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

International Tables for Crystallography (2006). Vol. C, ch. 4.2, p. 215

Section 4.2.3.2.3. Requirements for the absolute measurement of μl or (μ/ρ)

D. C. Creaghb

4.2.3.2.3. Requirements for the absolute measurement of μl or (μ/ρ)

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The following prescription should be followed if accurate, absolute measurements of [\mu_l] and [(\mu/\rho)] are to be obtained.

  • (i) X-ray source and X-ray monochromatization. The energy of the incident photons should be measured directly using reflections from a single-crystal silicon monochromator, and the energy spread of the beam should be measured. Measurements should be made of the state of polarization, since X-ray-polarization effects are known to be significant in some measurements (Templeton & Templeton, 1982[link], 1985a[link], 1986[link]). The results of a survey on X-ray polarization were given by Jennings (1984[link]). If a single-crystal monochromator is employed, it should be placed between the sample and the detector.

  • (ii) Collimation. It is of some advantage if both the incident-beam- and the transmitted-beam-defining slits can be varied in width.

    Should it be necessary to combat the effects of Laue–Bragg scattering in a single-crystal specimen, an incident beam with a high degree of collimation is required (Gerward, 1981[link]).

    To counter the effects of small-angle X-ray scattering, it may be necessary to widen the detector aperture (Chipman, 1969[link]). That these effects can be marked has been shown by Parratt, Porteus, Schnopper & Watanabe (1959[link]), who investigated the influence collimator and monochromator configurations have on X-ray-attenuation measurements.

  • (iii) Detection. Detectors that give some degree of energy discrimination should be used. Compromise may be necessary between sensitivity and energy resolution, however, and these factors should be taken into account when a choice is being made between proportional and solid-state detectors.

Whichever detection system is chosen, it is essential that the system dead-time be determined experimentally. For descriptions of techniques for the determination of system dead-time, see, for example, Bertin (1975[link]).

References

Bertin, E. P. (1975). Principles and practice of X-ray spectrometric analysis. New York: Plenum.
Chipman, D. R. (1969). Conversion of relative intensities to an absolute scale. Acta Cryst. A25, 209–214.
Gerward, L. (1981). X-ray attenuation coefficients and atomic photoelectric absorption cross sections of silicon. J. Phys. B, 14, 3389–3395.
Jennings, L. D. (1984). The polarization ratio of crystal monochromators. Acta Cryst. A40, 12–16.
Parratt, L. G., Porteus, I. O., Schnopper, H. W. & Watanabe, T. (1959). X-ray absorption coefficients and geometrical collimation of the beam. Rev. Sci. Instrum. 30, 344–347.
Templeton, D. H. & Templeton, L. K. (1982). X-ray dichroism and polarized anomalous scattering of the uranyl ion. Acta Cryst. A38, 62–67.
Templeton, D. H. & Templeton, L. K. (1985a). X-ray dichroism and anomalous scattering of potassium tetrachloroplatinate(II). Acta Cryst. A41, 356–371.
Templeton, D. H. & Templeton, L. K. (1986). X-ray birefringence and forbidden reflections in sodium bromate. Acta Cryst. A42, 478–481.








































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