Instrument Description

Guide hall n°2, cold guide H512
 
Monochromator
velocity selector Anatole Δλ/λ = 10 % (standard)
wavelength 4.5 < λ/Å < 40 (for Δλ/λ = 10 %)
Collimation
8 guide sections 55 x 40 mm
source-to-sample distances / m 1.4, 2.0, 2.8, 4.0, 5.6, 8.0,
11.2, 14.4, 17.6, variable
apertures at 19.1
Sample area
maximum flux at sample
(for Δλ/λ = 10%)
1.2x108 n cm-2 s-1
typical sample size 10 to 300 mm2
Detector
distances 1.1 ... 17.6 m
rotation -2° < 2Θ <22°
horizontal offset -5 ... 50 cm
area 102.4 x 98 cm2
pixel size 8 x 8 mm2
maximum counting rate 5 MHz
electronic noise 2 Hz for the whole multidetector

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).

Wavelength selection

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'.

Collimation

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.