Dispersion, detector

Figure Bl.24.14. A schematic diagram of x-ray generation by energetic particle excitation, (a) A beam of energetic ions is used to eject inner-shell electrons from atoms in a sample, (b) These vacancies are filled by outer-shell electrons and the electrons make a transition in energy in moving from one level to another this energy is released in the fomi of characteristic x-rays, the energy of which identifies that particular atom. The x-rays that are emitted from the sample are measured witli an energy dispersive detector.

The construction of a TXRF system, including X-ray source, energy-dispersive detector and pulse-processing electronics, is similar to that of conventional XRF. The geometrical arrangement must also enable total reflection of a monochromatic primary beam. The totally reflected beam interferes with the incident primary beam. This interference causes the formation of standing waves above the surface of a homogeneous sample, as depicted in Fig. 4.1, or within a multiple-layered sample. Part of the primary beam fades away in an evanescent wave field in the bulk or substrate [4.28],... 

A diagram of the Multi-Wavelength Dispersive Detector is shown in figure 7. 

Diffraction spectra of a-quartz, recorded by energy dispersive and angle dispersive detectors, contrasting the different resolutions. The energy dispersive spectrum was recorded in 5 minutes while the angle dispersive record required 54 minutes. 

Figure 7.7 Schematic set-up for measuring X-ray fluorescence with an energy-dispersive detector as in EDX. Irradiation of a bulk sample activates a pear-shaped volume from which X-rays are emitted. The chance of secondary processes is considerable and requires correction of the measured X-ray yields secondary effects are much less important if the sample is a thin film.

Most of the routine work in structural analysis is performed with D5000 Siemens diffractometer equipped with a Gobel mirror and an energy-dispersive detector. Raman micro-spectrometry has been recently introduced with a Labram infinity spectrometer with two laser sources, fitted with a horizontal output adapted to the investigation of vertical items like paintings or statues. For the most fine structural investigations, experiments are conducted with EXAFS, XANES or diffraction lines from various synchrotron facilities (ESRF at Grenoble, BESSY at Berlin, LURE at Paris). 

In the early 1970s when the energy dispersive detectors of semiconductor origin were commercially introduced, they actually revolutionized elemental analysis in the electron microscope. X-ray... 

Other analytical methods can also be applied for the detection of F in archaeological artefacts, especially when it is possible to take a sample or to perform microdestructive analysis. These are namely the electron microprobe with a wavelength-dispersive detector (WDX), secondary ion mass spectrometry (SIMS), X-ray fluorescence analysis under vacuum (XRF), transmission electron or scanning electron microscopy coupled with an energy-dispersive detector equipped with an ultrathin window (TEM/SEM-EDX). Fluorine can also be measured by means of classical potentiometry using an ion-selective electrode or ion chromatography. 

Transmission electron or scanning electron microscopy coupled with an energy-dispersive detector... 

Source Absorption Dispersing Detector Display cell element... 

There are two basic types of multi-wavelength detector, the dispersion detector and the diode array detector, the former being the more popular. In fact, today very few dispersion instruments are sold but there are many still used in the field and so their characteristics will be discussed. Both types require a broad emission light source such as deuterium or the xenon lamp, the use of the deuterium lamp being the most widespread. 

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