Links in this section:

Introduction
Evolution of WDS technique
Basics of WDS
Diffraction
Crystals
Detectors & Geometry
Comparison of EDS and WDS
Qualitative Analysis
Quantitative Analysis
Mapping
Summary

Detectors and Geometry

Detectors

Detectors used in WDS are usually of the gas proportional counter type (Fig. 3). Generally, X-ray photons are diffracted into the detector through a collimator (receiving slit), entering the counter through a thin window. They are then absorbed by atoms of the counter gas. A photoelectron is ejected from each atom absorbing an X-ray. The photoelectrons are accelerated to the central wire causing further ionization events in the gas, so that an “avalanche” of electrons drawn to the wire produces an electrical pulse. The detector potential is set so that the amplitude of this pulse is proportional to the energy of the X-ray photon that started the process. Electronic pulse height analysis is subsequently performed on the pulses to filter out noise. There are two types of gas proportional counters: sealed (SPC) and gas flow (FPC). Generally, SPCs are used for high-energy X-ray lines, while FPCs are used for low energy lines. Sealed proportional counters have a relatively thick window (i.e. beryllium, ~50µm thick), in order to prevent leakage of the gas (usually xenon or a xenon-CO₂ mixture) which is sealed into the detector. Gas flow proportional counters commonly use ultrathin (0.5 -1µm thick) mylar or polypropylene windows, and the counter gas (usually P-10: argon with 10% methane) flows through the detector at a constant rate.

Spectrometer geometry

To maintain the correct geometrical relationship between specimen, crystal and detector for the full range of diffracted angles, it is necessary to maintain all three on the Rowland circle. This is accomplished by a mechanical goniometer which moves the crystal and detector so that correct diffraction conditions are maintained. If a spectrometer is mounted with the Rowland circle vertical (as is typical on dedicated microprobes), it is necessary to position the sample height very accurately to maintain diffraction geometry. This is achieved by adjusting the height of the sample and observing focus using an optical microscope. In the SEM, accurate vertical positioning is not critical because the spectrometer is mounted with the Rowland circle horizontal and inclined, thus eliminating the need for an optical microscope. To analyze for a particular element it is important that the crystal and detector are positioned accurately and associated counting electronics are set up correctly. In the past this was a tedious and complex procedure, but automation and PC control have made WD operation very straightforward, routine and reliable.

Back | Next