Instrument Description

Amongst all classical constant-wavelength pin-hole small-angle neutron scattering instruments, D22 is the one with the highest flux at the sample in a wavelength range of 0.45 to 4 nm.
This is due to:

• the brilliant horizontal cold source;
• the short rotor and high transmission of its velocity selector;
• the relatively large beam cross-section (55 x 40 mm).

A relatively narrow wavelength band (normally 10 %) is chosen by a Dornier (now Astrium) velocity selector from the neutrons leaving the horizontal cold source. Other wavelength resolutions (from 8 to 20 %) can be obtained by manually rotating the selector. The selector has a 25 cm long rotating drum with helical lamellae shaped in a three-dimensional mould. The maximal speed is 28300 rpm filtering a wavelength of about 0.45 nm. The selector can be manually rotated around its vertical axis for tuning the resolution and/or reaching shorter wavelengths. This option is very rarely used, though.
The primary neutron guide system is prepared for a chopper system for running D22 in time-of-flight mode. Better wavelength resolution could be achieved with optional devices, e.g. a 5 % selector or a double monochromator replacing neutron guide sections in the 'casemate'.

The virtual source-to-sample distance is chosen by a collimation system consisting of eight sections. Each section comprises three tubes, any one of which can be positioned on the beam axis. One tube contains a neutron guide of 40 x 55 mm; the second is equipped with an antiparasitic aperture; the third tube bears fixations for future neutron-optical equipment.

The source is seen from the sample within a solid angle that decreases by a factor of about two with each guide section which is removed. The collimation system is prepared for mounting coils allowing one to apply a guide field for polarized neutrons. A space for a polarizer is foreseen.

A removable transmission polarisation mirror has been installed in the selector bunker, and a radio-frequency spin flipper close to the sample zone. Tests with a 3He polariser cell have been perform to allow for polarisation anlysis. For details, see here.

The sample equipment consists of a remotely controlled XYZ and rotation table and a Eulerian cradle for mounting devices for working in air (e.g. sample changer, shear and stopped-flow apparatus, electro-magnets) or in vacuum.

The installation of a new fast detector, built in the framework of ILL's Millennium Programme, enabled us to improve the investigation of smaller objects (below 2 nm) and to perform kinetic experiments on smaller particles than before.

D22 possesses the largest area multidetector (³He) of all small-angle scattering instruments (active area 1 m²), with a pixel size of 0.8 x 0.8 cm, i.e. 16 K resolution elements. The detector consists of 128 linear sensitive Reuter-Stokes® detector tubes arranged vertically with a spacing of 8 mm. Each tube has a dead time of 2 μs. It moves inside a 2.5 m wide and 20 m long vacuum tube providing sample-to-detector distances of 1.1 m to 17.6 m. D22 thus covers a total q range of 4 x 10^-4 to 0.44 Å^-1 (no detector offset) or 0.85 Å-1 (with detector offset) in standard conditions.

In the case of isotropic scattering, i.e. such that does not depend on the azimuth, the D22 detector can be shifted laterally by 50 cm, and rotated by 20º for reducing parallax. In this configuration, a large q range is covered simultaneously. This is of advantage for instance for investigating real-time processes or unique samples that cannot be prepared identically for successive experiments. In a setting optimized for a maximal dynamic q range, a q_max/q_min ratio of as much as 50 can be thus be reached when a beam stop is not required. Typical values are 20 for the detector completely offset with a beamstop in place, and 10 without detector offset.

The VME memory card of 32 Mb can store 30 Mb of data, i.e. more than 400 frames of 16K of 4 byte words allowing for time-resolved measurements.