Initial Publication Date: May 17, 2007

Back-scattered Electron Detector (BSE)

John Goodge, University of Minnesota at Duluth.

Interaction of an accelerated electron beam with a sample target produces a variety of elastic and inelastic collisions between electrons and atoms within the sample. Elastic scattering changes the trajectory of the incoming beam electrons when they interact with a target sample without significant change in their kinetic energy. In this case, we can think of elastic scattering in terms of a "billiard-ball" model of small particles (electrons) colliding with large particles (atoms). Larger atoms (with a greater atomic number, Z) have a higher probability of producing an elastic collision because of their greater cross-sectional area. Consequently, the number of backscattered electrons (BSE) reaching a BSE detector is proportional to the mean atomic number of the sample. Thus, a "brighter" BSE intensity correlates with greater average Z in the sample, and "dark" areas have lower average Z. BSE images are very helpful for obtaining high-resolution compositional maps of a sample and for quickly distinguishing different phases. They are often used in conjunction with spot probe analyses by either EDS or WDS methods.

How it Works—BSE

BSE detectors are commonly integrated into either an SEM or EPMA instrument. They are typically placed above the sample in the sample chamber based on the scattering geometry relative to the incident beam. BSE detectors are solid-state devices, often with separate components for simultaneous collection of back-scattered electrons in different directions. Detectors above the sample collect electrons scattered as a function of sample composition, whereas detectors placed to the side collect electrons scattered as a function of surface topography.

Strengths

BSE images can be obtained nearly instantly, depending on scan rate, and at any magnification within the instrument range. Thus, they are a quick means of determining the number of phases in a material and their mutual textural relationships.
BSE images can be captured on film or digitally and serve as sample maps for locating spot analyses.

Limitations

BSE images are limited to a grayscale range because they only record one variable, average Z (a combination of all the elements in a sample). Thus, they do not convey as much information as can be obtained by elemental composition mapping using EDS or WDS x-ray detectors.

Results

In most cases, a user scans a polished sample in back-scatter mode to produce a BSE image of the surface. The gain of the detector can be tuned to maximize contrast between phases with similar Z. In silicate rocks, for example, quartz and feldspars will appear dark or nearly black, Fe-Mg silicates will be brighter, and Fe-Ti oxides or rare-earth minerals will be very bright. BSE images can also be converted to false color, which often helps the human eye distinguish subtle variations by expanding the range of visible hues.

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Colorized BSE image. Details

References

Krinsley, David H., 1998, Backscattered scanning electron microscopy and image analysis of sediments and sedimentary rocks,Cambridge University Press
Goldstein, J. (2003) Scanning electron microscopy and x-ray microanalysis. Kluwer Adacemic/Plenum Pulbishers, 689 p.
Reimer, L. (1998) Scanning electron microscopy : physics of image formation and microanalysis. Springer, 527 p.
Egerton, R. F. (2005) Physical principles of electron microscopy : an introduction to TEM, SEM, and AEM. Springer, 202.
Clarke, A. R. (2002) Microscopy techniques for materials science. CRC Press (electronic resource)