International
Tables for Crystallography Volume C Mathematical, physical and chemical tables Edited by E. Prince © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. C. ch. 7.3, p. 652
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A good diffractometer on a reactor is supposed to give point by point for each solid-angle element dΩ an exact image of the scattered intensity. This is never perfectly achieved in practice and necessitates some corrections. In all cases, it is very important to be aware that additional background might be created when part of the shielding or of the collimator intersect the monochromatic neutron beam. We suppose for the following discussion that this effect has been corrected or avoided. The wavelength dependence of the detector response (which is needed for inelasticity corrections or TOF measurements) is generally computed from the theoretical detector law [see equation (7.3.2.1)]. At each measuring point, the collected intensity is renormalized by the integrated incident neutron flux, which is measured by the monitoring device.
In the case of TOF measurements on a spallation source, the measured intensity must also be renormalized by the wavelength spectrum of the source, obtained from the measurement of an isotropic scatterer such as vanadium.
For up to 10% of dead time in the counting rate, the correction for the dead-time loss is generally considered as linear. If Δt is the electronic dead time for one neutron (1–10 µs for the gas detectors) and n the number of counts per second, the dead-time correction factor is 1/(1 − nΔt).
In the case of a bank of detectors used for a powder diffractometer in a reactor, one has to calibrate the relative positions of the detectors and their response to the neutron intensity by scanning the detectors through a Bragg pattern.
In the case of TOF measurements, the detector banks are installed at fixed angles. For each detector, the measured intensity depends on the detector type, size, and distance to the sample. The neutron and γ background depends moreover on the detection angle. After background corrections, the intensities measured by each detector bank are calibrated and matched using the overlaps between spectra.
References
Berliner, R., Mildner, D. F. R., Sudol, J. & Taub, H. (1983). Position-sensitive detectors and data collection systems at the University of Missouri Research Reactor Facility. Position-sensitive detection of thermal neutrons, edited by P. Convert & J. B. Forsyth, pp. 120–128. London: Academic Press.Google Scholar