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)

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)

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)