Sealed proportional counters

Detector copper—xenon-sealed proportional counter silver, gold, and lead—flow proportional counter using 10% methane and 90% argon. 

Of the three commonly used X-ray detectors—(1) Geiger counter, (2) scintillation counter, and (3) proportional counter—the latter is used most frequently for electron-probe microanalysis. In the wavelengths from 1 to 10 A, sealed proportional counters may be used. For longer-waveleiigtli analysis—in the range from 10 to 93 A—the thinnest possible detector window is required to limit spectral attenuation. Nitrocellulose windows have proved successful. Nondispersive detection systems using cooled Li-dnfted Si are also applicable. 

Sealed proportional counter tube and DC voltage at plateau. 

Sealed proportional counter tube and DC voltage at plateau Pulse height selection El 5 volts Ey-out... 

Sealed proportional counter tube and DC voltage at plateau Pulse height selection El-5 volts Eu - out. Rate meter T.C. 4 2000 C/S full scale Chart speed - 1 in./5 min. 

Sealed Proportional Counter. A sealed proportional counter is shown schematically in Fig. 8.24(b). The windows are thicker, so they do not leak. Window materials include polymers, mica, aluminum, and beryllium. The filler gas used in a sealed proportional counter may be Ne, Kr, or Xe. Window and gas combinations are optimized for the wavelength of radiation to be detected A1 and Ne would be best for light elements, for example. 

Figure 8.24 Schematics of (a) a flow proportional counter and (b) a sealed proportional counter. (From Helsen and Kuczumow, used with permission.)...

Figure 8.38 Schematics of (a) a flow proportional counter and (b) a sealed proportional counter. (From Helsen, L.A. and Kuczumow, A., in Van Griekin, R.E. Markowicz, A.A. (eds.), Handbook of X-Ray Spectrometry, 2nd edn., Marcel Dekker, Inc., New York, 2002. Used with permission.) (c) Schematic view of a flow proportional counter.

A gas (usually 90%Ar- -10%CH4) filled proportional counter is an alternative ED detector (except at short wavelengths where a Xe sealed proportional counter is preferred) particularly in industrial applications. This detector provides resistance to vibrations, mechanical reliability, and minimal temperature dependency. A proper choice of the filling gas of proportional counter is of great importance in minimizing the background caused by the wall effect. Although the resolution of this detector is relatively very poor, the detection limit in the ppm range can be obtained. 

Proportional counters may consist of a sealed cylinder serving as cathode, a thin wire as anode and a thin window, but they are often constructed as flow counters, as shown in Fig. 7.10. In this type of counter a gas, preferably methane or a mixture of argon and methane, flows through the counter during operation and the sample is brought into the counter. The operational voltage depends on the nature and the pressure of the gas and varies between about 2 and 4 kV. 

A specific type of proportional counter that is used for accurate counting of a and P activity on smear samples, is the gas flow proportional counter. In some types of gas flow proportional counters, the sample is put inside the detector for greater sensitivity—there is no structural material to absorb the radiation before it can be detected. Figure 5.24 shows a diagram of the detector of such a gas flow proportional counter. Other gas flow proportional counters have very thin Mylar windows sealing off the detector s gas chamber. During counting, samples are positioned very close to the window. 

The scintillation counter (Sec. 7-7) and the sealed gas proportional counter (Sec. 7-5) are both used in spectrometry. The scintillation counter is better for the very short wavelength region because of its greater efficiency there (Fig. 7-12) in the 1 A-2 A range either counter is suitable. In the long-wavelength region a gas-flow proportional counter is required, because of its thin low-absorption window. 

Energy distributions in gas proportional counters— whether gas flow or sealed counters— undergo a noticeable shift to lower energies when X-ray counting rates (and, concomitantly, detector gas ionization rates) increase. The exact origin of this phenome-... 

The first type is a manual detector, either with a thin (typically 80 qg/cm ) window beneath which the sample is placed, or without a window but with a sample tray that slides under the detector and becomes the detector bottom. Use of the internal proportional counter eliminates external attenuation of low-energy beta particles but introducing the source into the detector can cause contamination or reduce the potential difference to a value below the applied voltage. The detection volume may be cylindrical (typically about 1 cm high and 3 cm in radius), or may be a hemisphere in the classical 27rgroportional counter. For either a thin window or no window, continuous gas flow is maintained. A thicker window (typically 0.5 mg/cm ) can seal the counting gas in the detector. 

Avoiding a loss of intensity by absorption, XRF analysis is realized under vacuum. The intensities of the fluorescence radiation are measured by flow or sealed proportional and scintillation counters. For quantification, measured intensities are compared with intensities of known standard samples. Based on the measured raw intensities, element concentrations are calculated using the... 

WDXRF systems commonly use one or more of the following detectors gas flow proportional counter (flow counter [FC]), sealed gas-proportional counter, and scintillation counters (SC). 

Detectors used in X-ray astronomy include proportional counters, microchannel plates, and charge-coupled devices and other solid-state detectors. Proportional counters were the first type of X-ray detector used in astronomy and are the most common astronomical X-ray detector in use today. Proportional counters consist of an electrically neutral gas (usually argon or xenon) in a sealed chamber. As an X-ray enters the chamber, it can photoionize a gas atom, producing a photoelectron that can then be amplified and detected. Modern proportional counters can detect the position of the incident X-ray along with its time of arrival... 

The choice of source material is a difficult problem. Hard radiation is required (to achieve high penetration through the electrolyte solution) but hard X-ray photons are difficult to detect in a proportional counter, requiring the use of high pressure and expensive xenon gas. This is acceptable with a sealed detector but it means that the construction materials must be carefully chosen to be clean, or the detector gas will soon become contaminated. Softer radiation is much easier to detect, and, for example, for Cu Ka photons reasonable detection efficiencies are obtained with Ar at atmospheric pressure. All the results reported here were obtained with a Cu Ka source, which limits the solution pathlength to a few tenths of a millimeter. This clearly makes cell design critical. For Mo Ka radiation, considerably thicker solution layers would be acceptable. 

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