The gas-flow proportional counters typically are used for low-energy lines, have ultrathin windows (0.5-1 m mylar or polypropylene windows), and use a counter gas (generally P-10; Ar gas with 10 methane) that flows through the detector at a constant rate.A wavelength-dispersive spectrometer uses the characteristic X-rays generated by individual elements to enable quantitative analyses (down to trace element levels) to be measured at spot sizes as small as a few micrometers.WDS can also be used to create element X-ray compositional maps over a broader area by means of rastering the beam.Together, these capabilities provide fundamental quantitative compositional information for a wide variety of solid materials.
This technique is complementary to energy-dispersive spectroscopy (EDS) in that WDS spectrometers have significantly higher spectral resolution and enhanced quantitative potential. Many SEM and EPMA instruments have EDS systems mounted to the column, and an EPMA typically has an array of several WDS spectrometers for simultaneous measurement of multiple elements. In typical EPMA applications, EDS is used for quick elemental scans to find out what a material contains, and WDS is then used to acquire precise chemical analyses of selected phases. There may be a single WD spectrometer horizontally mounted on an electron column (more typical in SEM instruments) or 4-5 spectrometers may be mounted vertically in sequence around the sample chamber (more typical of EPMA ). ![]() The geometry of the X-ray generating sample and the analytical crystal is such that they maintain a constant take-off angle. When X-rays encounter the analytical crystal at a specific angle, only those X-rays that satisfy Braggs Law are reflected and a single wavelength is passed on to the detector. The wavelength of the X-rays reflected into the detector may be varied by changing the position of the analyzing crystal relative to the sample i.e. X-ray source-crystal distance is a linear function of the wavelength. Consequently, X-rays from only one element at a time can be measured on the spectrometer and the position of a given analytical crystal must be changed in order to adjust to a wavelength characteristic of another element. The first type, the Johann geometry, is constructed by bending the analytical crystal to a radius of 2R, where R is the radius of the focusing circle, the Rowland circle (Fig. The second type, the Johansson geometry, is bent to a radius 2R and then ground to radius R, so that all of points of reflection lie on the Rowland circle, which maximizes the collection efficiency of the spectrometer. There is commonly more than a single analytical crystal in a WD spectrometer and, in the case of most EPMA instruments, there are typically multiple spectrometers with a suite of analytical crystals. Several different analytical crystals with dissimilar crystal lattice spacings (d-spacings) are normally used for WDS so that the spectrometers can reach all of the elemental wavelengths of interest and it will optimize performance in different wavelength ranges. An EPMA configured with 5 spectrometers, for example, might be set up to run WDS in a sequence to acquire 10 elements from one unknown sample by cycling through each crystal during analysis. The sample, crystal, and detector must lie on the Rowland circle and remain on it for all wavelengths of interest in order to focus X-rays efficiently. Because the sample and take-off angle of the X-rays are fixed, the analytical crystal and detector must both move to remain on the Rowland circle. In addition, the analytical crystal rotates as the X-ray source-crystal distance changes (requiring clever and precise engineering). Detectors used in WD spectrometers are most commonly gas proportional counter types, in which incoming X-rays enter the detector through a collimator (slit) and thin window, are absorbed by atoms of the counter gas, and then a photoelectron is ejected by each atom absorbing an X-ray. Hitachi ras 09ch1 manualThe photoelectrons are accelerated to a central wire such that additional ionization produces an electrical pulse which has an amplitude proportional to the energy of the original X-ray photon. The two general types of detector use sealed and gas-flow proportional counters. The sealed proportional counters are typically used for high-energy X-ray lines and have a relatively thick window (50 m thick Be window) that prevents leakage of Xe or Xe-CO 2 gas in the detector.
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