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

International Tables for Crystallography (2018). Vol. H, ch. 1.1, pp. 15-16

Section Background from extraneous sources

R. E. Dinnebiera* and S. J. L. Billingeb,c

aMax-Planck-Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany,bDepartment of Applied Physics and Applied Mathematics, Columbia University, 500 West 120th Street, Room 200 Mudd, MC 4701, New York, NY 10027, USA, and cCondensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, PO Box 5000, Upton, NY 11973–5000, USA
Correspondence e-mail: Background from extraneous sources

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The most commonly observed extraneous, or parasitic, scattering is from the sample container (such as a capillary) that holds the sample during the measurement. Another large contribution may come from air scattering, which originates principally from scattering of the direct beam by molecules in the air in the beam path, both before and after the sample. Air-scattering effects can be minimized by enclosing as much of the beam path as possible in a tube which may be evacuated or where the air is replaced by a weakly scattering gas (such as He in the case of X-rays). Air scattering that is detected by the detector can also be reduced by careful collimation of the beams and then shielding the detector from detecting radiation that does not originate from the sample position. Collimating the incident beam is straightforward and results in a big reduction in air scattering. For point detectors it is also straightforward to collimate the scattered beam, but the modern trend towards using linear and area detectors makes this more difficult. There is sometimes a trade-off between collimating the scattered beam to reduce background and having uniform backgrounds that do not vary with angle because of incomplete angle-dependent collimation. Incomplete angle-dependent collimation can be very difficult to correct when trying to measure diffuse scattering quantitatively and the current trend is to have minimal secondary collimation.

There is increasing interest in carrying out in situ experiments under extreme conditions of pressure, temperature, magnetic field and so on (see Chapters 2.6[link] to 2.8[link] ). These experiments inevitably introduce additional scattering from the environment. Again, there is a balance between finding creative ways to reduce these backgrounds, and simply making them less problematic in the data analysis. For example, in a diamond-anvil cell, where the beam accesses the sample through the diamond, one can drill a hole part way through the diamond to accommodate the direct beam and make the direct beam small enough to fit in the hole. This increases the complexity of the measurement as alignment becomes harder, but it is usually worth it. Shielding structural parts of the environment cell with an absorbing material, such as lead for X-rays or a borated material for neutrons, can help to reduce unwanted background intensity a lot, as can making thin, transparent windows for the incident and scattered beams.

An additional source of background in the signal does not come from scattering at all, but from electrical noise in the detector electronics. For some types of detectors it may be important to measure `dark' exposures with the X-rays turned off and subtract these carefully from the experimental data. It is also possible to detect signals from cosmic rays, which can leave tracks in two-dimensional detector signals.

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