Detectors true counters

Hence, the photographic film vaguely resembles a ratemeter because the intensity is extracted from the degree of darkening of the spots found on the film - the darker the spot, the higher the corresponding intensity because the larger number of photons have been absorbed by the spot on the film surface. The three most commonly utilized types of x-ray detectors today are gas proportional, scintillation, and solid-state detectors, all of which are true counters. 

Intensity measurements are simplified when a detector always gives one electrical pulse for each x-ray quantum absorbed the detector remains linear so long as this is true. For low intensities, when the rates of incidence upon the detector are low, the Geiger counter fulfills this condition. As this rate increases above (about) 500 counts per second, the number of pulses per second decreases progressively below the number of quanta absorbed per second. This decrease occurs even with electronic circuits that can handle higher counting rates without appreciable losses. 

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 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. 

Since the detector and electronics are relied upon to accurately analyze the x-ray spectrum in the Si(Li) spectrometer, the performance requirements on the associated electronics are far more demanding than for the NaI(Tl) detector or the proportional counter. This is particularly true since the energy resolution of the Si(Li) detector is much better than the resolution of the other two detectors. Furthermore, it becomes more important for spectroscopists to understand the operating limitations peculiar to the Si(Li) x-ray spectrometer, so that they can avoid faulty data. The purpose of Sec. 4.3.1 to 4.3.9 is to delineate the operating parameters of the system so that the spectroscopist becomes sensitive to where the boundaries between reliable operation and faulty answers may lie for a particular instrument. 

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