Comparing detectors
Three views of a zircon crystal

Three images of the same crystal viewed with different detectors

Comparison of three views of the same zircon crystal viewed with different detectors in the SEM (8.7 Mb image)

Comparing 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 crystal of the mineral zircon compares the same crystal viewed with the three most common types of detectors in any scanning electron microscope.

(For the non-geologists)  Zircon is a mineral that crystallizes from molten magma or grows during recrystallization of rocks in the heat and pressure of metamorphism. The composition of pure zircon is ZrSiO4, but the crystal structure usually incorporates minor amounts of uranium, rare earth elements, hafnium, niobium, scandium, yttrium, and titanium.  Those “impurities” can subtly change some of the characteristics of zircon, depending on the type and amount of impurities present.  (8.7 Mb original image)

Secondary Electron Detector

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.  You can see in this secondary electron image that the zircon crystal has a couple of holes eroded out of the surface, and that the zircon is harder than the surrounding material (because the zircon crystal casts a shadow around the edges since it’s a high mesa).

We have two secondary electron detectors.  This image was taken with the Everhart-Thornley detector located inside the sample chamber.  This is the typical detector in most SEMs.  We also have an in-lens detector that is located in the column that's best for really high magnification.   (2.1 Mb original image)

secondary electron image of a zircon crystal showing a hole in the middle

Secondary electron image of zircon crystal with hole in the middle (2.1 Mb image)

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.  Zircon has zirconium, which has a high atomic number (40), so backscatters more electrons and shows up very light gray compared to the surrounding grains.

We have two backscatter electron detectors.  This image was taken with the standard type detector (BSD1).  We also have an in-lens backscatter detector that is located in the column that allows us to select the energy level of electrons we detect.   (1.6 Mb original image)

backscatter electron image of a zircon crystal showing it's made of heavy elements

Backscatter electron image of the same zircon crystal showing composition differs from surrounding grains (1.6 Mb image)

Variable Pressure Secondary Electron Detector

The VPSE detector is used for observing the shapes of samples that would build up static electric charge because they're non-conductors.  This detector is for samples that are in some way special so we cannot coat them with a thin layer of conductive metal (e.g., an archeological artifact that is unique in the world, or some forensic evidence which cannot be modified in any way).

The VPSE detector actually “sees” photons of light rather than directly detecting electrons.  We let a little nitrogen gas into the sample chamber.  When the electron beam hits the sample, secondary electrons shoot off the surface and cause the gas to glow.  The VPSE detects that light.

Some materials directly glow visible light when struck by electrons - a property is called “cathodoluminescence”  (cathodo- refers to electrons, and -lumin refers to glowing light).  Zircon is a cathodoluminescent mineral.  Because zircon crystals incorporated different amounts of impurities each time they grow a new layer (like tree rings!), each growth layer glows differently.  An expert in crystal growth can tell a lot about the history of the crystal from this (e.g., times when the crystal started to partially dissolve, and times when it grew quickly).  A geochemist with a special lab for analyzing isotopes can even tell exactly how old each individual layer in the crystal is by detecting the concentrations of lead formed when the radioactive uranium decayed.   (2.2 Mb original image)

cathodoluminescence image of a zircon crystal showing concentric crystal growth rings

Cathodoluminescence image of the same zircon crystal showing growth zones within the crystal (2.2 Mb file)