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, pp. 649-650
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With the advantages of speed and simultaneity of data collection over broad angular ranges, position-sensitive detectors (PSDs) have seen considerable development (Convert & Chieux, 1986). They are based on the fundamental detection processes described in Section 7.3.3. Their dimensions, the introduction of various systems for position encoding and decoding, and the multiplication of the number of detection chains (with necessary adjustments and controls) have produced detection systems of a complexity that can no longer be grasped by a researcher only occasionally involved in neutron-scattering experiments. The following lines give only a very crude introduction to PSDs at a descriptive level.
A PSD is always mounted in a shield that has an opening limited by the cone defined by the sample size and the detection area. Given the large opening in the shield, the PSD is very sensitive to parasitic neutrons, especially those coming from materials in the main monochromatic beam. However, if sufficient care is taken with the sample environment and beam-stop design, the signal-to-noise ratio may be as good as or even better than in a single detector. The scattering angle is determined by the angular position of the origin of the PSD, the sample-to-detector distance, and the location of the neutron capture in the PSD.
In a PSD, each individual neutron-capture reaction in the continuous converter is localized via an internal encoding system followed by electronic decoding. There are three types of PSD: the gas resistive wire, the gas multielectrode, and the Anger camera (scintillation).
Table 7.3.5.1 gives the characteristics of some PSDs, each example being a good compromise of detector characteristics adapted to the instrument needs, powder diffraction, single-crystal diffraction, and small-angle neutron scattering (SANS). From this table, it seems that the characteristics of the various types of PSD that have been presented are nearly equivalent. The homogeneity of neutron detection over the PSD sensitive area is better than 5% in all cases. Among those PSDs, the gas PSDs, which have been installed since the early 1970s, are the most commonly used. The definite advantage of the gas multi-electrode PSD is to digitize the encoding, thus offering very good stability and reliability of the neutron localization. The resistance-encoding gas PSD has less complex electronics and permits a choice of pixel size (elementary detection area) and thereby definition. The Anger camera also offers the same flexibility in the choice of the pixel size and, because of the small thickness of the scintillator (1–2 mm), has very little parallax and is well adapted to TOF measurements. However, the γ sensitivity of the scintillators is much higher than that of the gas PSDs.
MURR: Missouri University Research Reactor, Columbia, Missouri, USA; ORNL: Oak Ridge National Laboratory, Tennessee, USA; ILL: Institut Laue–Langevin, Grenoble, France; ANL: Argonne National Laboratory, Argonne, Illinois, USA; CENG: Centre d'Etudes Nucléaires de Grenoble, Grenoble, France.
References: (1) Berliner, Mildner, Sudol & Taub (1983); (2) Abele, Allin, Clay, Fowler & Kopp (1981); (3) Institut Laue–Langevin (1988); (4) Roudaut (1983); (5) Ibel (1976).; (6) Schaefer, Naday & Will (1983); (7) Strauss, Brenner, Chou, Schultz & Roche (1983). |
PSDs permit a simultaneous measurement of intensities over relatively large areas. As compared with the scanned single detector, this opens the way to two possibilities, either shorter counting time (e.g. real-time experiments) or improved statistical precision. However, the high counting rates achieved, as well as the complexity of the detection, increase the effective dead time and the occurrence of defects. The efficient use of a PSD therefore requires a detailed understanding of its operation (detection process, encoding–decoding, data storage, etc.) and periodic tests and calibrations.
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