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Double Focusing Thermal Triple-Axis Spectrometers
Contact: Jeff Lynn, (301)975-6246, Jeff.Lynn@nist.gov
As part of the modernization of the thermal neutron spectrometers, a new state-of-the-art triple-axis instrument is being commissioned at the BT-7 thermal beam port, and a second spectrometer is under development to be installed at the BT-1 beam port. The new instruments will take full advantage of the large 16 cm diameter beam tube, with 20×20 cm2 double focusing monochromators. Two monochromators are available, a pyrolytic graphite monochromator for both instruments, and Cu(220) for BT-7 and Ge(311) for BT-1. The analyzer system also provides horizontal focusing capability, and together these new capabilities will provide signals that are two orders-of-magnitude larger than available with the original thermal triple-axis instruments. The analyzer system can also be used as a flat crystal array coupled with a position-sensitive detector to allow the simultaneous collection of data over a range of (Q,E). A schematic of the spectrometer is shown in Fig. 1.
Figure 1. Cut-away layout of the new instrument. The size of the beam exiting from the source can be varied by a set of slits, and will pass through a filter and a system with a selection of horizontal collimations, before striking the double-focusing monochromator. The monochromatic beam impinges on the sample table that is cantilevered from the monochromator drum. The analyzer system is on air pads.
Source Beam Optics
On the new thermal instruments, the experimental beam shutter is contained within the biological shield of the source, and simply has two positions, open and closed. Outside the in-pile shutter, all the thermal beam holes are (nominally) identical. The beam shutter opening is 6.4 cm wide at the exit, opening to 9 cm on the end toward the source. The vertical opening is 16 cm. The shutter opens or closes in about ten seconds.
The next element is the variable apertures to reduce the size of the beam from 6.
4x16 cm2 to a smaller size when needed. The blades are composed of compressed lithium fluoride on the surface toward the source, with a 10 cm thick aluminum frame holder behind this filled with B4C. We did not use any gamma shielding (such as lead to tungsten) on these blades, which greatly reduces the load on the translation system.The next element is the filter, which is a tunable pyrolytic graphite (PG) filter system. No cryogenically cooled filters are included in the design at this time, but an option for additional filters is included in the design.
Rotating Collimator Selector
The rotating collimator-exchange system has been designed, built, and installed. There are three Söller-slit collimations, 10', 25', and 50', along with an open position for horizontal energy focusing.
Option for 3He polarizer or other capabilities.
Additional room has been included in the design to allow for future options. This also has the benefit of moving the monochromator drum out away from the source, so that higher take-off angles for the monochromator drum can be achieved.
Monochromator Drum
The monochromator drum is 213 cm in diameter, with a 40.6 cm inner diameter to accommodate the monochromator systems. The angular range of the drum is from the straight-through position (for optical alignment purposes) to 115 degrees scattering angle. The practical angular range will be from the smallest angle possible due to radiation considerations (~16°), to the largest angle possible given the geometrical constraints imposed by interference with other instruments and facilities. This will be ~75° on BT-7, and the full 115° on BT-1.
There are three additional components that have been designed for the monochromator drum. One is the stationary "pipe" that goes from the edge of the drum on the source side, to the center post. This is an essential piece of shielding, as it determines to a substantial extent the lowest achievable drum angle that can be used on the working instrument. It also contains a vertical magnetic field for polarized neutrons if a white-beam polarizer (such as 3He) becomes available. The size of the neutron beam onto the monochromator is 20 cm high 11 cm wide. The drum design itself can accommodate a beam considerably wider than 11 cm, and this will allow substantial neutron and gamma shielding here.
The second item is the double-focusing monochromator systems, and the support and shielding systems associated with these. Initially we have two monochromator systems, a PG graphite system, and either Cu(220) or Ge(311)monochromator. Each one consists of squares that are 2x2
cm2, with a total height of 20 cm and a width of 20 cm. The monochromator systems are on an elevator and can be interchanged remotely.The third component is the beam optics for the monochromatic beam to the sample. This has the following capabilities:
a) Guide field, for polarized neutron beam operation.
c) Söller-slit collimations, 10', 25', and 50', and open. The beam width with collimation will be 3.8 cm.
d) Horizontal beam focusing. Without collimators, there will be an option where the beam will be converging horizontally (and vertically, of course) onto the sample, with a maximum sample size of 2.5 cm 5 cm high. This is a "pie-shaped" shielding plug in order to achieve the desirable low instrumental background. Changes in collimation will be partially automated, and can be accomplished without the need to remove the sample and environmental system from the sample position.
e) Beam limiters. At least two pairs, one on the monochromator side (which may double as a beam shutter), and one on the sample side. These will move symmetrically about the nominal beam center both horizontally and vertically.
f) Sample flux monitor.
g) Spin Flipper option, in the case of polarized beam operation. He3 polarizers before and after the sample are available.
h) Room for other options, such as an extra PG filter, or cold Be filter after the sample, which are available.
i) An option for optically aligned Söller slit collimation in the vertical direction. This will probably be best achieved by using a specially designed Söller slit, and will be used to align the tilt of the monochromator so that the incident beam is level to within 0.1 degrees or better.
Sample
The sample table system is fixed to the monochromator drum (see Fig. 1). The sample table has a travel of 35 cm along the monochromatic beam direction (toward the monochromator). This will allow the experimenter to vary the maximum scattering angle from the sample, and to allow some flexibility when adding ancillary equipment. This motion is computer controlled.
The sample table has a horizontal angular motion (rotation), and a second concentric horizontal motion is available to allow the independent rotation of a (horizontal field) magnet. The goniometers mounted on an elevator, which has a vertical travel of +2.5 cm above the standard height, and 7.5 cm below.
With the exception of the incident beam and the analyzer/detector system, the sample will be surrounded by neutron absorbing shielding to reduce experimental background, as well as to prevent access to the sample area when the beam is on. There is a beam stop following the sample to "catch" the neutron and gamma beam transmitted by the sample. This will be retractable to allow for low-angle operation of the analyzer system. It must provide adequate shielding for health physics purposes, and adequate neutron shielding so as not to cause an instrumental background problem on BT-8.
Analyzer/Detector Systems
For the analyzer portion of this machine several different types of systems have been proposed. Each system will be attached to the 2q arm of the sample table through a hinge mechanism, which will allow modest vertical displacements while providing the lateral rigidity necessary to assure the proper angular precision. The connection will be designed in a “quick” coupling modular fashion so that other analyzer/detector systems can be accommodated and quickly interchanged. To achieve this rapid interchangeability, the detector electronics and motor command movements must be integrated into each analyzer unit so that the only connections are power, a communications cable, and compressed air. The unit will move along the floor on an air-pad suspension to accommodate the weight and varied footprint of the sample-analyzer distance. The floor is a poured epoxy base covered by anodized aluminum tooling-plate tiles.
The first new analyzer system installed on BT-7 is a horizontally focused pyrolytic graphite analyzer system as shown in Fig. 2. The analyzer crystal system consists of 13 pyrolytic graphite elements, each 2 cm wide and 15 cm high, with an individual detector where the 13 blades of the analyzer focus, or a position-sensitive detector, at the operator’s choice. This is the modern equivalent of our present analyzer systems, and is expected to be the workhorse for the new thermal triple axis instruments. The blades of the analyzer can be freely rotated by 360 degrees and individually positioned, while the entire unit can be rotated as a whole to achieve the focusing condition. A straight-through beam monitor is incorporated into the shielding behind the analyzer crystals to continuously monitor the flux of neutrons entering the analyzer system. A separate diffraction detector is also provided, which can be moved in front of the analyzer if the energy-integrated signal is to be measured, and for alignment purposes. The general design philosophy is to make the instrument as user friendly as possible while still meeting all the desired operating criteria, which includes various beam defining equipment such as magnetic guide field for polarized beam operation, collimators, beam apertures, spin flippers, and filters. Conventional and radial collimators are included within the analyzer system, eliminating the need to change collimation by hand.
Fig. 2 Horizontally focused pyrolytic graphite analyzer system.
A second type of analyzer system consists of a series of up to 30 individual and isolated analyzer/detector systems. For the thermal triple-axis instruments each analyzer/detector would be limited to 75° for the detector. This style of analyzer has now been adopted for the NG-0 (MACS) double-focusing cold triple axis instrument, but will employ a double-crystal system for each detector in order to accommodate higher detector scattering angles needed for cold neutrons. Other analyzer options, to be developed in the future, include incorporating a velocity selector into the analyzer system, and developing a "conventional" double-focusing analyzer with a single, well-shielded detector.
Last modified 21-February-2008
