Instrument Description

Reactor hall, thermal beam H11
 
Monochromator
vertically focussed

graphite (002)
germanium (11n), (335)

flat

copper (220), (331)

 copper (111), (224)
Bragg angle 2θm 42.7°, 70° or 90°
wavelength 0.8 < λ/Å <2.4
Collimation
before monochromator 60' (natural)
Sample
max. beam size 10 mm
flux 107 ... 108 n cm-2s-1
angular range -60° < γ <140°
-45° < ω <210°
+56° < χ <205°
-180° < φ <180°
Detector
120° x 30° position-sensitive detector
detector gas: 3He (5 atm) + CF4 (1 atm)
Nominal resolution for fixed sample-detector distance of 76 cm 0.19° horiz. x 0.12° vert.

D19 is a single-crystal and fibre diffractometer, which incorporates a very large (120° x 30°) position-sensitive detector (PSD), and works best in the wavelength range 0.8 to 2.4 Å. D19 works most efficiently for unit cell volumes between 1000 and 100,000 Å3 i.e. cell edges from say 10 to 50 Å. Experiments requiring wavelengths below 0.8 Å are best performed on the hot-source D9 diffractometer, unless D19’s large PSD is required. D19 shares the H11 thermal beam tube with the two powder diffractometers D2B and D20. Important features of D19 are: - D19 “sees” the neutrons first (from part of the H11 beam tube: diameter 20 cm at the core end) - the D19 monochromator position is relatively close to the core giving high flux, and also has high resolution due to a maximum monochromator take-off angle of 90° - any one of four monochromators may be rotated into diffracting position using an ILL box-shaped monochromator mount so that the wavelength choice can be matched to the user’s sample.

The new very large banana PSD is of innovative design and works at high gas pressure (5 atm 3He + 1 atm CF4) for high efficiency coupled with high resolution (2.50 mm horizontally and 1.56 mm vertically; 640 x 256 pixels; 0.19° x 0.12° at 76 cm).

The horizontally mounted "banana" detector is set symmetrically about the equatorial plane, with the sample at the centre of horizontal curvature. The detector rotates on a gamma-arm (γ=2θ in the equatorial plane), on air cushions. The present sample orienter is a C-shaped Eulerian cradle with offset phi axis. This allows a larger range in omega than a conventional cradle, and easy installation of a compact 2-stage Displex which can cool below 20K. Installation of a kappa cradle is planned to further reduce blind regions.

In addition to its mainstream single crystal applications, D19 is unique for high-resolution studies of biological and industrial fibres; here also its high flux and large area detector allow the efficient coverage of large regions of reciprocal space. Data are collected for a range of fibre tilt angles, effectively eliminating the blind region in the meridional direction (fibre axis). For many biological fibres (eg DNA, polysaccharides, filamentous viruses), water is a key environmental parameter. The amount of H2O in the sample and the H2O/D2O substitution in biological samples is controlled by a purpose built automatic humidity control system.The desired relative humidity of the sample environment can be set by the user and maintained or adjusted by the instrument control software.

Finally, with D19's high-resolution PSD, texture studies including pole-figure determination, e.g. on Alpine rocks and magnetic samples, can be performed very efficiently.