Counters energy resolution

Solid-state detectors based on silicon- or germanium-diodes possess better resolution than gas counters, particularly when cooled with liquid nitrogen, but they allow only very low count rates. PIN diodes have also recently become available and have been developed for the instruments used in the examination of Martian soils (Sects. 3.3 and 8.3). A very recent development is the so-called silicon-drift detector (SDD), which has very high energy resolution (up to ca. 130 eV) and large sensitive detection area (up to ca. 1 cm ). The SNR is improved by an order of magnitude compared to Si-PIN detectors. Silicon drift detectors may also be used in X-ray florescence spectroscopy, even in direct combination with Mossbauer spectroscopy (see Sects. 3.3 and 8.3). 

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. 

We will be interested in three aspects of counter behavior losses, efficiency, and energy resolution. These are defined below and made more specific in later sections on particular counters. 

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. 

Compared to the conventional method (single wavelength, moving counter), energy-dispersive diffractometry is much faster, because the diffraction pattern is acquired simultaneously rather than serially. Typically, the entire pattern can be recorded in 1 to 5 minutes, whereas the conventional technique requires over an hour. However, the resolution of closely spaced diffraction lines is inferior to that of the conventional technique. Also on the debit side are the added cost of an MCA and the inconvenience of cooling the Si (Li) counter. 

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. 

If the last extractant is also a scintillator such as ETRAC s STRONEX, the equilibrated organic phase can be counted directly in a beta-liquid-scintillation counter or in a PERALS spectrometer. The carboxylic acid is not colored and does not quench. The PERALS spectrometer provides better beta-energy resolution and has only slightly lower counting efficiency for betas and thus may offer some advantage if both Sr and Sr are required in the same sample. The PERALS spectrometer will provide better separation of the 0.546 MeV Sr, the 1.48 MeV Sr, and the 2.28 MeV Y. In addition, if radium is present, the pulse shape discrimination feature of the PERALS spectrometer can be used to reject the contribution from radium alphas. 

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. 

These are used on single counter four-circle diffractometers. The detector is often sodium iodide. The scintillator is used in conjunction with a photomultiplier tube. Thallium activated sodium iodide has an energy resolution of about 40% at lOkeV. Hence, the attractiveness of such a detector lies not only with its counting of individual photons but also its... 

There is no detector that satisfies all these requirements. Few detectors have 100 percent efficiency. In practice, it is not feasible for gamma and neutron detectors to have all the energy of the particle deposited in the counter. Because of statistical effects, there is no detector with ideal energy resolution. What should one do ... 

The most important advantage of the semiconductor detectors, compared to other types of radiation counters, is their superior energy resolution the ability to resolve the energy of particles out of a polyenergetic energy spectrum (energy resolution and its importance are discussed in Chaps. 9, 12-14). Other advantages are... 

To discuss the effect of the statistical fluctuations on energy resolution, consider a monoenergetic source of charged partieles being detected by a silicon semiconductor detector. (The discussion would apply to a gas-filled counter as well.) The average energy w needed to produce one electron-hole pair in silicon is... 

It can be seen from Eq. 9.7 that the resolution is better for the detector with the smaller average energy needed for the creation of a charge carrier pair (and smaller Fano factor). Thus, the energy resolution of a semiconductor detector (w 3 eV, F < 0.1) should be expected to be much better than the resolution of a gas-filled counter (w = 30 eV, F 0.2), and indeed it is (see Chaps. 12 and 13). 

Assume that the energy resolution of a scintillation counter is 9 percent and that of a semiconductor detector is 1 percent at energies around 900 keV. If a source emits gammas at 0.870 MeV and 0.980 MeV, can these peaks be resolved with a scintillator or a semiconductor detector ... 

Of all the scintillators existing in the market, the Nal crystal activated with thallium, NaI(Tl), is the most widely used for the detection of y-rays. Nal(Tl) scintillation counters are used when the energy resolution is not the most important factor of the measurement. They have the following advantages over Ge(Li) and Si(Li) detectors ... 

A disadvantage of all scintillation counters, in addition to their inferior energy resolution relative to Si(Li) and Ge(Li) detectors, is the necessary coupling to a photomultiplier tube. 

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. 

For Ge detectors other than the well-type, the efficiency is low, relative to Na(Tl) scintillation counters. This statement holds true for Si(Li) detectors as well (see Sec. 12.9). Lower efficiency, however, is more than compensated for by the better energy resolution of the semiconductor detector. Figure 12.32 illustrates the outstanding resolution characteristics of a semiconductor detector by showing the same spectrum obtained with a Nal(Tl) and a Ge(Li) detector. Notice the tremendous difference in the FWHM. The Ge(Li) gives a FWHM =... 

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. 

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