3.5.2. Spectrometer Geometry

Unlike an X-ray fluorescence spectrometer (XRF) that uses a goniometer, microprobe spectrometers use linear-focusing drives. The sample, crystal, and detector must lie on a circle called the Rowland circle and remain on it for all wavelengths of interest to focus X-rays efficiently. Since the sample is fixed in place, the crystal and detector must both move to remain on the Rowland circle (Figure 3.5.2a).

Figure 3.5.2a. The geometry of a linear, fully focusing X-ray spectrometer. This type of spectrometer is used in most commercial wavelength-dispersive electron microprobes (after Williams 1987).

This arrangement is mechanically complex, since to maintain the appropriate geometry the crystal must rotate as source-crystal distance changes. However, because the Rowland circle radius and the take-off angle do not change, the mechanical problems are not insurmountable. Precision internal gearing controls the movement of the detector along a complex path and rotation of the detector. The crystal and detector movement is driven by worm gears connected to stepping motors. Each step corresponds to 10000 x sinq. Thus a position where sinq = 0.45 would be entered as 45000.

A larger diameter Rowland circle is more efficient than a small diameter circle in providing peak separation at given angle. The diameter of the Rowland circle of the Cameca MBX and SX-50 microprobes is 16 cm; the JEOL 733 Superprobe has a 14 cm circle. Thus, the Cameca microprobes provide better peak resolution. However, larger Rowland circles require larger spectrometer housings with the attendant vacuum and positioning problems. A very large Rowland circle could provide better peak resolution, but it is not practical.

Spectrometers may be mounted vertically (Figure 3.5.2b) or inclined (Figure 3.5.2c) relative to the sample.

Figure 3.5.2b. Vertical spectrometer on the Cameca MBX microprobe. The x-ray path is shown in green and the Rowland circle is dashed.
	Figure 3.5.2c. Inclined spectrometer on the Cameca MBX microprobe. The x-ray path is indicated in green. The secondary-electron detector is shown beneath the spectrometer.

Inclined spectrometers (Figure 3.5.2d) are less sensitive to changes in sample focus and topography (Figure 3.5.2e), but, because of their orientation, only a few can be mounted together around the electron column.

	Figure 3.5.2d. Photograph of the interior of an inclined spectrometer. X-rays emerge from the sample through the circular hole into the column and are diffracted by the analyzing crystal (located below the gray cable) into the detector located in the center of the photograph. The high voltage feed and gas-flow lines trail off from the detector towards the lower right corner.
Figure 3.5.2e. The effect of a small variation in sample height on the focusing of the crystal is very small in an inclined spectrometer, as is the corresponding variation of the Bragg angle. For clarity, only the front part of the Rowland circle is shown (after Maurice et al. 1979).

Vertical spectrometers (Figure 3.5.2f) are more sensitive to changes in focus (Figure 3.5.2g), but many can be mounted together. For example, the old Smithsonian microprobe had 12 vertical spectrometers! NAU's MBX microprobe has one inclined spectrometer (#1) and two vertical spectrometers (#2, #3).