Counter proportional

The correct voltage at which to operate the counter is found as follows. Position the counter to receive an x-ray beam of constant intensity. Measure the 

7-16 Effect of voltage applied to proportional counter on observed counting rate at constant x-ray intensity (schematic). 

The proportional counter is essentially a very fast counter and has a linear counting curve up to about 10,000 cps. This ability to separate closely spaced pulses is due to the fact that the avalanche triggered by the absorption of an x-ray quantum is confined to an extremely narrow region of the counter, 0.1 mm or less, and does not spread along the counter tube (Fig, 7-17). The rest of the counter volume is still sensitive to incoming x-rays. 

The electric field near the end of the anode wire is not uniform. Most proportional counters are now made with a side window, rather than the end window shown in Fig. 7-17, so that x-ray absorption can take place in a region of uniform field. 

In x-ray spectroscopy (Chap. 15), but not in diffraction, there is a need to measure soft x-rays of wavelength about 5-20 A. Because ordinary windows would almost totally absorb such radiation, thin sheet plastic is used as a window, so thin that it leaks. To allow for this, a stream of counter gas is continuously passed through the counter, which is then called a gas-flow proportional counter [G.29, 7-10]. 

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There are many types of electronic detector. The original fomi of electronic detector was the Geiger counter, but it was replaced many years ago by the proportional counter, which allows selection of radiation of a particular type or energy. Proportional counters for x-rays are filled witii a gas such as xenon, and those for... 

Propofol Propofol [2078-54-8] Proportional counters Proposition 65 Propoxides Propoxur [114-26-1]... 

The concentration of is determined by measurement of the specific P-activity. Usually, the carbon from the sample is converted into a gas, eg, carbon dioxide, methane, or acetylene, and introduced into a gas-proportional counter. Alternatively, Hquid-scintiHation counting is used after a benzene synthesis. The limit of the technique, ca 50,000 yr, is determined largely by the signal to background ratio and counting statistics. 

Alpha counting is done with an internal proportional counter or a scintiUation counter. Beta counting is carried out with an internal or external proportional gas-flow chamber or an end-window Geiger-MueUer tube. The operating principles and descriptions of various counting instmments are available, as are techniques for determining various radioelements in aqueous solution (20,44). A laboratory manual of radiochemical procedures has been compiled for analysis of specific radionucHdes in drinking water (45). Detector efficiency should be deterrnined with commercially available sources of known activity. 

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. 

In X-Ray Fluorescence (XRF), an X-ray beam is used to irradiate a specimen, and the emitted fluorescent X rays are analyzed with a crystal spectrometer and scintillation or proportional counter. The fluorescent radiation normally is diffracted by a crystal at different angles to separate the X-ray wavelengths and therefore to identify the elements concentrations are determined from the peak intensities. For thin films XRF intensity-composition-thickness equations derived from first principles are used for the precision determination of composition and thickness. This can be done also for each individual layer of multiple-layer films. 

At present, the Geiger counter is the most popular x-ray detector in analytical chemistry. Although it is yielding ground to the proportional counter and the scintillation counter, it will be remembered for having greatly accelerated the use of x-ray emission spectrography in analytical chemistry. 

Fig. 2-4. Mean pulse height versus quantum energy, to illustrate pulse-height selection of characteristic lines. Side window tube, 3-in. diameter 0.005-in. wire operating at 1275 v filled with argon plus 10% ethylene total pressure, 15 cm of mercury (proportional counter) gain, 3.5 X 104. (After Friedman, Birks, and Brooks, A STM Spec. Tech. Publ., No. 157, page 3. Copyright 1954. American Society for Testing Materials.)...

When a proportional counter is. used in conjunction with a pulse-height selector, the occurrence of an escape peak may vitiate the results. Assume that the counter filling contains argon, whose K edge is at 3.87 A, and suppose that the pulse-height selector is set to select an x-ray line 3f shorter wavelength the intensity of which is to be measured. This line will excite the K lines of argon. To the extent that these lines are... 

It has always been difficult to do quantitative work with the characteristic x-ray lines of elements below titanium in atomic number. These spectra are not easy to obtain at high intensity (8.4), and the long wavelength of the lines makes attenuation by absorption a serious problem (Table 2-1). The use of helium in the optical path has been very helpful. The design of special proportional counters, called gas-flow proportional counters,20 has made further progress possible, and it is now possible to use aluminum Ka (wavelength near 8 A) as an analytical line (8.10). 

In these gas-flow proportional counters, the windows are exceedingly thin, fragile,. and unavoidably leaky. Such a window is satisfactory only if a steady flow of. the filling gas is maintained at minimum pressure differential against the helium atmosphere in the optical path. The purging of impurities from the counter is an incidental benefit derived from the gas flow. Hendee, Fine, and Brown20 look upon the gas-flow proportional counter as a steppingstone on the road toward a windowless counter. 

It is often advantageous to place the window in the side of a proportional counter so that the x-ray beam passes perpendicularly to the central wire even though this necessarily shortens the path length.21 A second window may be placed opposite the entrance window to permit escape of the unabsorbed x-rays, which might otherwise excite characteristic lines upon being absorbed by the counter walls. 

In the phosphor-photoelectric detector used as just described, the x-ray quanta strike the phosphor at a rate so great that the quanta of visible light are never resolved they are integrated into a beam of visible light the intensity of which is measured by the multiplier phototube. In the scintillation counters usual in analytical chemistry, on the other hand, individual x-ray quanta can be absorbed by a single crystal highly transparent to light (for example, an alkali halide crystal with thallium as activator), and the resultant visible scintillations can produce an output pulse of electrons from the multiplier phototube. The pulses can be counted as were the pulses-from the proportional counter. 

The tube of Figure 2-2 can be operated as an ionization chamber, as a proportional counter, or as a Geiger counter. The tube output differs radically from one case to another. Because of these differences, the electronic circuitry associated with the tube must also be different for each case if the pulses from the tube are to be reliably selected and counted. In particular, the circuitry will have to differ in characteristics such as stability, amount of amplification, and time of response. In all cases, linear amplification (amplifier output always proportional to tube output) is desirable. 

Recent papers from the Philips Laboratories37 40 contain thorough discussions of the Geiger counter, the proportional counter, and the scintillation counter, and significant performance data for all three, the emphasis being placed throughout upon x-ray applications. The detection system employed by Parrish and Kohler was particularly noteworthy in that it could conveniently accommodate any one of four detectors. ... 

Proportional counter in special cases where it gives the most favorable ratio of Nt to NB. 

The tubes run cool when operated at capacity. No contaminant gives a line at intensity exceeding 2% of the strongest characteristic line from the target. (2) For reasons given in Chapter 2, the preferred detectors are scintillation and gas-flow proportional counters. (3) The com-... 

The apparatus as modified for x-ray emission spectrograph is also shown in Figure 11-1. The proportional counter may be used alone (pulse-height analysis Section 2.13) or a curved-crystal spectrometer can be employed to achieve better resolution. Analytical results were comparable to those quoted above, but localization of the area analyzed was considerably less sharp than the micron-diameter spot achieved in differential absorptiometry. 

The second feature, the use of a secondary radiator, entails a loss of intensity because it introduces a second x-ray excitation process, but this loss is, offset to a large extent by the increased absorption of the characteristic lines from the radiator. The third feature also merits further comment. As Figure 11-7 shows, the proportional counter... 

Gas-flow proportional counter, 55 in aluminum analysis, 217 in comparison with other detectors, 65-67... 

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