International
Tables for Crystallography Volume F Crystallography of biological macromolecules Edited by M. G. Rossmann and E. Arnold © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. F. ch. 6.2, p. 136
Section 6.2.1.3.4. Velocity selectors
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 |
The relatively low speed of longer-wavelength neutrons (~600 m s−1 at 6 Å) enables wavelength selection by mechanical means (Lowde, 1960). In general, there are two classes of mechanical velocity selectors (Clark et al., 1966). Rotating a group of short, parallel, curved collimators about an axis perpendicular to the beam direction will produce a pulsed neutron beam with λ and determined by the speed of rotation. This is a Fermi chopper. An alternate method is to translate short, parallel, curved collimators rapidly across the neutron beam, permitting only neutrons with the correct trajectory to be transmitted. This is achieved in the helical velocity selector, where the neutron wavelength is selected by the speed of rotation and can be modified by changing the angle between the neutron beam and the axis of rotation (Komura et al., 1983). The neutron beam is essentially continuous, the resolution function is approximately triangular and the overall neutron transmission efficiency exceeds 75% in modern designs (Wagner et al., 1992).
References
Clark, C. D., Mitchell, E. W. J., Palmer, D. W. & Wilson, I. H. (1966). The design of a velocity selector for long wavelength neutrons. J. Sci. Instrum. 43, 1–5.Google ScholarKomura, S., Takeda, T., Fujii, H., Toyoshima, Y., Osamura, K., Mochiki, K. & Hasegawa, K. (1983). The 6-meter neutron small-angle scattering spectrometer at KUR. Jpn. J. Appl. Phys. 22, 351–356.Google Scholar
Lowde, R. D. (1960). The principles of mechanical neutron-velocity selection. J. Nucl. Energy, 11, 69–80.Google Scholar
Wagner, V., Friedrich, H. & Wille, P. (1992). Performance of a high-tech neutron velocity selector. Physica B, 180–181, 938–940.Google Scholar