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ENERGY DISPERSIVE X-RAY SPECTROSCOPY

DESCRIPTION OF TECHNIQUE

Energy Dispersive X-Ray Spectroscopy (EDS or EDX) is a chemical microanalysis technique used in conjunction with scanning electron microscopy (SEM). The EDS technique detects x-rays emitted from the sample during bombardment by an electron beam to characterize the elemental composition of the analyzed volume.

EDS Spectrum from a nonmetallic inclusion in Steel 
When the sample is bombarded by the SEM's electron beam, electrons are ejected from the atoms comprising the sample's surface. The resulting electron vacancies are filled by electrons from a higher state, and an x-ray is emitted to balance the energy difference between the two electrons' states. The x-ray energy is characteristic of the element from which it was emitted.

When the sample is bombarded by the SEM's electron beam, electrons are ejected from the atoms comprising the sample's surface. The resulting electron vacancies are filled by electrons from a higher state, and an x-ray is emitted to balance the energy difference between the two electrons' states. The x-ray energy is characteristic of the element from which it was emitted.




The EDS x-ray detector measures the relative abundance of emitted x-rays versus their energy. The detector for all ASPEX products is typically a silicon drift detector (SDD). When an incident x-ray strikes the detector, it creates a charge pulse that is proportional to the energy of the x-ray and is then converted to a corresponding voltage pulse.  The signal is then sent to multichannel analyzer where the pulses are sorted by voltage value.  The energy, as determined from the voltage measurement for each incident x-ray, is sent to a computer for display and further data evaluation.  The spectrum of x-ray energy versus counts is evaluated to determine the elemental composition of the sampled volume.

The main advantage of the SDD is that it has much lower electronic noise than a planar device at short shaping times, i.e. at high count rates. Lower noise implies better resolution, particularly at low energies. With an SDD, the resolution is good even for large areas. Overall, the resolution of the SDD is better than that of the planar detector, but the advantages are particularly important at low energies, high count rates, and for larger areas.

ANALYTICAL INFORMATION

Elemental Composition Analysis - The sample X-ray energy values from the EDS spectrum are compared with known characteristic x-ray energy values to determine the presence of an element in the sample. Elements with atomic numbers ranging from that of Boron to Uranium can be detected simultaneously. The minimum detection limits vary from approximately 0.1 to a few weight percent, depending on the element and the sample matrix.

Elemental Mapping




Elemental Mapping
– The lateral “two dimension” position on a sample is measured for characteristic x-ray intensities. Variations in x-ray intensity at any characteristic energy values at each point indicate the relative concentration for the applicable element across the surface. One or more maps are recorded simultaneously using image brightness intensity as a function of the local relative concentration of the element(s) present. About 1 μm lateral resolution is possible.

EDX Line Scan

Line Profile Analysis - The SEM electron beam is scanned along a preselected line segment across the sample while x-rays are detected at discrete positions along the line. Analysis of the resulting x-ray energy spectrum at each position provides plots of the relative elemental concentration variance for each element as a function of those position-point values along the given path.


TYPICAL APPLICATIONS

•         Foreign material analysis

•         Corrosion evaluation

•         Coating or boundary layer composition analysis

•         Rapid material alloy identification

•         Small component material analysis

•         Phase identification and distribution

 

 

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