Physical Principles of Electron Microscopy: An Introduction to TEM ...

Analytical Electron Microscopy 169
electron
beam
specimen
x-rays
Rowland
circle
+3kV
detector
crystal
Figure 6-6. Schematic diagram of an XWDS system. For simplicity, the Bragg-reflecting
planes are shown as being parallel to the surface of the analyzing crystal. The crystal and
detector move around the Rowland circle (dashed) in order to record the spectrum.
mechanical force) into a slightly-curved shape, which also provides a degree
of focusing similar to that of a concave mirror; see Fig. 6-6.
Although XWDS can be performed by adding the appropriate mechanical
and electronic systems to an SEM, there exists a more specialized
instrument, the electron-probe microanalyzer (EPMA), which is optimized
for such work. It generates an electron probe with substantial current, as
required to record an x-ray spectrum with good statistics (low noise) within a
reasonable time period. The EPMA is fitted with several analyzer/detector
assemblies so that more than a single wavelength range can be recorded
simultaneously.
6.7 Comparison of XEDS and XWDS Analysis
A big advantage of the XEDS technique is the speed of data acquisition, due
largely to the fact that x-rays within a wide energy range are detected and
analyzed simultaneously. In contrast, the XWDS system examines only one
wavelength at any one time and may take several minutes to scan the
required wavelength range. The XEDS detector can be brought very close
(within a few mm) of the specimen, allowing about 1% of the emitted x-rays