Electron-excited X-ray mapping is a key operational mode of the
scanning electron microscope (SEM) equipped with energy dispersive X-ray
spectrometry (EDS). The popularity of X-ray mapping persists despite the
significant time penalty due to the relatively low output count rates,
typically less than 25 kHz, that can be processed with the conventional
EDS. The silicon drift detector (SDD) uses the same measurement physics,
but modifications to the detector structure permit operation at a factor
of 5–10 times higher than conventional EDS for the same resolution.
Output count rates as high as 500 kHz can be achieved with 217 eV energy
resolution (at MnKα). Such extraordinarily high count rates make
possible X-ray mapping through the method of X-ray spectrum imaging, in
which a complete spectrum is captured at each pixel of the scan. Useful
compositional data can be captured in less than 200 s with a pixel density
of 160 × 120. Applications to alloy and rock microstructures,
ultrapure materials with rare inclusions, and aggregate particles with
complex chemistry illustrate new approaches to characterization made
practical by high-speed X-ray mapping with the SDD.Note: The Siegbahn notation for characteristic X-rays is
commonly used in the field of electron beam X-ray spectrometry and will be
used in this article. The equivalent IUPAC notation is indicated in
parentheses at the first use.In
this article, the following arbitrary definitions will be used when
referring to concentration (C) ranges: major: C > 0.1
(10 wt%), minor: 0.01 ≤ C ≤ 0.1 (1–10 wt%), and
trace: C < 0.01 (1 wt%).