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Providing cutting edge microscopy services and solutions for the life and physical sciences X-ray Analysis in the SEM X-ray Analysis in the SEM Operation X-ray analysis in a scanning electron microscope (SEM) provides quantitative information about the elemental composition of the sample. The xrays are produced when the electrons hit the sample surface. As a consequence the technique can be used to image the spatial variations in composition within the sample. Electron column Liquid nitrogen reservoir to keep detector cooled Each of the SEMs in CfAM is equipped with Xray analysis systems. The details of each system vary from instrument to instrument but each is an Oxford Instruments INCA system. Each SEM is equipped with an energy dispersive detector. The environmental SEM also has a wavelength dispersive system which runs in parallel with the energy dispersive detector. The key difference is the resolution, the wavelength system is considerably higher (peak widths as low as 2eV) than the energy dispersive detector (~ 150eV). However, this higher resolution comes at the price of much longer data accumulation times. X-ray detector The INCA systems have a powerful software system which is integrated with the hardware of the SEM. Since the X-rays are formed by the electron beam interaction with the sample surface, what ever area of the sample being imaged is analyzed. This allows the SEM to perform elemental analysis in very selected areas. The X-rays are emitted from a depth equivalent to how deep the secondary electrons are formed. Depending on the sample density and accelerating voltage of the incident beam, this is usually from 1/2 to 2 microns in depth. Detectibility limits can be as low as 0.2% for the higher atomic number elements. It is also possible to map the elements found in an SEM image by X-ray analysis. The INCA system can provide overlay maps mimics the SEM image, except the contrast is formed by the strength of the elemental composition. As well as maps, variations in compositions along lines or in selected areas can be displayed. The central image (right) compares the spectrum obtained with an energy dispersive detector with that from a wavelength dispersive system.. The INCA energy dispersive system can detect x-rays from all elements from atomic number 5 (boron) up to 92 (uranium) and analyze them simultaneously. How does it work? The incident electron beam is composed of highly energized electrons. If one of these electrons collides with a sample atom electron, it will knock the electron out of its shell. This electron is called a secondary electron and is weak in energy (nearly 100 volts). If these secondary electrons are close enough to the sample surface, they can be collected to form an SEM image. How does it work continued? If the innermost shell (the K shell) electron of an iron atom is replaced by an L shell electron, a 6.4 keV K alpha X-ray is emitted from the sample. Whereas if the electron is replaced by an M shell electron or an L shell electron, a 7.057 keV K beta or a 0.704 keV L alpha X-ray is emitted. Since lower atomic number elements have fewer filled shells, they have fewer X-ray peaks. Carbon for example, has only one peak, a K alpha X-ray at 0.282 keV. When the incident beam passes through the sample creating secondary electrons, it leaves thousands of the sample atoms with holes in the electron shells where the secondary electrons used to be. If these "holes" are in inner shells, the atoms are not in a stable state. To stabilize the atoms, electrons from outer shells will drop into the inner shells, however, because the outer shells are at a higher energy state, to do this the atom must lose some energy. It does does this in the form of X-rays. Schematics from www.seallabs.com Essentially, each element has characteristic X-ray line(s) that allow a sample's elemental composition to be identified by a nondestructive technique. For more information contact 0118 378 6118 [email protected] or www.reading.ac.uk/cfam