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
Kα Kβ
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.
Certain events such as Mα, Lβ, and Kγ are only possible in atoms of sufficient atomic weight
There are a wide variety of subsets ofX-rays since each electron shell has multiple orbitals
An X-ray spectrum for a sample is composed of all An 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) and These will differ in terms of energies (KeV) and probabilities (likelihood) scored as number of such probabilities (likelihood) scored as number of such signals collected over a given period of time.signals collected over a given period of time.
# C
ou
nts
X-ray Energy in KeV
Each element has a family of characteristic X-rays Each element has a family of characteristic X-rays associated with it associated with it
Positive identification of an element is best done by Positive identification of an element is best done by evaluating the entire family of peaks for a given evaluating the entire family of peaks for a given
element.element.
"Bremsstrahlung" means "braking radiation" and comes from "Bremsstrahlung" means "braking radiation" and comes from the original German to describe the radiation which is emitted the original German to describe the radiation which is emitted when electrons are decelerated or "braked" when they interact when electrons are decelerated or "braked" when they interact with the specimen. with the specimen.
Although they contribute to the total X-ray signal they contain Although they contribute to the total X-ray signal they contain no useful information because their energies are nonspecific no 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 .
Bremsstrahlung X-rays are the major part of the Bremsstrahlung X-rays are the major part of the continuum X-ray signal that can escape from the continuum X-ray signal that can escape from the deepest portion of the interaction region. deepest portion of the interaction region.
Chrysotile Asbestos FibersChrysotile Asbestos Fibers
Bullet fragments (blue) can be identified on cloth Bullet fragments (blue) can be identified on cloth fibers and distinguished from other metal pieces by fibers and distinguished from other metal pieces by their elemental compositiontheir elemental composition
Gunshot Residue (GSR) AnalysisGunshot Residue (GSR) Analysis
Gunshot Residue (GSR) AnalysisGunshot Residue (GSR) Analysis
•Particles are very Particles are very characteristic, therefore characteristic, therefore presence of these particles presence of these particles forms evidence of firing a forms evidence of firing a gun. gun.
•Particles normally consist Particles normally consist of Pb (lead), Sb (antimony) of Pb (lead), Sb (antimony) and Ba (barium). and Ba (barium).
•New ammunition: New ammunition: environmentally friendly (no environmentally friendly (no Sb). Sb).
The proportion of The proportion of elements present in elements present in GSR differ slightly GSR differ slightly and databases of and databases of GSR from different GSR from different manufacturers can manufacturers can be used to identify be used to identify what ammunition what ammunition was used in a crime.was used in a crime.GSR is often found GSR is often found on criminals and also on criminals and also on victims if shot at on victims if shot at close range.close range.
X-ray MappingX-ray Mapping
X-ray analysis of paint fragmentsX-ray analysis of paint fragments
The combined The combined (a) backscatter image (a) backscatter image and X-ray maps of and X-ray maps of (b) Au, (b) Au, (c) Ba (c) Ba (d) Ca (d) Ca
Different layers of Different layers of paint can be identifiedpaint can be identified
EDS = Energy EDS = Energy Dispersive Dispersive Spectroscopy Spectroscopy
WDS = Wavelength WDS = Wavelength Dispersive Dispersive Spectroscopy Spectroscopy
X-ray DetectionX-ray Detection
ED
S
WD
S
Pulse ProcessorMeasures the electronic signals to determine the energy of each X-ray detected
X-ray DetectorDetects and convertsX-rays into electronic signals
AnalyzerDisplays and interpretsthe X-ray data
Cut-away diagram showingCut-away diagram showingthe construction of a typical the construction of a typical EDS detector.EDS detector.
FET
Crystal
Window
Collimator
Lithium doped Silicon (SiLi) crystal detectorLithium doped Silicon (SiLi) crystal detectoracts as a semiconductor that carries current in a rate acts as a semiconductor that carries current in a rate proportional to the number of ionization events and proportional to the number of ionization events and acts as an indirect measurement of the energy acts as an indirect measurement of the energy
contained in the X-raycontained in the X-ray..
Absorbed X-rays create an ionization event Absorbed X-rays create an ionization event similar to that of a scintillator similar to that of a scintillator
Each ionized atom of silicon absorbs 3.8 eV of Each ionized atom of silicon absorbs 3.8 eV of energy, so an X-ray of 3.8 KeV will ionize energy, so an X-ray of 3.8 KeV will ionize
approximately 1000 silicon atomsapproximately 1000 silicon atoms.
CollimatorCollimator to limit BSE and stray X-rays to limit BSE and stray X-rays
WindowWindow usually made of beryllium (limited to sodium, atomic usually made of beryllium (limited to sodium, atomic number 11) or thin plastic to detect down to boron (Atomic number 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
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. This 3.8 eV from an X-ray, produce an electron and hole. This produces a pulse of current, the voltage of which is produces a pulse of current, the voltage of which is proportional to the X-ray energy. Must keep the crystal at LN proportional 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 the The field effect transistor is positioned just behind the detecting crystal. It is the first stage of thedetecting crystal. It is the first stage of theamplification process that measures the charge liberated in amplification process that measures the charge liberated in the crystal by an incident X-ray and converts it to a voltage the crystal by an incident X-ray and converts it to a voltage output.output.
FET
Crystal
Window
Collimator
Multichannel Analyzer (MCA)Multichannel Analyzer (MCA)
The changes in conductivity of the SiLi crystal can be The changes in conductivity of the SiLi crystal can be counted for a given time and displayed as a histogram counted for a given time and displayed as a histogram using a multichannel analyzer. using a multichannel analyzer.
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 which Newer systems employ a digital pulse processor which converts the signal on the fly converts the signal on the fly
ThenThen NowNow
Factors affecting signal collectionFactors affecting signal collection
Distance between detector and X-ray sourceAngle at which detector is struckVolume of signal collected.
For a given angle of electron incidence, the length of For a given angle of electron incidence, the length of the absorption path is directly proportional to the the 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
Solid AngleSolid Angle
The solid angle The solid angle ΩΩ of a detector is defined as angle of a detector is defined as angle of the of the conecone of signal entering the detector. The of signal entering the detector. The greater the size of the detector surface area the greater the size of the detector surface area the greater will be the solid angle. greater will be the solid angle.
Larger SiLi crystals will be able to sample a larger Larger SiLi crystals will be able to sample a larger volume of signal (better volume of signal (better ΩΩ) ) but because of but because of imperfections in the crystal they have slightly greater imperfections in the crystal they have slightly greater noise and thus slightly lower resolution.noise and thus slightly lower resolution.
One can also increase the solid angle by placing One 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 angle One then tries to maximize both the solid angle and the take-off angle.and the take-off angle.
One reason that the final lens of an SEM is conical in One reason that the final lens of an SEM is conical in shape is so that the EDS detector can be positioned at shape is so that the EDS detector can be positioned at a high take-off angle and inserted close to the a high take-off angle and inserted close to the specimen for a high solid angle.specimen for a high solid angle.
William Henry BraggWilliam Henry Bragg 1862 – 19421862 – 1942Nobel Prize in Physics Nobel Prize in Physics 19151915
X-ray diffraction in a crystal.X-ray diffraction in a crystal.Like an electron beam an X-ray Like an electron beam an X-ray has its own wavelength which is has its own wavelength which is proportional to its energyproportional to its energy
Crystal:Crystal: A solid formed by A solid formed by the solidification of a the solidification of a chemical and having a chemical and having a highly regular atomic highly regular atomic structure. May be structure. May be composed of a single composed of a single element (C = diamond) or element (C = diamond) or multiple elements.multiple elements.
CubicCubic HexagonalHexagonal
If a wavelength enters a crystal at the appropriate angle it will If a wavelength enters a crystal at the appropriate angle it will be diffracted rather than being absorbed or scattered by the be diffracted rather than being absorbed or scattered by the crystal crystal
For a given wavelength For a given wavelength λλ there is a specific angle there 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-spacing Bragg’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….).
A WDS detector takes advantage of the fact that an X-ray of a 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 given 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 form To better accomplish this crystals are bent and ground to form a curved surface which will bring all the diffracted X-ray a curved surface which will bring all the diffracted X-ray wavelengths to a single focal point, thus the crystal acts as a wavelengths to a single focal point, thus the crystal acts as a focusing lens. focusing lens.
To change the Bragg’s angle the diffracting crystal and To change the Bragg’s angle the diffracting crystal and detector can be moved together relative to the detector can be moved together relative to the stationary specimen along a circle known as the stationary specimen along a circle known as the Roland CircleRoland Circle. .
WDS detectors are quite large and must be positioned around WDS detectors are quite large and must be positioned around the specimen chamber at an angle to take advantage of the specimen chamber at an angle to take advantage of maximum take-off angle and maximum solid anglemaximum take-off angle and maximum solid angle
A microprobe is a specialized SEM that is outfitted A microprobe is a specialized SEM that is outfitted with an EDS detector and array of several WDS with an EDS detector and array of several WDS detectors.detectors.
Different diffracting crystals can Different diffracting crystals can only diffract certain wavelengths only diffract certain wavelengths (even with the changes in Bragg’s (even with the changes in Bragg’s angle) so an array of detectors angle) so an array of detectors must be used if one is to be able must be used if one is to be able to detect K, L, and M events for to detect K, L, and M events for many different elements. Since many different elements. Since WDS detectors do not need to be WDS detectors do not need to be cooled they are windowless and cooled they are windowless and can detect down to Beryliumcan detect down to Berylium
LiF = Lithium fluoride; PET =Pentaerythritol; and TAP = Thallium acid phthalate.
Specimen preparation for WDSSpecimen preparation for WDS
Samples must be conductive since high KeV is used Samples 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 of Samples must be flat (polished) as geometry of sample to detector is crucial and also minimizes sample to detector is crucial and also minimizes artifacts when doing quantitative measurements. artifacts when doing quantitative measurements.
A comparison of two A comparison of two spectra collected with spectra collected with EDS and WDS shows EDS and WDS shows how peak overlap and how peak overlap and energy spread can energy spread can serve to obscure the serve to obscure the information in an EDS information in an EDS spectrumspectrum
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 MicroanalysisApplications of X-ray Microanalysis
Secondary Secondary Electron imageElectron image
EDS can be added as a component of a TEMEDS can be added as a component of a TEMRequires an angled detector (for take-off angle) and Requires an angled detector (for take-off angle) and scan coils in the column to function as a Scanning scan coils in the column to function as a Scanning Transmission Electron Microscope or STEM.Transmission Electron Microscope or STEM.
EDS can be used to EDS can be used to identify elements present identify elements present vacuoles or inclusions. vacuoles or inclusions. Must take into account Must take into account elements present in the elements present in the embedding mediumembedding medium
