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DCS Design Features and Principal Specifications

The Primary Spectrometer
High energy neutrons and gammas from the cold neutron source are removed using an optical filter supplemented by a cooled graphite filter.
A cleanpulsed monochromatic neutron beam is produced using seven disk choppers.
Incident wavelengths range from ~0.23 to ~1.0 nm, i.e. ~2.3 to ~10 Å
Chopper speeds may be varied from 1200 to 20000 rpm.
Each of the pulsing and monochromating choppers has three slots of different widths. This permits three choices of resolution at a given wavelength and master chopper speed.
The chopper phase stability is excellent, of order 100-200 ns at 20000 rpm.
A white beam fission chamber monitors the beam at the local shutter, downstream from the crystal filter, upstream of the choppers.
A low efficiency 3He beam monitor is placed at the entrance to the sample chamber.

The Sample Area
The inside diameter of the sample chamber is ~870 mm.
The sample chamber is readily accessible from above and from the side.
The beam at the sample position is 30 mm wide (in the "low" resolution mode of operation), 100 mm high.
An oscillating radial collimator, inside radius 200 mm, outside radius 300 mm, blade separation 2°, is used to reduce the scattering from sample environment structures.

The Secondary Spectrometer
There are 913 six atmosphere 3He detectors, covering ~0.65 steradians and arranged in three banks:
The middle bank detector scattering angles range from -30° to -5° and from +5° to +140°
The upper and lower bank detector scattering angles range from -30° to -10° and from +10° to +140°
There is continuous coverage in detector scattering angle; there are no gaps due to support structural members.
The flight distance from sample to detectors is 4010 mm.
The flight chamber, inside radius 615 mm, outside radius 3905 mm, is purged with argon.
The window between the sample chamber and the flight chamber is made of 0.001" (0.025 mm) aluminum foil.
The window between the flight chamber and the detectors is made of 0.00035" (0.009 mm) aluminum foil.
A rectangular tube, internally and externally lined with cadmium, and ~3260 mm in length is placed in the space between the windows of the flight chamber, in line with the incident neutron beam.
A fission chamber beam monitor is installed in line with the incident neutron beam, 4054 mm from the sample position. A beam stop is placed immediately downstream of this fission chamber.

Data Acquisition Hardware and Software
VME-based data acquisition boards generate 32-bit event words with time channel and detector channel information. Each event word is passed to the data acquisition computer which increments the appropriate address in a 2-dimensional data histogram.
The histogram is stored in a 4 Mbyte reflective memory module that communicates via fibre optic cables with a second 4 Mbyte reflective memory in the instrument control computer.
The data acquisition software allows users to define sequences that include "runs", wavelength changes, temperature changes and wait periods. A run is broken into "cycles"; at the end of each cycle the accumulated dataset for the completed cycles is backed up to the disk.

General Remarks
Critical distances, such as the distances from the choppers to the sample, and from the sample to the detectors, are accurately known.
The detectors are mounted on racks in such a way that they can be precisely and reproducibly positioned.

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Last modified 19-June-2012 by website owner: NCNR (attn: Bill Kamitakahara)