DCS Instrument Details
The NG4 neutron guide incorporates an
which removes almost all of the high energy neutrons and
gamma-rays in the initial section of the guide.
After an auxiliary cooled graphite filter
and local shutter there are
seven disk choppers
that collectively produce clean monochromatic bursts of
neutrons at the sample position. The sample is located in
sample chamber, accessible from above and from
the side; a radial collimator reduces scattering from the
sample environment. The flight chamber
is filled with argon gas at atmospheric pressure to reduce
the possibility of gas scattering.
Each of the 15 detector racks holds three rows of
and amplifier-discriminators. The
data acquisition system timestamps each neutron
event, attaches the detector number, and histograms the information
in up to 1024 time channels.
The Neutron Guide and Optical Filter
The NG4 neutron guide within the confinement building is 60 mm wide
and 150 mm high. Approximately 12 m from the start of the guide, at B, an
asymmetric taper is introduced as shown in the attached figure, such
that after a further ~6.9 m, at C, the guide width is reduced to 30 mm.
Thereafter the guide has parallel sides once again, but its direction has
been rotated through ~0.25°. All points beyond point D, which is ~25.3 m
from C, are such that there
is no line of sight to the guide entrance. The guide ends at E, ~26.0 m
downstream from C.
The top and bottom surfaces of the guide are coated with a supermirror coating
that has twice the critical angle of natural nickel at any given wavelength.
The side surfaces are coated with a multilayer coating such that
the critical angle for total reflection is about 1.2 times that of
natural nickel at any given wavelength.
The idea of the optical filter is that since there is no line of sight to the
source from the end of the guide neutrons and gamma rays cannot reach the
end of the guide unless they have been reflected or scattered.
The Cooled Graphite Filter
Notwithstanding theory, which says that no neutron with wavelength less
than ~ 0.6 Å can be transmitted from the cold source to the end of the
guide (point E above) by specular reflection, slightly shorter wavelength
neutrons have been observed, and we now understand why;
see publication 5(d).
To remove these neutrons we have installed a liquid nitrogen cooled
pyrolytic graphite filter made of eleven 40 mm wide by 110 mm high
"filter grade" ("ZYH") blocks, stacked with their short dimension
parallel to the beam. The total length of the filter is 100 mm.
We have installed a cooled graphite
filter in the white beam, roughly 0.5 m upstream from the local shutter.
This permits measurements at wavelengths down to about 2.3 Angstroms but
measurements at wavelengths near 3.335 and 6.67 Angstroms will not be
The DCS uses a set of seven chopper disks and a partitioned guide to produce
pulses of neutrons of a single wavelength at the sample position. There are
three slots in each of the disks belonging to the first and last
counter-rotating chopper pairs. By appropriately phasing these disks the
resolution of the instrument can be changed, without having to change
the incident wavelength or the speed of the choppers.
The Sample Chamber
The sample chamber is a double-walled aluminum vessel. It is accessible
from above and from the side through a large gas-tight shielded door.
A cover plate can be installed to seal the top opening.
The chamber can be purged with a breathable gas (80% Ar, 20% O2).
The sample table has 3 translation stages and 3 rotation stages.
In its home position the sample table is 170 mm below the beam center line.
A radial collimator is separately installed. Its inside and outside radii are
200 mm and 300 mm. Its inside height is 250 mm.
It has 84 Gd2O3-painted blades at 2° intervals.
The Flight Chamber
Most of this section has yet to be written.
A new window, separating the argon-filled flight chamber from the
detectors, has recently been installed. It is a very thin (0.00035",
i.e. ~9 micron) 1100 aluminum foil. The window between the sample chamber and
the flight chamber has also been changed, from a 3 mm 6061 Al alloy plate
to a 0.076 mm (0.003") 1100 Al foil.
This section is yet to be written.
The Data Acquisition System
This section is yet to be written.
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Last modified 11-March-2004 by website owner: NCNR (attn: Bill Kamitakahara)