A unique wavelength-dispersive soft X-ray emission spectrometer for electron probe X-ray microanalyzers

摘要

Wavelength-dispersive spectrometers (WDS) have been around for over 60 years. Their longevity is a testament to their effectiveness at measuring X-rays reproducibly, with high efficiency, and with very high energy spectral resolution. The spectrometers have two main components: (1) an analyzing crystal that diffract X-rays conforming to Bragg's Law (nλ = 2d sinθ; where λ is the wavelength, d is the lattice spacing, and 6 is the incident angle of the X-ray); and (2) a counter that records each diffracted X-ray (Figure 1 A). The analyzing crystal and counter have to be moved synchronously to a new position for each different X-ray wavelength. This changes the incident angle of the X-ray striking the analyzing crystal, diffracting a different X-ray wavelength, and insuring that diffracted X-rays enter the counter. Each analyzing crystal is only able to diffract a certain range of X-ray wavelengths (or energies). Three analyzing crystals (TAP, PET and LIF) are required to diffract the range of X-rays with energies between 0.5 and 14 keV. For lower energy X-rays, layered synthetic microstructures are available. These are constructed of interlayers of high and low atomic number materials on a scale of 6-20 nm. These in turn diffract long-wavelength (low-energy) X-rays, just as the natural crystalline material does for the short-wavelength (high-energy) X-rays. Using layered synthetic microstructures, one can analyze X-rays down to the Be Kα line (～109 eV). Even though wavelength-dispersive spectroscopy has been around for over 60 years, it is still the preferred X-ray analytical system for many, because of its high spectral resolution. For comparison, an energy dispersive spectrometer (EDS) has an energy resolution of around 130 eV (when measured as the FWHM of Mn Kα) as compared to a traditional wavelength-dispersive spectrometer (WDS), which has an energy resolution of around 16 eV, almost an order of magnitude improvement.