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, pp. 618-619

Section 7.1.2. Geiger counters2

W. Parrishf and J. I. Langforde

7.1.2. Geiger counters2

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Geiger-Müller counters (Geiger & Müller, 1928[link]) are now obsolete for data collection, but are still used in portable monitors for X-rays. A cross section of a once-popular type is shown in Fig. 7.1.2.1(a)[link] . The cathode C is a cylinder made of a metal such as chrome-iron, about 2 cm in diameter and 10 cm long. The anode A is a tungsten wire about 0.7 mm in diameter mounted coaxially with C and terminated by a bead to prevent destructive electrical discharges from its tip. About 1400 V DC is applied between C and A. X-rays enter at a low-absorption end window W, made of mica about 0.013 mm thick or other suitable material; beryllium would now be used. The gas filling may be argon at a pressure of about 55 cm Hg or krypton at a lower pressure. A small amount of halogen (∼0.4% of chlorine or bromine) helps to avoid destructive discharges. Separating the anode and the window is a dead space in which X-rays are absorbed but not detected.

[Figure 7.1.2.1]

Figure 7.1.2.1 | top | pdf |

Detectors used for diffractometry: (a) Geiger counter, (b) side-window proportional counter, (c) end-window scintillation counter. The arrows X show direction of incident X-ray beam, W thin window, C cathode, A anode, SC scintillation crystal, PT photomultiplier tube.

The quantum-counting efficiency varies with wavelength; for Cu K and its neighbours, it is about 50% and, for Mo K, it is about 10%. For the longer wavelengths, it is limited by absorption in the window and the dead space, so it is important to keep these as thin as practicable. For the shorter wavelengths, it is limited by the transparency of the gas in the sensitive volume.

The tube is not uniformly sensitive across its diameter. The maximum sensitivity is confined to the cylindrical volume shown cross-hatched in the figure. The diameter of this sensitive area depends on the gas filling and the geometry of the tube. For maximum efficiency, the X-ray beam should be directed along and close to the anode, but should not strike it. Geiger counters are not critically temperature-dependent. Linearity of response is limited by the dead-time following the discharge initiated by the absorption of a quantum and the magnification of the few hundred ions produced to some millions by their acceleration under the electric field. To produce this amplification, a certain minimum threshold voltage is required. Above this minimum, there is a plateau extending for several hundred volts within which the number of quanta detected is essentially independent of the applied voltage and the size of the pulses is essentially independent of the energy of the absorbed quantum.

Geiger counters are simple to use and show little deterioration even after prolonged use. However, since the pulses are all of about the same size, pulse-height discrimination cannot be used, and the long dead-time limits linearity of response unless special monitoring circuits are used (Eastabrook & Hughes, 1953[link]). They have been almost completely superseded by other types of counter, described in Sections 7.1.3.[link][link][link][link][link]–7.1.8[link].

References

Eastabrook, J. N. & Hughes, J. W. (1953). Elimination of dead-time corrections in monitored Geiger-counter X-ray measurements. J. Sci. Instrum. 30, 317–320.
Geiger, H. & Müller, W. (1928). Das Elektronenzählrohr. Z. Phys. 29, 839–841.








































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