Proportional counter energy resolution

Benchtop X-ray energy dispersive analyzer BRA-17-02 based on a gas-filled electroluminescent detector with an x-ray tube excitation and range of the elements to be determined from K (Z=19) to U (Z=92) an electroluminescent detector ensures two times better resolution compared with traditional proportional counters and possesses 20 times greater x-ray efficiency compared with semiconductor detectors. The device is used usually for grits concentration determination when analysing of aviation oils (certified analysis procedures are available) and in mining industry. 

Scintillation counters usually consist of a sodium iodide crystal doped with 1% thallium. The incident X-ray photons cause the crystal to fluoresce producing a flash of light for every photon absorbed. The size of the light pulse is proportional to the energy of the photon and is measured by a photomultiplier. A deficiency associated with scintillation counters is that they do not provide as good energy resolution as proportional or solid state detectors. 

Many types of detectors, such as Geiger-Miiller counters, proportional counters and scintillation detectors, are used for charged particle detection. The selection is made on the basis of resolution and range of particle in the gas or scintillator. In some cases, the particles are not completely stopped within the detector for an energy measurement, but deposit only a portion of their energy. This is related to the relative ionization of the particle and can be used to identify different kinds of particles. 

Gas proportional counters have relatively good resolution, so the heights of current pulses can be analyzed and discriminated to eliminate pulses that appear due to Kp photons and due to low and high energy white radiation photons. The pulse height discrimination is often used in combination with a p-filter to improve the elimination of the Kp and white background photons. 

Developed in the 1960s, semiconductors are the newest form of counter. They produce pulses proportional to the absorbed x-ray energy with better energy resolution than any other counter this characteristic has made them of great importance in spectroscopy (Chap. 15). Although they have had little application in diffraction, it is convenient to describe them here along with the other counters. 

The excellent energy resolution of a Si(Li) counter is shown in Fig. 7-19. The width W of the pulse distribution is so small that the Si(Li) counter can resolve the Kol and lines of manganese, which the other two counters cannot do. Put another way, the resolution R = W/V of the Si(Li) counter is 2.7 percent or some six times better than that of the proportional counter. For any kind of counter, both W and WjV vary with V, i.e., with the energy hv of the incident x-rays. Therefore any description of counter performance must specify the x-ray energy at which it is measured the 5.90 keV energy of the Mn Ka line is the usual standard reference. The width W, incidentally, is often written as FWHM (full width at half maximum) in the literature of this subject. 

The utility of this kind of spectrometer is based on two properties (1) the excellent energy resolution of the Si(Li) counter (Sec. 7-8), which is far better than that of any other type of proportional counter, and (2) the ability of the MCA to perform rapid pulse-height analysis (Sec. 7-9). The latter feature makes this spectrometer very much faster than a single-channel crystal spectrometer. The MCA can measure the intensities of all the spectral lines from the sample in about a minute, unless elements in very low concentrations are to be determined. 

This type of detector is one of the oldest and most often used in X-ray diffraction. Its response time is extremely low (= 0.2 pS). This feature is its main advantage compared to proportional gas detectors. It is sensitive to the energy of X-rays but has a poor energy resolution. Its efficiency is almost 100%. Compared to proportional gas counters, the background noise is more significant and scintillating crystals deteriorate in a humid atmosphere. 

The energy resolution of these counters is such that the FWHM is 1-2 keV at 20 keV. Thus, proportional counters are superior to scintillation counters in this energy range. 

The width F is indicated as electronic noise in Fig. 12.41. Of the three types of X-ray detectors mentioned—scintillation, proportional, and semiconductor counters—the Si(Li) detector has the best energy resolution for X-rays. This fact is demonstrated in Fig. 12.42, which shows the same energy peak obtained with the three different detectors. Notice that only the Si(Li) detector can resolve and lines, an ability absolutely necessary for the study of fluorescent X-rays for most elements above oxygen. The manganese fluorescence spectrum obtained with a Si(Li) detector is shown in Fig. 12.43... 

Figure 12.42 Demonstration of the superior energy resolution of Si(Li) detectors by showing the same peak recorded with a NaKTl) scintillator and a gas-filled proportional counter (from Ref. 55).

It should be pointed out that the X-ray detector used with the crystal spectrometer need not have an extremely good energy resolution, because it is the resolution of the analyzing crystal that determines the spectroscopic capabilities of the system, not that of the detector. The X-ray counter may be a proportional counter or a Si(Li) detector. 

Finite resolution of proton detector. The resolution of a proportional counter for monoenergetic protons is derived from two factors. One is a statistical broadening that depends on the number of ion pairs produced. The other is a mechanical broadening due to imperfections in the design of the counter and impurities in the filling gas. At an energy of 615 keV, the energy resolution is of the order of 4 percent, but it deteriorates to about 60 percent at 1 keV. 

The Nal(Tl) scintillation detector is most useful for short-wavelength X-rays, <2 A (Z > 27), so it complements the proportional counter. It also has the potential for escape peaks caused by the iodine K line (about 30 keV or 0.374 A). Incoming X-rays with wavelengths less than 0.374 A will result in escape peaks about 30 keV lower in energy than the true energy. The major disadvantage of the Nal(Tl) detector is that its resolution is much worse than that of the proportional counter. This is due to the wider pulse height distribution that results in the output pulse because of the multiple steps involved in the operation of this detector. 

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. 

Modified versions of the gas proportional counter are used for spectral analysis of X rays in the energy region of a few kiloelectron volts. Detectors are constructed with a greater depth and a window with a low-Z material such as Be, and with a high-Z counting gas such as Xe at apressure of several atmospheres. The resolution is about 0.6 keV at 5 keV (Knoll 1989). 

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