Neutron guides

Schoenborn, B. P.; Knott, R.

doi:10.1107/97809553602060000666

International
Tables for
Crystallography
Volume F
Crystallography of biological macromolecules
Edited by M. G. Rossmann and E. Arnold

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International Tables for Crystallography (2006). Vol. F. ch. 6.2, p. 136 | 1 | 2 |

Section 6.2.1.3.5. Neutron guides

B. P. Schoenborn^a ^* and R. Knott^b

^a Life Sciences Division M888, University of California, Los Alamos National Laboratory, Los Alamos, NM 8745, USA, and ^bSmall Angle Scattering Facility, Australian Nuclear Science & Technology Organisation, Physics Division, PMB 1 Menai NSW 2234, Australia
Correspondence e-mail: schoenborn@lanl.gov

6.2.1.3.5. Neutron guides

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In order for a collimator to be effective, its walls must absorb all incident neutrons. The angular acceptance is strictly determined by the line-of-sight geometry. Neutron guides can be used to improve this acceptance dramatically and to transport neutrons with a given angular distribution, almost without intensity loss, to regions distant from the source (Maier-Leibnitz & Springer, 1963). The basic principle of a guide is total internal reflection. This occurs for scattering angles less than the critical angle, $[\theta_{c}]$ , given by $[\theta_{c} = 2(1 - n)^{1/2},]$ where n is the (neutron) index of refraction related to the coherent scattering length, b, of the wall material, viz, $[n = 1 - (\lambda^{2} \rho b/2\pi),]$ where ρ is the atom number density (in $[\hbox{cm}^{-3}]$ ). Among common materials, Ni with $[b = 1.03 \times 10^{-12}\;\hbox{cm}]$ , in combination with suitable physico-chemical properties, provides the best option, with a critical angle $[\theta_{c} = 0.1 \lambda]$ (in Å). The dependence of $[\theta_{c}]$ on λ implies that guides are more effective for long-wavelength neutrons. With the introduction of supermirror guides with up to four times the $[\theta_{c}]$ of bulk Ni, both thermal and cold neutron beams are being transported and focused with high efficiency (Böni, 1997).

While a straight guide transports long wavelengths efficiently, it continues to transport all neutrons within the critical angle, including non-thermal neutrons emitted within the solid angle of the guide. This situation may be modified significantly by introducing a curvature to the guide. Since a curved neutron guide provides a form of spectral tailoring (cutoff or bandpass filters), simulation is a distinct advantage in exploring the impact of guide geometry on neutron-beam quality (van Well et al., 1991; Copley & Mildner, 1992; Mildner & Hammouda, 1992).

References

Böni, P. (1997). Supermirror-based beam devices. Physica B, 234–236, 1038–1043.Google Scholar

Copley, J. R. D. & Mildner, D. F. R. (1992). Simulation and analysis of the transmission properties of curved–straight neutron guide systems. Nucl. Sci. Eng. 110, 1–9.Google Scholar

Maier-Leibnitz, H. & Springer, T. (1963). The use of neutron optical devices on beam-hole experiments. J. Nucl. Energy, 17, 217–225.Google Scholar

Mildner, D. F. R. & Hammouda, B. (1992). The transmission of curved neutron guides with non-perfect reflectivity. J. Appl. Cryst. 25, 39–45.Google Scholar

Well, A. A. van, de Haan, V. O. & Mildner, D. F. R. (1991). The average number of reflections in a curved neutron guide. Nucl. Instrum. Methods A, 309, 284–286.Google Scholar

International Tables for Crystallography (2006). Vol. F. ch. 6.2, p. 136