The new guides will view the existing liquid hydrogen cold source
through the "Cryogenic Tube West" (CTW) beam port that is currently
occupied for the development of the
MACS
spectrometer. MACS will move to beam port BT-9 which will contain
the new cold source.
FIGURE 1. New guides (in the foreground and in various colors)
viewing the cold source (dark blue). The existing guides are on the
left (pale blue).
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FIGURE 2. Perspective view of new guides, from the beam ends
looking towards the source. Note that NG-B splits into
lower and upper guides.
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Viewed in the horizontal plane (top view), the guides have a curved
section near the source end followed by a straight section. The
curved sections permit five beams to be extracted from the restricted
angle of the CTW opening and also reduce or eliminate the
need for crystal filters upstream usually required in straight
guides to remove unwanted short wavelength neutrons and gamma
rays. The straight portions help to minimize beam asymmetries
after transmission through the curved sections.
Again, viewed from the top, in order to promote total
reflection from the outer curve for the minimum wavelength
required by the instruments served, a greater critical angle
is needed compared to the inner curve. Thus the supermirrors
there have a higher m value (the effective supermirror
critical wavevector transfer to that of natural Ni).
In cases where the beam divergence must be restricted (e.g.,
for the SANS or NSE instruments), lower critical angle, high
reflectivity coatings are used on the straight portions and
inner radii of the guides (guides NG-A and NG-B, see
"General Layout" below),
For high flux guides (NG-C and NG-D), high m supermirrors
are used. Because the reflectivity of these supermirrors
usually decreases with increasing angle and m value,
focusing and defocusing sections have been used to
increase the transmission efficiency of these guides as
well as for beam reshaping. One extreme example is the
use of a quasi-elliptical vertical guide profile for
guide NG-C shown in the figure below.
FIGURE 3. A conceptual design for guide NG-C.
A conceptual design for guide NG-C is shown above in which a random
selection of 4 Angstrom wavelength trajectories make at least one
reflection from the top or bottom surface. (Straight-through
trajectories are omitted for clarity.) The colored portions
represent supermirror regions having different "m" values
(reflectivity cutoffs).
In the top view the curved guide sections near the source contain
elements approximating a parabolic curve, increasing the beam
width to 11 cm. The vertical profile is quasi-elliptical
with smaller "m" supermirror coatings used near the center, where
beam divergence is minimal.
Four initial guides become five:
- NG-A, which is designed for optimal intensity within a ~4 cm x ~4 cm
region at the sample. This guide is reserved for the
neutron spin-echo spctrometer,
which is to be relocated from its current position at NG-5.
The guide also allows the existing velocity selector to be used.
- NG-B, which will be split vertically into two guides: the lower one
will be directed to the
10 m SANS and the
upper one will be directed to
the 30 m SANS. The final straight sections of these guides will use
natural Ni coatings and will have 5 cm x 5 cm cross-sections.
- NG-C, which is being developed for the combined neutron physics station
aCORN and
neutron imaging facility NIF.
This guide is designed to have an 11 cm x 11 cm beam
at its exit.
- NG-D, which is directed to the vertical sample reflectometer relocated
from NG-1, and the new instrument
MAGIk.
The guide then converges to provide small area, high flux, beams for
prompt gamma activation analysis
(PGAA) and
cold neutron depth profiling
(CNDP).
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