Wavelength-dispersive crystal spectrometer

An X-ray tube with a tungsten target and which serves as the source in an X-ray wavelength dispersive fluorescence spectrometer is equipped with a crystal of ethylenediamine tartrate. The plane of reflection in use corresponds to an inter-reticular distance of d — 4.404 A. 

X-Ray Fluorescence analysis (XRF) is a well-established instrumental technique for quantitative analysis of the composition of solids. It is basically a bulk evaluation method, its analytical depth being determined by the penetration depth of the impinging X-ray radiation and the escape depth of the characteristic fluorescence quanta. Sensitivities in the ppma range are obtained, and the analysis of the emitted radiation is mosdy performed using crystal spectrometers, i.e., by wavelength-dispersive spectroscopy. XRF is applied to a wide range of materials, among them metals, alloys, minerals, and ceramics. 

Before the development of semiconductor detectors opened the field of energy-dispersive X-ray spectroscopy in the late nineteen-sixties crystal-spectrometer arrangements were widely used to measure the intensity of emitted X-rays as a function of their wavelength. Such wavelength-dispersive X-ray spectrometers (WDXS) use the reflections of X-rays from a known crystal, which can be described by Bragg s law (see also Sect. 4.3.1.3)... 

Figure 5.5. A wavelength-dispersive spectrometer showing the specimen, crystal and detector all lying on...

X-ray spectroscopy is nowadays applied mostly in the form of X-ray fluorescence, where scanning monochannel machines, sequence spectrometers and simultaneous spectrometers are used in wavelength dispersive X-ray fluorescence. The introduction of bent analyser-crystals extended the method to smaller samples, thus marking another step toward microprobe analysis. 

The wavelength dispersive x-ray (WDX) spectrometer is based on the Bragg law and employs curved crystals that can be rotated so as to reflect the x-rays emitted by the specimen and focus them, one wavelength at a time, onto a detector. Although this technique allows high resolution of one wavelength from another, it has a number of practical disadvantages ... 

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)...

Summary. The various differences between wavelength-dispersive and energy-dispersive spectrometry have been described in this chapter. All in all, wavelength dispersion by a crystal spectrometer is superior, chiefly because of its better resolution for most elements of interest, for the quantitative determination of several elements in a complex sample with high accuracy. Energy dispersion has a special place in microanalysis, in portable spectrometers, or wherever fast, semi-quantitative analyses are required. 

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. 

A Jeol-35 Scanning Electron Microscope equipped with energy-and wavelength- dispersive X-ray spectrometers was used for elemental analysis. Spot analyses were carried out these covered approximately 0.5 pm for the pyrite and 1 pm for the coal. The standard atomic number, absorption and fluorescence (ZAF) corrections were applied to all analyses, using counting times of 200 seconds, an accelerating voltage of 15 kV and a pyrite crystal as standard. 

A Philips PW-14(X) wavelength dispersive spectrometer equipped with four crystals (LiF, Ge, PET, and TIAP) was used for the analysis. The rhodium target tube was operated at 2.5 kW (50 kV and 50 mA). The sample holders were 32 mm in diameter with a copper mask. The elements determined and their respective peak and background times are shown in Table 2. The intensity ratio method was employed using a synthetic monitor specimen. 

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. 

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