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Guides

(Last modified June 2011)


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.
New guides viewing the 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).
Perspective view of new guides
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.
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.

Perspective view of new guides
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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Last modified 19-October-2011 by website owner: NCNR (attn: John Copley)