X-ray proportional counter

By separating the coating from the substrate after deposition, the unique coating features of parylenes, especially continuity and thickness control and uniformity, can be imparted to a freestanding film. Applications include optical beam splitters, a window for a micrometeoroid detector, a detector cathode for an x-ray streak camera, and windows for x-ray proportional counters. 

X-ray proportional counters are usually cylindrical with a very thin beryllium window located either on the side or at the front end. They use a gas that is a mixture of a noble gas—He, Ne, Ar, Kr, Xe—with methane, at a pressure of about 1 atm. 

Several review articles on the x-ray proportional counter have been written [7,10,12]. They are highly recommended for further reading. Reference 6 may be consulted for further details on the associated electronic instrumentation. 

The Bragg crystal spectrometer consists of (1) an entrance slit or collimator, (2) the diffracting element, (3) an X-ray proportional counter, and (4) a... 

The detector system just described is analogous to a gas-filled detector used for x-rays. At low applied voltage it is like an ionization chamber, and when there is amplification, like a proportional counter. An x-ray proportional counter has a limited response rate, of about 10 kHz, but the x-ray systems are normally operated at pressures close to atmospheric, with local fields up to 100 MV m that give amplifications of several thousand times. These conditions can produce very high concentrations of positive ions by the central wire. The ions reduce the applied field, and the response rate is limited by the need for them to diffuse away. In the case of the HPSEM, the field is more uniform, the pressure and the field are lower. All these factors tend to reduce local ion concentrations and thus increase the response frequency. 

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... 

Figure 8.28 shows how the X-rays fall on the solid or liquid sample which then emits X-ray fluorescence in the region 0.2-20 A. The fluorescence is dispersed by a flat crystal, often of lithium fluoride, which acts as a diffraction grating (rather like the quartz crystal in the X-ray monochromator in Figure 8.3). The fluorescence may be detected by a scintillation counter, a semiconductor detector or a gas flow proportional detector in which the X-rays ionize a gas such as argon and the resulting ions are counted.

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. 

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). 

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. 

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. ... 

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... 

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