SEM: Variable Pressure (VP) Imaging

The VP detector is the front-most on the left. This specialty detector is designed to give images much like the SE detector, but it works under (as it’s variously called) “variable pressure” “environmental” or “low vacuum” conditions. One advantage of VP conditions is that the samples don’t have to be coated, because the ambient gas, ionized by secondary and backscattered electrons, carries away the beam charge. The disadvantage is that the ambient gas scatters the electron beam, reducing contrast and resolution. The detector works by sensing light emitted by the ionized gas. For the best resolution, use the shortest practical working distances and be sure to read about VP SEM methods. You also need to insert an aperture at the bottom of the conical pole magnet.

Because the VP detector senses light, it can also be used as a cathodoluminescence detector under high-vacuum conditions on carbon-coated samples. Is your sample luminescent? Switch to the VP detector and see! It’s much easier to use than the specialized CL system, but much less sensitive and only produces grayscale images.

The VP detector is on the left, in front of the SE detector.

Coprolite matrix. Thin fibers are fungal hyphae, spherical objects may be casts and molds of bacterial cell walls. Sorry about the quality of the image, I’m sure better can be gotten. Like with other forms of high-quality imaging, experience and patience are important for the best results.

This is a polished, carbon-coated grain mount of cathodoluminescent materials (calcite, strontianite, witherite). The image was made with the VP detector sensing luminescence. Among other things, calcite, quartz, feldspars, apatite, and zircon can be cathodoluminescent. Try it out, you might be surprised. Here is the detector sensitivity curve.