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. 2.3, pp. 86-88

Section 2.3.4.2.2. Detection

C. J. Howarda* and E. H. Kisia

aSchool of Engineering, University of Newcastle, Callaghan, NSW 2308, Australia
Correspondence e-mail:  chris.howard@newcastle.edu.au

2.3.4.2.2. Detection

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All the neutron detector types discussed in Section 2.3.4.1.3[link] are capable of detecting the scattered neutrons in a TOF pattern. Gas-filled proportional counters such as BF3 and 3He detectors have efficiencies governed by the neutron energy (or wavelength). Detectors on CW diffractometers are optimized for the narrow band of wavelengths available when using a crystal monochromator, say 1–2.4 Å. The wavelength range in TOF diffraction is generally much wider; as much as 0.2–6 Å or more, and proportional detectors need to be specifically optimized. There is of course the added complexity of tracking the arrival time of each neutron and this has worked against the use of multi-wire proportional detectors and microstrip detectors as described in Section 2.3.4.1.3[link]. Instead, there is extensive use of scintillation detectors, which are usually based on the 6Li(n;t,α) reaction (Section 2.3.4.1.3[link]). When doped into the ZnS film of a scintillator, the 6Li provides excellent detection sensitivity and energy range. Discrimination against fast neutrons and γ-ray contamination in the incident beam is easily accommodated as these have different velocities to the thermal and epithermal neutrons used for TOF diffraction and are therefore readily excluded by the chopper system and detector electronics.

The detector electronics on older instruments recorded the diffraction pattern in a fixed set of time channels or bins; typically 1024 to begin with and progressively more as electronic and computational advances occurred. More recently, the technique has shifted to recording the data to memory in a continuous stream known as event mode, where the arrival time of each neutron is recorded. The user may then bin (and re-bin) the data into time channels to suit the resolution of the diffraction pattern, which may differ significantly from the instrument resolution because of microstructural features of the sample. Such features are discussed at length in Chapters 5.1 and 5.2[link] .

In a new development, a neutron-sensitive microchannel plate detector has been developed (Tremsin, McPhate, Vallerga, Siegmund, Feller et al., 2011[link]). Microchannel plate detectors (MCPs) are divided into discrete pixels and record the arrival time of each neutron in each pixel. Initially used for high-resolution radiography at pulsed neutron sources, it was quickly realized that MCP detectors can be used for diffraction via the Bragg-edge phenomenon (Tremsin, McPhate, Vallerga, Siegmund, Kockelmann et al., 2011[link]). The resolution is typically 55 µm due to the data-acquisition electronics but can be sharpened to less than 15 µm using centroiding techniques. This type of detector opens the door to spatially resolved neutron powder diffraction in materials as well as strain-imaging applications on TOF neutron diffractometers.

References

Tremsin, A. S., McPhate, J. B., Vallerga, J. V., Siegmund, O. H. W., Feller, W. B., Lehmann, E., Butler, L. G. & Dawson, M. (2011). High-resolution neutron microtomography with noiseless neutron counting detector. Nucl. Instrum. Methods Phys. Res. A, 652, 400–403.Google Scholar
Tremsin, A. S., McPhate, J. B., Vallerga, J. V., Siegmund, O. H. W., Kockelmann, A., Steuwer, A. & Feller, W. B. (2011). High-resolution neutron counting sensor in strain mapping through transmission Bragg edge diffraction. IEEE Sens. J. 11, 3433–3436.Google Scholar








































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