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High-Resolution Diffractometers

Powder neutron diffractometers are very large in comparison to laboratory X-ray instruments as shown by the scale in the figure below, which shows the layout of the high-resolution powder neutron diffractometer D2B at the ILL. The term high-resolution refers not to the smallest peak width in the powder diffraction pattern, but to the fact that the instrumental resolution function results in narrow peaks at high scattering angle. This is achieved by having large monochromator take-off angle as shown in the figure, which for this instrument is 135°. The layout of the instrument is "Debye-Scherrer" in the sense that it uses a cylindrical sample with transmission geometry.

With the detector bank placed so that the configuration of incident, monochromated, and diffracted beams is "Z" shaped, a focussing effect occurs: this produces a minimum in the resolution function for the diffractometer at high scattering angle as shown in the next figure. The resolution function is determined from the full-width of the peaks at half-maximum height (FWHM) as a function of scattering angle from a standard sample. (Note that if the detector bank was positioned on the other side of the diffractometer, which corresponds to the "negative" 2θ angles, the diffraction peak widths would simply increase as a function of increasing scattering angle.)


The large value of the monochromator angle is required so that the wavelength dispersion, Δλ, of the incident beam is kept small. In order to achieve high resolution, the instrument must be able to count neutrons with a very precise Bragg angle. This is achieved by the use of Soller collimators placed in front of each detector. Finally, given the relatively low flux available at neutron sources, instrument efficiency is greatly improved by using many detectors at the same time: the early powder neutrons diffractometers used only a single detector; the first high-resolution powder instrument at the ILL known as D1A had a bank of 10 detectors (though this has been increased in recent years to 25); while the highest-resolution powder diffractometer at the ILL, D2B, was designed with 64 detectors so as to cover virtually the entire 2θ range.

The instruments function in a similar way to that of a laboratory diffractometer in that the detector bank of n detectors is step scanned in 2θ. In principle, a complete diffraction pattern can be obtained by just stepping through a 2θ range that is equal to the detector separation angle, which is 2.5° for D2B, and 6° for the older instrument D1A.


The photograph shows the medium- to high-resolution diffractometer D1A at the ILL as seen throughout the 1980's and early 1990's. The geometry is the mirror image of the top figure, with the monochromated neutron beam coming from the far right-hand side (near the yellow radiation sticker). The beam stop is the large dark-yellow cylindrical object seen on the far left of the picture. The detectors and Soller collimators are inside the black detector bank, mounted on the yellow 2θ arm of the diffractometer. The bright orange device in the middle of the photograph is a liquid helium cryostat, which is routinely used to achieve sample temperatures as low as 1.5 K. To the right of the yellow radiation sign, is a set of slits and beam monitor; horizontal slits are used to match the incident beam diameter to that of the sample container. The instrument is sited on the thermal guide tube H22; the minimum wavelength is 0.95 Å. With a take-off angle of 122.7°, the Ge monochromator (113), (115), and (117) planes provide the useful wavelengths 2.994, 1.911, and 1.390 Å, respectively. Since the wavelength cut-off for the neutron guide H22 is just below 2.994/3 Å, then the wavelength from the (113) planes is very slightly contaminated by λ/3 from the co-parallel (339) planes.

The use of Soller collimators on the high-resolution diffractometers enables cryostats and furnaces to be used without any problems from spurious scattering off the walls of these devices, e.g. Bragg scattering from the front and back aluminium walls of the cryostat tail is directly eliminated, the collimation having its maximum effect at 2θ equal to 90°, and no effect at 0° and 180°.

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© Copyright 1997-2006.  Birkbeck College, University of London. Author(s): Jeremy Karl Cockcroft