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

International Tables for Crystallography (2006). Vol. C, ch. 7.5, p. 667

Section 7.5.5. Complicating phenomena

A. J. C. Wilsona

aSt John's College, Cambridge CB2 1TP, England

7.5.5. Complicating phenomena

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7.5.5.1. Dead time

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After a count is recorded, the detector and the counting circuits are `dead' for a short interval, and any ionizing event occurring during that interval is not detected. This is important if the dead time is not negligible in comparison with the reciprocal of the counting rate, and corrections have to be made; these are large for Geiger counters, and may sometimes be necessary for counters of other types. The need for the correction can be eliminated by suitable monitoring (Eastabrook & Hughes, 1953[link]); other advantages of monitoring are described in Chapter 2.3[link] .

7.5.5.2. Voltage fluctuations

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Mains-voltage fluctuations, unless compensated, and unsmoothed high-tension supplies may affect the sensitivity of detectors and counting circuits, and in any case cause the probability distribution of the arrival of counts to be non-Poissonian. Backlash in the diffractometer drives may be even more important in altering the observed counting rates. As de Boer (1982[link]) says, the ideal distributions represent a Utopia that experimenters can approach but never reach. He observed erratic fluctuations in counting rates, up to ten times as big as the expected statistical fluctuations. When care is taken, the instabilities observed in practice are much less than those of the extreme cases described by de Boer. Stabilizing an X-ray source and testing its stability are discussed in Subsection 2.3.5.1[link] .

References

Boer, J. L. de (1982). Statistics of recorded counts. Crystallographic statistics, edited by S. Ramaseshan, M. F. Richardson & A. J. C. Wilson, pp. 179–186. Bangalore: Indian Academy of Sciences.
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.








































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