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

International Tables for Crystallography (2006). Vol. C, ch. 2.5, p. 84

Section 2.5.1.2. Incident X-ray beam

B. Burase and L. Gerwardb

2.5.1.2. Incident X-ray beam

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(a) Bremsstrahlung from an X-ray tube. Bremsstrahlung from an X-ray diffraction tube provides a useful continuous spectrum for XED in the photon-energy range 2–60 keV. However, one has to avoid spectral regions close to the characteristic lines of the anode material. A tungsten anode is suitable because of its high output of white radiation having no characteristic lines in the 12–58 keV range.

A drawback of Bremsstrahlung is that its spectral distribution is difficult to measure or calculate with accuracy, which is necessary for a structure determination using integrated intensities [see equation (2.5.1.7)[link]]. Bremsstrahlung is strongly polarized for photon energies near the high-energy limit, while the low-energy region has a weak polarization. The direction of polarization is parallel to the direction of the electron beam from the filament to the anode in the X-ray tube. Also, the polarization is difficult to measure or calculate.

(b) Synchrotron radiation. Synchrotron radiation emitted by electrons or positrons, when passing the bending magnets or insertion devices, such as wigglers, of a storage ring, provides an intense smooth spectrum for XED.

Both the spectral distribution and the polarization of the synchrotron radiation can be calculated from the parameters of the storage ring. Synchrotron radiation is almost fully polarized in the electron or positron orbit plane, i.e. the horizontal plane, and inherently collimated in the vertical plane. Full advantage of these features can be obtained using a vertical scattering plane. However, the mechanical construction of the diffractometer, the placing of furnaces, cryogenic equipment, etc. are easier to handle when the X-ray scattering is recorded in the horizontal plane. Recent XED facilities at synchrotron-radiation sources have been described by Besson & Weill (1992[link]), Clark (1992[link]), Häusermann (1992[link]), Olsen (1992[link]), and Otto (1997[link]).

References

Besson, J. M. & Weill, G. (1992). EDX station for high pressure at LURE (DCI). High Press. Res. 8, 715–716.
Clark, S. M. (1992). A new white beam single crystal and powder diffraction facility at the SRS. Rev. Sci. Instrum. 63, 1010–1012.
Häusermann, D. (1992). New techniques for new sources: a fresh look at energy-dispersive diffraction for high-pressure studies. High Press. Res. 8, 647–654.
Olsen, J. S. (1992). Instrumentation for high-pressure X-ray diffraction research at HASYLAB. Rev. Sci. Instrum. 63, 1058–1061.
Otto, J. W. (1997). A facility for high-pressure X-ray diffraction at HASYLAB. Nucl. Instrum. Methods, A384, 552–557.








































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