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WDS and EDS for EM

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WDS and EDS for EM WDS and EDS for EM Presentation Transcript

  • 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.
  • 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
  • Energy Dispersive Spectroscopy (EDS or EDX)
  • When an electron from a K-shell is replaced by one from the next closest shell (L), it is designated as a Kα event
  • When an electron from a K-shell is replaced by one from the second closest shell (M), it is designated as a Kβ event K  K 
  • L  - 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.
  • Certain events such as M α , L β , and K γ are only possible in atoms of sufficient atomic weight
  • There are a wide variety of subsets of X-rays since each electron shell has multiple orbitals
  • An X-ray spectrum for a sample is composed of all the possible signals for that given set of elements. These will differ in terms of energies (KeV) and probabilities (likelihood) scored as number of such signals collected over a given period of time. # Counts X-ray Energy in KeV
  • Each element has a family of characteristic X-rays associated with it
  • Positive identification of an element is best done by evaluating the entire family of peaks for a given element.
  • "Bremsstrahlung" means "braking radiation" and comes from the original German to describe the radiation which is emitted when electrons are decelerated or "braked" when they interact with the specimen. Although they contribute to the total X-ray signal they contain no useful information because their energies are nonspecific and therefore are considered as part of the background .
  • Bremsstrahlung X-rays are the major part of the continuum X-ray signal that can escape from the deepest portion of the interaction region.
  • Chrysotile Asbestos Fibers
  • Bullet fragments (blue) can be identified on cloth fibers and distinguished from other metal pieces by their elemental composition
  • Gunshot Residue (GSR) Analysis
  • Gunshot Residue (GSR) Analysis
          • Particles are very characteristic, therefore presence of these particles forms evidence of firing a gun.
          • Particles normally consist of Pb (lead), Sb (antimony) and Ba (barium).
          • New ammunition: environmentally friendly (no Sb).
  • The proportion of elements present in GSR differ slightly and databases of GSR from different manufacturers can be used to identify what ammunition was used in a crime. GSR is often found on criminals and also on victims if shot at close range.
  • X-ray Mapping
  • X-ray analysis of paint fragments The combined (a) backscatter image and X-ray maps of (b) Au, (c) Ba (d) Ca Different layers of paint can be identified
  • EDS = Energy Dispersive Spectroscopy WDS = Wavelength Dispersive Spectroscopy X-ray Detection
  • EDS WDS
  • 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
  • Cut-away diagram showing the construction of a typical EDS detector. FET Crystal Window Collimator
  • Lithium doped Silicon (SiLi) crystal detector acts as a semiconductor that carries current in a rate proportional to the number of ionization events and acts as an indirect measurement of the energy contained in the X-ray .
  • Absorbed X-rays create an ionization event similar to that of a scintillator
  • Each ionized atom of silicon absorbs 3.8 eV of energy, so an X-ray of 3.8 KeV will ionize approximately 1000 silicon atoms .
  • Collimator to limit BSE and stray X-rays Window usually made of beryllium (limited to sodium, atomic number 11) or thin plastic to detect down to boron (Atomic number 5) protects cooled crystal from air. FET Crystal Window Collimator
  • Detector : crystal silicon wafer with lithium added in. For each 3.8 eV from an X-ray, produce an electron and hole. This produces a pulse of current, the voltage of which is proportional to the X-ray energy. Must keep the crystal at LN temperature to keep noise to a minimum. FET : The field effect transistor is positioned just behind the detecting crystal. It is the first stage of the amplification process that measures the charge liberated in the crystal by an incident X-ray and converts it to a voltage output. FET Crystal Window Collimator
  • Multichannel Analyzer (MCA) The changes in conductivity of the SiLi crystal can be counted for a given time and displayed as a histogram using a multichannel analyzer.
  • Multichannel Analyzer (MCA) MCA consists of an analog to digital converter which “scores” the analog signal coming from the field effect transistor (FET). Newer systems employ a digital pulse processor which converts the signal on the fly Then Now
  • Factors affecting signal collection Distance between detector and X-ray source Angle at which detector is struck Volume of signal collected.
  • For a given angle of electron incidence, the length of the absorption path is directly proportional to the cosecant of the take-off angle, φ Take-off Angle
  • Solid Angle The solid angle Ω of a detector is defined as angle of the cone of signal entering the detector. The greater the size of the detector surface area the greater will be the solid angle.
  • Larger SiLi crystals will be able to sample a larger volume of signal (better Ω ) but because of imperfections in the crystal they have slightly greater noise and thus slightly lower resolution.
  • One can also increase the solid angle by placing the detector closer to the source. One then tries to maximize both the solid angle and the take-off angle.
  • One reason that the final lens of an SEM is conical in shape is so that the EDS detector can be positioned at a high take-off angle and inserted close to the specimen for a high solid angle.
  • William Henry Bragg 1862 – 1942 Nobel Prize in Physics 1915 X-ray diffraction in a crystal. Like an electron beam an X-ray has its own wavelength which is proportional to its energy
  • Crystal: A solid formed by the solidification of a chemical and having a highly regular atomic structure. May be composed of a single element (C = diamond) or multiple elements.
  • Cubic Hexagonal
  • If a wavelength enters a crystal at the appropriate angle it will be diffracted rather than being absorbed or scattered by the crystal
  • For a given wavelength λ there is a specific angle θ (Bragg’s angle) at which diffraction will occur. Bragg’s angle is determined by the d-spacing (interplanar spacing) of the crystal and the order of diffraction (n = 1, 2, 3….).
  • A WDS detector takes advantage of the fact that an X-ray of a given wavelength can be focused by a crystal if it encounters the crystal at the proper Bragg’s angle. To better accomplish this crystals are bent and ground to form a curved surface which will bring all the diffracted X-ray wavelengths to a single focal point, thus the crystal acts as a focusing lens.
  • To change the Bragg’s angle the diffracting crystal and detector can be moved together relative to the stationary specimen along a circle known as the Roland Circle .
  • WDS detectors are quite large and must be positioned around the specimen chamber at an angle to take advantage of maximum take-off angle and maximum solid angle
  • A microprobe is a specialized SEM that is outfitted with an EDS detector and array of several WDS detectors.
  • Different diffracting crystals can only diffract certain wavelengths (even with the changes in Bragg’s angle) so an array of detectors must be used if one is to be able to detect K, L, and M events for many different elements. Since WDS detectors do not need to be cooled they are windowless and can detect down to Berylium LiF = Lithium fluoride; PET =Pentaerythritol; and TAP = Thallium acid phthalate.
  • Specimen preparation for WDS Samples must be conductive since high KeV is used (Carbon coating if not naturally conductive) Samples must be flat (polished) as geometry of sample to detector is crucial and also minimizes artifacts when doing quantitative measurements.
  •  
  • A comparison of two spectra collected with EDS and WDS shows how peak overlap and energy spread can serve to obscure the information in an EDS spectrum
  • 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 correction of the data
  • 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 midpoint of another.
  • 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.
  • Applications of X-ray Microanalysis Secondary Electron image
  • EDS can be added as a component of a TEM Requires an angled detector (for take-off angle) and scan coils in the column to function as a Scanning Transmission Electron Microscope or STEM.
  • EDS can be used to identify elements present vacuoles or inclusions. Must take into account elements present in the embedding medium