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)

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)

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)

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)