Comparing detectors
Four views of an iron drop in slag

Four SEM images of the same droplet of iron in slag

Comparison of different kinds of views of the same iron droplet in slag from Lyon Mountain, New York (17.3 Mb file)

More than just electron detectors

Detectors are the senses of a scanning electron microscope.

We humans perceive the color of an orange with our eyes, smell the fragrance of the spraying zest with our nose, hear the tearing of the leathery peel with our ears as we open the fruit, taste the sweet juice with our tongues, and feel the stickiness on our lips as we savor the whole experience.

Similarly, the SEM can see different aspects of materials by using different kinds of detectors.  This composite image of a droplet of crystallized iron in slag illustrates how difference kinds of detectors can work together to create a more complete picture of a sample.

(For the non-geologists)  Slag is a waste product generated when we separate metal from ore rocks.  Most metals occur in rock bonded to other elements like sulfur or oxygen.  One process for separating the metal from the other elements to form pure metal is called smelting.  Concentrated ore minerals from rock are heated with quartz and other ingredients to form an artificial lava.  The metals sink to the bottom as a heavy liquid metal and the elements we don't want either burn off or stay dissolved in the artificial lava.  The artificial lava with the unwanted elements is dumped so that it quenches into a glassy artificial rock called slag.  (17.3 Mb original image)

Secondary Electron Detector (low keV)

The SE2 detector mostly “sees” electrons dislodged from the very top surface of the sample, so the SE2 detector is great for seeing the shapes of the outermost surface of the sample.  We can adjust how deeply the electron beam penetrates into the sample by adjusting the beam energy, measured in kilo electron volts (keV).  When we only want to see the tiniest details on the topmost surface, we use a 1 keV or 2 keV beam.

This image was taken using a 5 keV beam because the beam can traverse the 8.5 mm gap between the column and the sample to give good surface detail (WD in the legend stands for Working Distance)With gentle beams like this, most features in the image are similar in brightness (except the holes). (10.1 Mb original image)

SEM image of surface of slag sample

Low-energy secondary electron image of Lyon Mountain slag showing surface texture (10.1 Mb file)

Secondary Electron Detector (high keV)

This is another image taken using the same SE2 secondary electron detector in the first image, but using a higher energy beam (15 keV).  Our SEM goes up to a maximum 30 keV, but we never use our SEM at that high of an energy setting for surface imaging because so much of the beam penetrates deeply into the sample.

You probably note that the heart-shaped blob and round blob are lighter gray in this image.  That is because those are materials made of higher atomic number atoms (iron) than the surrounding stuff (mostly magnesium and silicon).  High atomic number elements cause more backscattering (see below), and the SE2 detector cannot tell the difference between secondary and backscatter electrons, so heavy elements appear brighter. (10.6 Mb original image)

high-energy secondary electron image of slag

High-energy secondary electron image of the surface of Lyon Mountain slag showing surface detail and some backscatter shading (10.6 Mb file)

Backscatter Electron Detector

The BSD detector sees only electrons that shoot into the sample from the beam and get thrown back with equal energy – a process called “backscattering.”  Materials made of elements with higher atomic numbers have bigger atomic nuclei with more protons, so those materials boomerang (backscatter) more electrons to the BSD detector.  For this reason, different crystals will appear different shades of gray, depending on their composition.

Note how we lost most of the surface texture detail with the BSD detector that was so nicely shown by the SE2 detector.  The bright heart and circle are clearly made of something much heavier than the surrounding slag.  These are two frozen droplets of pure iron that did not sink out of the artificial lava (slag).  Smelting is an imperfect process.  If you look carefully, you can see that there are crystals inside the iron heart that were invisible to the SE2 detector (9.6 Mb original image)

backscatter SEM image of slag

Backscatter electron image of Lyon Mountain slag showing how detector highlights differences in composition (9.6 Mb file)

EDS chemical composition map

In addition to electrons shooting off the sample, the electron beam also causes atoms within the sample to glow x-rays!  While that may sound dangerous, the amount of x-rays coming off the sample is infinitesimal, and there's a 3-cm (1¼-inch) thick wall of steel around the sample that absorbs it all.

As with visible light occurring as a spectrum of colors related to the energy level of each color, x-rays also come in a spectrum of "colors" that differ in energy.  Each element glows a specific "color" of x-ray, so the EDS x-ray detector senses which elements are present in the sample by what "colors" of x-rays glow from the surface.  The EDS x-ray detector also measures how brightly each "color" of x-ray glows, from which information the computer calculates the chemical composition.

This is an example of multi-element chemical composition map of the same spot in the images above.  Each color corresponds to a high concentration of a specific element:  Fe = iron, Ti = titanium, K = potassium, and Mg = magnesium.  The composition map also records the concentration of every other element present, but I only had it show these four elements on this map to highlight the different types of crystals present.

The hot pink heart is pure iron.  The small, square, teal-colored crystals are titanium metal that appears to have crystallized from the molten liquid and stuck to the sides of the iron droplet.  The green crystals are a potassium-aluminum silicate called feldspar, and the blue crystals are a magnesium silicate called pyroxene.  (3.8 Mb original image)

X-ray analysis image of slag showing chemical composition variations

EDS X-ray map of the chemical composition variations across surface of Lyon Mountain slag showing iron droplet with titanium ring in silicate matrix (3.8 Mb image)