Scanning Electron Microscopes Laboratory
The SEM instruments provide very high-resolution imaging, semi-quantitative elemental analysis, and qualitative X-ray elemental maps that combine to provide important morphological and compositional insights.
Two scanning electron microscopes (SEM) reside in ARES, the 5910LV and the 7600F. They both provide imaging and chemical analysis services for investigations of meteorites, lunar samples, IDPs (interplanetary dust particles) and experimental samples synthesized in ARES high-temperature and high-pressure experimental labs. Scanning electron microscopes produce two types of electron images - secondary electron images (SEI) that show topographic features (sample morphology) - and backscattered electron images (BEI) that show compositional features. The SEMs are also used to examine samples from the space shuttle surface that have been hit by high-velocity impactors. We endeavor to identify the nature of the impactor (micrometeoroid or orbital debris) based on the chemical composition of condensates or residues from the impacting particle.
The 5910LV microscope is easier to use and can accommodate larger samples. The new field emission SEM (7600F) produces ultra high-resolution electron images using a thermal field emission electron gun. This type of gun is a significant advance over earlier types of SEM electron guns because of the remarkably fine (~1-2 nm) electron beam delivered to the sample surface. The tightly focussed beam enables us to record electron images with 2-3 nm spatial resolution. The 7600F SEM includes two different SEI detectors and two BEI detectors. For both detector types, one is in-lens and one is in the sample chamber. The in lens detectors enable our researchers to position samples very close to the bottom of the electron column, and this allows the acquisition of images with a low-energy beam. The advantage of a low-energy beam is that very near-surface features are emphasized.
The 7600F is equipped with a SDD type x-ray detector system. This type of detector is a significant advance over earlier Si(Li) detectors in that it can acquire and process >100,000 x-ray counts per second. This high count rate permits us to produce high quality x-ray maps of planetary samples in reasonable times. In the example shown below, an x-ray map of a portion of a chondritic meteorite, Mg is red, Ca is green, and S is blue.