Thus, characteristic X-rays are produced for each element in a mineral that is "excited" by the electron beam. As the excited electrons return to lower energy states, they yield X-rays that are of a fixed wavelength (that is related to the difference in energy levels of electrons in different shells for a given element). X-ray generation is produced by inelastic collisions of the incident electrons with electrons in discrete ortitals (shells) of atoms in the sample. Secondary electrons and backscattered electrons are commonly used for imaging samples: secondary electrons are most valuable for showing morphology and topography on samples and backscattered electrons are most valuable for illustrating contrasts in composition in multiphase samples (i.e. These signals include secondary electrons (that produce SEM images), backscattered electrons ( BSE), diffracted backscattered electrons ( EBSD that are used to determine crystal structures and orientations of minerals), photons ( characteristic X-rays that are used for elemental analysis and continuum X-rays), visible light ( cathodoluminescence-CL), and heat. Fundamental Principles of Scanning Electron Microscopy (SEM)Īccelerated electrons in an SEM carry significant amounts of kinetic energy, and this energy is dissipated as a variety of signals produced by electron-sample interactions when the incident electrons are decelerated in the solid sample. The design and function of the SEM is very similar to the EPMA and considerable overlap in capabilities exists between the two instruments. The SEM is also capable of performing analyses of selected point locations on the sample this approach is especially useful in qualitatively or semi-quantitatively determining chemical compositions (using EDS), crystalline structure, and crystal orientations (using EBSD).
Areas ranging from approximately 1 cm to 5 microns in width can be imaged in a scanning mode using conventional SEM techniques (magnification ranging from 20X to approximately 30,000X, spatial resolution of 50 to 100 nm).
In most applications, data are collected over a selected area of the surface of the sample, and a 2-dimensional image is generated that displays spatial variations in these properties. The signals that derive from electron-sample interactions reveal information about the sample including external morphology (texture), chemical composition, and crystalline structure and orientation of materials making up the sample.
The scanning electron microscope (SEM) uses a focused beam of high-energy electrons to generate a variety of signals at the surface of solid specimens. A typical SEM instrument, showing the electron column, sample chamber, EDS detector, electronics console, and visual display monitors.
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