International
Tables for
Crystallography
Volume H
Powder diffraction
Edited by C. J. Gilmore, J. A. Kaduk and H. Schenk

International Tables for Crystallography (2018). Vol. H, ch. 2.2, p. 51

Section 2.2.1. Introduction

A. Fitcha*

aESRF, 71 Avenue des Martyrs, CS40220, 38043 Grenoble Cedex 9, France
Correspondence e-mail: fitch@esrf.fr

2.2.1. Introduction

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X-rays produced at a synchrotron source are exploited in a wide range of applications in crystallography and structural science, and this includes studies by powder diffraction. Many synchrotron-radiation facilities have one or more dedicated powder-diffraction beamlines or end stations in regular user service. The high intensity, collimation and wavelength tunability of the radiation allow instruments to be designed whose performance and flexibility surpass what is possible with conventional laboratory apparatus. The majority of instruments operate with monochromatic radiation and an angle-dispersive diffractometer, but the polychromatic nature of synchrotron radiation means that an energy-dispersive setup is also possible. The general properties of synchrotron radiation include:

  • (a) High brightness, i.e. a highly collimated, intense X-ray beam and small source size.

  • (b) High flux of photons delivered to the sample.

  • (c) A range of wavelengths is available, possibly extending from the soft to the hard X-ray regimes, depending on the facility.

  • (d) Polarized source: synchrotron radiation is linearly polarized with the electric vector lying in the plane of the synchrotron orbit, but becomes progressively less polarized out of the plane.

  • (e) Pulsed source: the distribution of the bunches of electrons circulating in the storage ring allows the time structure to be exploited for specialized experiments.

Further information about the nature of synchrotron radiation can be found in texts by, for example, Margaritondo (1988[link]), Als-Nielsen & McMorrow (2001[link]) and Kim (2001[link]).

Synchrotrons are usually user facilities, where scientists from external laboratories visit to perform experiments that have been approved by a peer-review or other procedure, and are supported by the scientific and technical staff for the beamlines. Most facilities have regular rounds in which users submit proposals for beam time, with special arrangements for access to carry out proprietary research. Arrangements can also usually be made for urgent access to the facility (when justified), and some beamlines run a routine mail-in service, allowing samples to be measured under defined conditions without the user needing to attend.

For any powder X-ray diffraction experiment, the wavelength of the radiation to be used is of high importance. The wavelength, λ, is a measure of the photon energy, [epsilon], and the terms `photon energy' and `wavelength' tend to be used interchangeably at synchrotron beamlines. They can readily be converted by[\varepsilon = h\nu = hc/\lambda,]where h is the Planck constant, ν is the frequency of the radiation and c is the speed of light. If expressed in convenient units with λ in Å and [epsilon] in keV then[\varepsilon\ [{\rm keV}] = hc/e\lambda \times 10^7\ [{\rm \AA}] \simeq 12.3984/\lambda \ [{\rm \AA}]\simeq 12.4/\lambda \ [{\rm \AA}],]where e is the elementary charge.

References

Als-Nielsen, J. & McMorrow, D. (2001). Elements of Modern X-ray Physics. New York: Wiley.Google Scholar
Kim, K.-J. (2001). Characteristics of synchrotron radiation. X-ray Data Booklet, edited by A. C. Thompson & D. Vaughan. Lawrence Berkeley National Laboratory, USA. http://xdb.lbl.gov/Section2/Sec_2-1.html .Google Scholar
Margaritondo, G. (1988). Introduction to Synchrotron Radiation. Oxford University Press.Google Scholar








































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