We once owned this fine instrument; it has since been sold and is to be installed in California, for the use of another consulting firm, and is also to be made available to Berkeley students in Archaeology.  We include this description as a tribute to the makers of this instrument and also to Mr. Ken Converse of Quality Images, who maintained the instrument in great condition during the entire time that it was in Amenex hands.

Our former ETEC Autoprobe is a scanning electron microscope that is dedicated to the elemental analysis of solid specimens by wavelength-dispersive X-ray spectroscopy (WDXA). There are three diffractometer-type spectrometers that make use of the wave nature of X-ray photons. There is also a Kevex energy-dispersive spectrometer that makes use of the particle nature of the X-ray photons (EDXA). The wavelength-dispersive spectrometers can detect any element above nitrogen in the periodic table, whereas the energy-dispersive technique can detect elements above oxygen. The EDXA technique is fast but only performs semi-quantitatively, whereas WDXA can be quite accurate for even small amounts of an element.

The wavelength-dispersive spectrometers can be linked to the image-forming process so as to make elemental maps of the same area that is scanned by the electron beam. That capability is especially handy for finding out the microscopic distribution of elements in a structure. The spatial resolution of the WDXA mapping technique is considerably less than that of the electron beam, so the mapping technique works best for magnifications between about 150X and 2000X. The elemental resolution of the WDXA technique is essentially perfect, because every element can be separated spectrographically from its neighbors; there are no overlapping wavelengths that would interfere with distinguishing one element from another.

For the energy-dispersive spectrometer the situation is different, because the X-ray energies of different electronic transitions in the inner shells of the elements so sometimes overlap. In such cases, we use WDXA to distinguish between the two interfering elements.

X-ray generation and electron imaging: X-rays are produced when a high-energy electron beam strikes a target (i.e., the specimen) for several reasons. One is that the energy is suddenly given up and dissipated; this produces photons of a wide range of energies (and wavelengths) known as white radiation, which forms the background radiation, of no use for distinguishing anything about composition. Some of the elctrons strike inner electrons in the shells of the elements in the target, knocking them eitherout of the atoms altogether or at least to higher-energy orbits. When these energetic electrons (or others) fall back into the lower energy states, radiation that is characteristic of the element is produced, and its energy and wavelength can be used to measure or identify that element. Some of the electrons that strike the target simply bounce back off the target; these are called backscattered electrons. The backscattered electrons are detected close to the specimen and are used for imaging. The backscattered electron image is sensitive to atomic number, because elements with greater numbers of electrons (higher in the periodic table) scatter the incident electrons more strongly. The heavier elements therefore appear brighter in the backscattered electron image. Electrons to which so much energy have been imparted that they escape the target are called secondary electrons; they do not follow ballistic paths like the backscattered electrons. they are collected by a different detector to form a secondary electron image which is more sensitive to the surface topography of the target.

Useful specimens and specimen sizes: The ETEC's stage is designed to accommodate four, one-inch-tall, one-inch-diameter polished specimens, but it can also take a pair of one-inch-tall, 1.5 inch diameter specimens instead. Shorter specimens can be handled easily, but taller ones may get in their own way. Long, thin specimens can be examined obliquely if they are two inches in length or less. Awkward specimens might be placed in the microscope's viewing chamber but the mobility will be extremely limited if the specimen can be imaged at all. In such cases we resort to the making of plastic replicas of the specimens which can be examined after evaporatively coating them with a refractory material such as carbon, gold, gold-palladium, aluminum, or just about any nontoxic element that can be evaporated more easily than the refractory tungsten used as the heating element.

Evaporative coating: Is used to preserve the surface of specimens which either do not conduct electrons or which degrade under electron bombardment. Nonconducting materials such as silica must be coated with a thin conductive film (usually carbon) to carry off the electron current from the incident beam; if not done, the electrons accumulate right where they strike, charging the surface and creating an unwanted lens or reflector which ruins image quality. Delicate materials such as cellulose acetate replicas can be imaged better and without damage if they are first coated with aluminum. The same aluminum coating also faciliates examination with reflected light in a metallograph. The evaporative coating process is carried out in a high vacuum within which the atoms evaporated from the heated filament fly outward in radial paths without striking any gaseous atoms in the remaining atmopsphere inside the coating chamber. The vacuum must be good enough that the mean free path within the chamber is longer than the dimensions of the chamber. The evaporator therefore has a pumping system just as good as the electron microscope's. Specimens which give off excessive gases are therefore difficult to coat and may be impossible to image as well. That leaves out volatile liquids and solids which degrade into gases under electron bombardment. Oils have been imaged, however inadvertently, so materials with low vapor pressures can indeed be examined even if they are amorphous or liquid.

Image formation: Is accomplished in the scanning electron microscope by demagnifying an image of the hot filament in the electron gun at the top of the instrument and bringing that spot into focus on the specimen. the smaller the defocused spot, the better the spatial resolution of the microscope. The focusing and lens actions are mostly done with magnetic lenses because the electron is a charged particle, and charged particles are deflected when they move through magnetic fields. The electron spot is scanned back and forth across the specimen surface with a set of beam-deflection coils which magnetically deflect the spot. Another, equivalent spot is scanned by the same time-varying signals across the phosphor of a cathode ray tube in the same raster pattern in order to map out the varying signal produced by the electron detector as variations in brightness on the viewing screen. That produces the scanned image.

Magnification: Is achieved simply by scanning the cathode-ray spot over larger distances on the viewing screen than the electron spot is scanned on the specimen. Low magnifications are therefore more difficult to achieve than high magnifications because the beam deflection coils have to deflect the electron beam more on the specimen at low magnifications; the cathode-ray beam always scans the same area, so it does not care what magnification is operative.

Spatial resolution: Is limited by the ability of the condensor and objective lenses in the microscope column to bring the image of the filament into sharp focus on the target. Magnetic lenses for electron microscopes work at very small apertures compared to those of optical microscopes because concentrated magnetic fields can have only simple shapes. Our ETEC Autoprobe has a useful range of magnifications from as low as 30X to as high as 5000X.

Depth of field: Is spectacular in any scanning electron microscope because of the necessarily tiny numerical aperture at which the objective lens functions. Images of fly eyes come to mind, of course, but fracture surfaces which are impossible to visualize with the light microscope become fields of towering mountain peaks in the scanning electron microscope.