Electron Microprobe Analysis and Scanning Electron by S. J. B. Reed

By S. J. B. Reed

Covers rules and strategies of EMPA and SEM for geological graduate scholars and postdoctoral staff.

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The working distance can be set by adjusting the final lens focus to the required setting, then moving the specimen stage vertically until the image is sharp. The focus setting depends on the accelerating voltage, and must be adjusted when this is changed. 4 Topographic images The principal function of the SEM is to produce images of three-dimensional objects (the main advantages over the optical microscope are greater depth of focus and higher resolution). Usually secondary-electron images are employed to show topographic contrast, but backscattered-electron images, though mostly used to show compositional differences, may also contain topographic information.

Is small, most being converted into heat in the target. The temperature rise ÁT can be estimated from the following expression: ÁT ¼ 4:8E0 i=ðkdÞ; (2:5) where E0 is the incident electron energy (keV), i the current (mA), k the thermal conductivity (W cmÀ1 KÀ1) and d the beam diameter (mm). For metals k is 20 Electron–specimen interactions 400 500 700 600 800 Wavelength (nm) (a) 450 500 550 600 Wavelength (nm) (b) Fig. 15. ; and (b) emission lines caused by rare earths (mainly Dy) in zircon. typically in the range 1–4 and under normal conditions ÁT is negligible.

The required current may therefore be obtained by adjustment of the condenser lenses, which, in a computercontrolled instrument with current monitoring, can be done automatically by means of a software command. ) Drift in beam current as a function of time is caused mainly by movement of the tip of the filament, which can be corrected using beam alignment coils (as discussed in the previous section). Such drift is a potential source of error in automated quantitative X-ray analysis sessions, which may extend over several hours.

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