International
Tables for
Crystallography
Volume C
Mathematical, physical and chemical tables
Edited by E. Prince

International Tables for Crystallography (2006). Vol. C, ch. 7.1, p. 619

Section 7.1.3. Proportional counters

W. Parrishf and J. I. Langforde

7.1.3. Proportional counters

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7.1.3.1. The detector system

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The commonest types of detector for both powder and single-crystal diffractometry are proportional counters and especially scintillation counters (Section 7.1.4[link]). The detector system consists of the detector itself, a high-voltage power supply, a single-channel pulse-height analyser, and a scaling circuit, as shown schematically in Fig. 2.3.3.5[link] . For position-sensitive detectors (Section 7.1.3.3[link]) and solid-state detectors (Sections 7.1.4.2[link] and 7.1.5[link]), multichannel analysers are necessary. The X-ray manufacturers and a number of electronic companies provide complete detector systems, often integrated with the computer data-collection system.

7.1.3.2. Proportional counters

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Proportional counters are available in various sizes and gas fillings. A typical detector is a metal cylinder about 2 cm in diameter and 8–10 cm long, with central wire anode and 0.13 mm Be side window, Fig. 7.1.2.1(b)[link]. Some have an opposite exit window to transmit the unabsorbed beam and thus avoid fluorescence from the wall. The tube may be filled with Xe to atmospheric pressure for high absorption, and a small amount of quenching gas such as CO2 or CH4 is added to limit the discharge. When an X-ray quantum is absorbed, the discharge current is the sum of the Townsend avalanches of the secondary electrons and the gas amplification is about 104. A charge-sensitive preamplifier is generally used. Some proportional counters are filled to several atmospheres pressure to increase the gas absorption. Very thin organic film windows are used for very long wavelengths as in fluorescence spectroscopy. They may transmit moisture, and gas may migrate through them so that flow counters are used to replenish the gas. This requires careful control of the pressure to avoid changes in the counting efficiency.

7.1.3.3. Position-sensitive detectors

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One variety of position-sensitive detector, in which the photon absorptions in different regions are counted separately, is a special type of proportional counter. The following description applies primarily to one-dimensional detectors for powder diffractometry; two-dimensional (area) detectors are treated in Section 7.1.6[link].

Position-sensitive detectors (PSD's) are being used in increasing number for various powder-diffraction studies. They have the great advantage of simultaneously recording a much larger region of the pattern than conventional counters. The difference in receiving apertures determines the gain in time. The position at which each quantum is detected is determined electronically by the system computer and stored in a multichannel analyser. There is a digital addition of each incident photon address and the angular address of the diffractometer.

The PSD's are available in short straight form and as longer detectors with curvature to match the diffractometer focusing circle. The short detectors can be used in a stationary position to cover a small angular range or scanned. Göbel (1982[link]) developed a high-speed method using a short (8° window) scanning PSD with 50 µm linear resolution in the diffractometer geometry shown in Fig. 2.3.1.12[link] (b). He was able to record at speeds of a hundred or more degrees a minute, and patterns with reasonably good statistical precision in several tens of degrees a minute. This is faster than conventional energy-dispersive diffraction and has the advantage of much higher resolution.

The PSD should be selected to match best the diffraction geometry. The detector is sensitive across the 1–2 cm gas-absorption path. If the diffracted rays are not perpendicular to the window, the parallax causes broadening and loss of resolution. This becomes important in the focusing geometries and can be minimized if the diffractometer and specimen focusing circles are nearly coincident. A large loss of resolution would occur in the conventional geometry, Fig. 2.3.1.3[link] , because only the central ray of a single reflection would be normal to the window. The problem is minimized in powder-camera geometry with a thin rod specimen, Fig. 2.3.4.1[link] (a), where the entire pattern can be recorded with a long, curved PSD (Ballon, Comparat & Pouxe, 1983[link]); see also Shishiguchi, Minato & Hashizume (1986[link]), Lehmann, Christensen, Fjellvåg, Feidenhans'l & Nielsen (1987[link]), Wölfel (1983[link]), and Foster & Wölfel (1988[link]).

7.1.3.4. Resolution, discrimination, efficiency

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The topics of energy resolution, pulse-height discrimination, quantum-counting efficiency, and linearity are common to proportional, scintillation and solid-state counters, and are treated in Subsections 7.1.4.3.[link][link]–7.1.4.5[link].

References

Ballon, J., Comparat, V. & Pouxe, J. (1983). The blade chamber: a solution for curved gaseous detectors. Nucl. Instrum. Methods, 217, 213–216.
Foster, B. A. & Wölfel, E. R. (1988). Automated quantitative multiphase analysis using a focusing transmission diffractometer in conjunction with a curved position sensitive detector. Adv. X-ray Anal. 31, 325–330.
Göbel, H. E. (1982). A Guinier diffractometer with a scanning position sensitive detector. Adv. X-ray Anal. 25, 315–324.
Lehmann, M. S., Christensen, A. N., Fjellvåg, H., Feidenhans'l, R. & Nielsen, M. (1987). Structure determination by use of pattern decomposition and the Rietveld method on synchrotron X-ray and neutron powder data; the structures of Al2Y4O9 and I2O4. J. Appl. Cryst. 20, 123–129.
Shishiguchi, S., Minato, I. & Hashizume, H. (1986). Rapid collection of X-ray powder data for pattern analysis by a cylindrical position-sensitive detector. J. Appl. Cryst. 19, 420–426.
Wölfel, E. R. (1983). A novel curved position-sensitive proportional counter for X-ray diffractometry. J. Appl. Cryst. 16, 341–348.








































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