Wavelength Dispersive Spectrometer WDS

The X-rays leave the specimen at a take-off angle 4 , are collimated by two slits Si and S2 before falling on to a crystal (bent to a radius 2R, where R is the 

The spectrometer is set to the appropriate Bragg angle 0 of the requisite characteristic wavelength, and only these X-rays will reach the detector and be counted. The detector employed is the gas proportional counter, which can operate at much faster count rates than the EDS crystal detector. 

The spectrometer is necessarily quite large, and a complicated mechanism has to be precision engineered in order to enable 0 to be altered while keeping both the crystal and the detector on the Rowland circle. In order to cover the whole X-ray spectrum a range of crystals with different lattice spacings is required, which may be interchanged automatically. 

WDSs have excellent resolving power, and the peak-to-background ratio of each line is much higher than can be achieved with a crystal detector. With a suitable crystal of large lattice spacing it is possible to detect and count X-rays as soft as boron K or even beryllium K , and this type of spectrometer is widely used when 

In the following sections we will consider in turn the instruments employed for 

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Wavelength dispersive X-ray fluorescence spectrometric (xrf) methods, 25 60 Wavelength dispersive spectrometer (WDS), 76 488, 26 433-434 Wavelength dispersive X-ray fluorescence (WDXRF)... 

As with X-ray fluorescence, the characteristic X rays are analysed using wavelength dispersive spectrometers (WDS) based on the selective reflection of radiation by a monochromator crystal. The related analytical performance levels are ... 

Table 6.4 Analyzing Crystals used in Wavelength Dispersive Spectrometers (WDS)...

Left Energy-dispersive X-ray system (EDX) with Si(Li) detector Right Wavelength-dispersive spectrometer (WDS) with bent crystal... 

Two different systems presently are utilized to detect and measure X-ray generation in electron beam instruments wavelength dispersive spectrometers (WDS) and energy dispersive spectrometers (EDS). 

On the other hand, its poorer resolution and the mediocre signal/noise ratio means it cannot compete with the wavelength dispersive spectrometer (electron microprobc) for quantitative analysis and the analysis of trace elements (the detection limits are generally 10 times higher than in WDS). 

The instrumentation for EM uses the same type of X-ray spectrometers discussed in detail in Chapter 8, with an electron beam as the source and a UHV system that includes the sample compartment. An ED X-ray spectrometer allows the simultaneous collection and display of the X-ray spectrum of all elements from boron to uranium. The ED spectrometer is used for rapid qualitative survey scans of sample surfaces. The wavelength dispersive spectrometer has much better resolution and is used for quantitative analysis of elements. The WD spectrometer is usually equipped with several diffracting crystals to optimize resolution and to cover the entire spectral range. The electron beam, sample stage, spectrometer, data collection, and processing are all under computer control. 

The phase compositions of the SHS products were characterized by using XRD, XRF and EPMA techniques. The morphologies of the products were characterized by electron probe microanalysis (EPMA, CAMECA SX-100) with using three WDS (Wavelength Dispersive Spectrometer) units. X-ray analyses of obtained alloys and slags were performed with Thermo Scientific Niton XL3t XRF device and PANalytical X Pert Pro PW3040/60 XRD device. 

An X-ray fluorescence spectrometer needs to resolve the different peaks, identify them and measure their area to quantify the data. There are two forms of X-ray spectrometers (Fig. 5.5), which differ in the way in which they characterize the secondary radiation - wavelength dispersive (WD), which measures the wavelength, and energy dispersive (ED), which measures the energy of the fluorescent X-ray (an illustration of the particle-wave duality nature of electromagnetic radiation, described in Section 12.2). 

Verify the statement in Sec. 15-8 regarding spectrometer resolution by wavelength dispersion (WD) and energy dispersion (ED) by calculating the percent resolution AA/A for each type and for wavelengths of 0.5, 1.0, and 1.5 A. For the WD spectrometer, assume a LiF crystal with 2d = 4.03 A (200 reflection) and line width B = 0.5°. For the ED spectrometer, assume a Si(Li)-FET counter and Eq. (7-5). Assume also that the line or pulse-distribution separation must be twice the breadth for adequate resolution. 

Wavelength dispersive X-ray spectrometry (WDS) for a more detailed elemental analysis of samples in the SEM. JEOL Four-Crystal Spectrometer attached to the JSM-35C SEM can be used for l-pm spot analysis, digital and analog line scans, and X-ray image mapping, elements detection from Be to U, minimum detection limit of 0.01% by weight, fully quantitative results by extended cp-p-z. 

The electron probe microanalyzer (EM or EPMA) uses a beam of high-energy electrons to bombard the surface of a solid sample. This results, as we have already seen, in the removal of an inner shell electron. As discussed in Section 14.2, this can result in the ejection of a photoelectron (the basis of ESCA) and the emission of an X-ray photon. The X-ray photons emitted have wavelengths characterishc of the elements present. The EPMA uses either a wavelength dispersive (WD) or energy dispersive (ED) X-ray spectrometer to detect and identify the emitted X-rays. This is very much analogous to XRE spectrometry... 

Energy Dispersive (ED) or Wavelength Dispersive (WD) XRF spectrometer 15,000-150,000 Very common... 

Commercial instruments use multiple tube or multiple slit collimator arrangements, often both before the analyzing crystal (the primary collimator) and before the detector (the secondary collimator). The collimator positions in a sequential WDXRF spectrometer are shown schematically in Figure 8.30. In many wavelength-dispersive (WD) instruments, two detectors are used in tandem, and a third auxiliary collimator may be required. Such an arrangement is shown in Figure 8.31. 

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