ray wavelengths to a single focal point, thus the crystal acts as awavelengths to a single focal point, thus the crystal acts as a focusing lens.focusing lens.
  44. 44. To change the Bragg’s angle the diffracting crystal andTo change the Bragg’s angle the diffracting crystal and detector can be moved together relative to thedetector can be moved together relative to the stationary specimen along a circle known as thestationary specimen along a circle known as the Roland CircleRoland Circle..
  45. 45. WDS detectors are quite large and must be positioned aroundWDS detectors are quite large and must be positioned around the specimen chamber at an angle to take advantage ofthe specimen chamber at an angle to take advantage of maximum take-off angle and maximum solid anglemaximum take-off angle and maximum solid angle
  46. 46. A microprobe is a specialized SEM that is outfittedA microprobe is a specialized SEM that is outfitted with an EDS detector and array of several WDSwith an EDS detector and array of several WDS detectors.detectors.
  47. 47. Different diffracting crystals canDifferent diffracting crystals can only diffract certain wavelengthsonly diffract certain wavelengths (even with the changes in Bragg’s(even with the changes in Bragg’s angle) so an array of detectorsangle) so an array of detectors must be used if one is to be ablemust be used if one is to be able to detect K, L, and M events forto detect K, L, and M events for many different elements. Sincemany different elements. Since WDS detectors do not need to beWDS detectors do not need to be cooled they are windowless andcooled they are windowless and can detect down to Beryliumcan detect down to Berylium LiF = Lithium fluoride; PET =Pentaerythritol; and TAP = Thallium acid phthalate.
  48. 48. Specimen preparation for WDSSpecimen preparation for WDS Samples must be conductive since high KeV is usedSamples must be conductive since high KeV is used (Carbon coating if not naturally conductive)(Carbon coating if not naturally conductive) Samples must be flat (polished) as geometry ofSamples must be flat (polished) as geometry of sample to detector is crucial and also minimizessample to detector is crucial and also minimizes artifacts when doing quantitative measurements.artifacts when doing quantitative measurements.
  49. 49. A comparison of twoA comparison of two spectra collected withspectra collected with EDS and WDS showsEDS and WDS shows how peak overlap andhow peak overlap and energy spread canenergy spread can serve to obscure theserve to obscure the information in an EDSinformation in an EDS spectrumspectrum
  50. 50. Quantitative X-ray Analysis If one wants to quantify the relative amounts of different elements present in a complex sample one has to account for a number of factors and carry out a
  51. 51. One must account for other elements present in the sample and whether their individual peaks overlap with each other creating a “shoulder” that can mask the presence of one element or distort the
  52. 52. Several methods to correct the spectra. ZAF takes into account the Atomic Weight (Z), effects of Absorbance (A) and effects of Fluorescence (F) in adjusting the data to give the correct values.
  53. 53. Applications of X-ray MicroanalysisApplications of X-ray Microanalysis SecondarySecondary Electron imageElectron image
  54. 54. EDS can be added as a component of a TEMEDS can be added as a component of a TEM Requires an angled detector (for take-off angle) andRequires an angled detector (for take-off angle) and scan coils in the column to function as a Scanningscan coils in the column to function as a Scanning Transmission Electron Microscope or STEM.Transmission Electron Microscope or STEM.
  55. 55. EDS can be used toEDS can be used to identify elements presentidentify elements present vacuoles or inclusions.vacuoles or inclusions. Must take into accountMust take into account elements present in theelements present in the embedding mediumembedding medium

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