EDXRF Detectors

The detector is the most crucial component of the EDXRF unit since it detects and sorts the incoming photons originating from the sample. The detector type and associated electronics determine the performance with respect to count rate, resolution, and detection efficiency. 

The count rate is the total of all photons detected and counted by the detector over the energy range being detected. 

Two type of detectors are used in commercially available units proportional detectors and semiconductor detectors such as silicon PIN, Si(Li), Ge(Li), and silicon drift detectors. The detectors used in EDXRF have very high intrinsic energy resolution. In these systems, the detector resolves the spectrum. The signal pulses are collected, integrated, and displayed by a multichannel analyzer (MCA). 

A cylindrical piece of pure, single-crystal silicon is used. The size of this piece is 4-19 nun in diameter and 3-5 mm thick. The density of free electrons in the silicon is very low, constituting a p-type semiconductor. If the density of free electrons is high in a saniconductor, then we have an n-type semiconductor. Semiconductor diode detectors always operate with a combination of these two types. 

For a similar Ge lithium-drifted detector, the energy required for ionization is 2.96 eV. This is much less than the energy required for ionization in a proportional counter or a Nal(Tl) scintillation detector. 

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The basic function of the spectrometer is to separate the polychromatic beam of radiation coming from the specimen in order that the intensities of each individual characteristic line can be measured. In principle, the wide variety of instruments (WDXRF and EDXRF types) differ only in the type of source used for excitation, the number of elements which they are able to measure at one time and the speed of data collection. Detectors commonly employed in X-ray spectrometers are usually either a gas-flow proportional counter for heavier elements/soft X-rays (useful range E < 6keV 1.5-50 A), a scintillation counter for lighter elements/hard X-rays (E > 6keV 0.2-2 A) or a solid-state detector (0.5-8 A). 

In EDXRF, trends are for miniaturisation, development and optimisation of high-resolution room-temperature detectors and extension of the application range towards the determination of light elements. 

The literature on XRF is abundant. Recent general reviews are refs [235,237] for sample preparation see ref. [247]. EDXRF was specifically dealt with in ref. [248] and an excellent X-ray detector overview is available [225]. Several recent XRF monographs have appeared [233,249,249a], also covering TXRF [250] and quantitative XRF [232,251]. 

In EDXRF the secondary X-ray emitted by the excited atom is considered to be a particle (an X-ray photon) whose energy is characteristic of the atom whence it came. The major development which has facilitated this technique is the solid state semiconductor diode detector. An EDXRF system consists of a solid state device which provides an electronic output that is... 

As discussed above, the measurement of characteristic y rays is very similar to the methods used in EDXRF. Early studies used a scintillation counter, typically a crystal of sodium iodide containing a small amount of thallium (Tite 1972). y ray absorption by these counters produces visible light, which is converted into an electrical pulse using a photosensitive detector. More recently semiconductor detectors have been used, either a lithium drifted germanium crystal, or, more typically, a pure ( intrinsic )... 

An on-line analyzer must be packaged much more robustly than a laboratory instrument to withstand the process environment which, for example, may have an explosive atmosphere and significantly variable ambient temperature. It must also be capable of continuous, unattended operation over long periods of time. Clearly, the simpler the instrument the better. Of the methods listed in Table 1, WDXRF, polarized EDXRF, and Pyro-microcoidometry have not been adsqrted to on-line process instrumentation, whereas the other methods have. The relative simplicity of Pyro-EC makes it particularly suitable for adaptation to process instrumentation. The sulfur dioxide sensor is a small, plug-in, low cost electrochemical cell, easily replaceable and with an expected lifetime of over one year. The UV lamp, UV optics, and photomultiplier used in Pyro-UVF are not required. The X-ray tube (or radioactive source). X-ray detector, and X-ray optics used in all the XRF instruments are not required. 

Another new instrument recently developed for sulfur by XRF determination is described by Wissmann (Spectra, Inc ). This method uses polarized EDXRF, considerably reducing background scatter, and achieving detection limit comparable to that of WDXRF. Recent developments in detector technology and in closed coupled static geometry have resulted in further improvement of sensitivity for this application. This method is also in the developmental stage for ASTM method designation. 

There are three major classes of X-ray detectors in commercial use gas-filled detectors, scintillation detectors, and semiconductor detectors. Semiconductor detectors will be discussed with EDXRF equipment. Both WDXRF and EDXRF detection makes use of a signal processor called a pulse height analyzer or selector in conjunction with the detector, and discussed subsequently. 

When an X-ray falls on a semiconductor, it generates an electron (—e) and a hole (+e) in a fashion analogous to the formation of a primary ion pair in a proportional counter. Based on this phenomenon, semiconductor detectors have been developed and are now of prime importance in EDXRF and scanning electron microscopy. The principle is similar to that of the gas ionization detector as used in a proportional counter, except that the materials used are in the solid-state. The total ionization caused by an X-ray photon striking the detector is proportional to the energy of the incident photon. 

Resolution in a semiconductor detector EDXRF system is a function of both the detector characteristics and the electronic pulse processing. The energy resolution of semiconductor detectors is much better than either proportional counters or scintillation counters. Their excellent resolution is what makes it possible to eliminate the physical dispersion of the X-ray beam without the energy resolution of semiconductor detectors, EDXRF would not be possible. 

Figure 8.31 Artifacts in an EDXRF spectrum. The spectrum of pure iron, measured with a Si(Li) detector, shows a peak lower in energy than the Fe Kq. peak by an amount exactly equal to the energy of the Si Kq, line. Some of the Fe photon energy is transferred to the Si detector atoms the amount of energy absorbed by an Si atom has escaped from the Fe photon. This t)rpe of peak is called an escape peak. Sum peaks also appear in EDXRF spectra when two intense photons arrive at the detector simultaneously. A sum peak from two Kq, photons is shown along with a sum peak from one Kq and one Kys photon. [Courtesy of Thermo ARE (www.thermo ARL.com).]...

Spectrum artifacts may appear in the energy dispersive spectrum. These are peaks that are not from elements in the sample. The Si escape peak, from the Si K line, results in an artifact peak 1.74 eV lower than the parent peak when a Si(Li) detector is used. Such an escape peak is shown in the EDXRF spectrum of an iron sample in Fig. 8.31. Similar escape peaks at different energies appear for Ge if a Ge detector is used. 

Look up the Ge line energies in Appendix 8.1 and predict where you would find a Ge escape peak in an EDXRF spectrum of pure Cu using a Ge(Li) detector. Where would the escape peak occur if a Si(Li) detector were used for the same sample ... 

The detectors used in the various forms of EDXRF are semiconductor detectors Conventionally, two types, i. e. lithium drifted silicon (Si(Li)) and hyperpure germa nium (HP-Ge) detectors are used. Their main advantages are their compact size the non-moving system components, and relatively good energy resolution which optimally is of the order of 120 eV at 5.9 keV. Because of their operation prin... 

Besides EDXRF spectrometers that are intended for use in the laboratory, a number of portable EDXRF instruments are also available. These devices are used in various fields for on-site analysis of works of art, environmental samples, forensic medicine, industrial products and waste materials etc. In their simplest form, the instraments consist of one or more radioisotope sources combined with a scintillation or gas proportional counter. However, combinations of radio-sources with ther-moelectronically cooled soHd-state detectors are also available in compact and lightweight packages (below 1 kg). In Fig. 11.21, schematics of various types of radiosource based EDXRF spectrometers are shown. In Fig. 11.21a, the X-ray source is present in the form of a ring radiation from the ring irradiates the sample from below while the fluorescent radiation is efficiently detected by a solid-state detector positioned at the central axis. Shielding prevents radiation from the source from entering the detector. In Fig. 11.21b and c, the X-ray source has another... 

Spectrum evaluation is a crucial step in X-ray analysis, as important as sample preparation and quantification. As with any analytical procedure, the final performance of X-ray analysis is determined by the weakest step in the process. Spectrum evaluation in EDXRF analysis is more critical than in WDXRF spectrometry because of the relatively low resolution of the solid-state detectors employed. 

In EDXRF spectrometry, there is no physical separation of the fluorescence radiation from the sample. There is no dispersing device prior to the detector. All the photons of all energies arrive at the detector simultaneously. Any EDXRF is designed around the following components—a source of primary excitation, a sample holder, and a detector, seen in Figure 8.14. 

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