Part 4: Conclusions and Future Directions 100
A.4 Furnace Simulation Template
captured in the simulation.
These examples demonstrate the successful introduction of a radial collimator component into the simulation. The radial collimator is likely to be used in routine operation of ARCS, and may be introduced in other SNS chopper spectrometers as well. For this reason, it is important to that this component to the simulation gives close agreement with experimental results.
Figure A.7: Schematic of the MICAS furnace (left) and the furnace itself, in storage on a yellow cart.
region in the schematic and the crinkled-foil region in the photo, is the region of the furnace that is in the path of the beam. For introduction of this furnace into the MCViNE simulation, it is this region that must be accurately described.
The region of the furnace in the beam contains the sample at the center, which is attached to the bottom of a stick and inserted into the furnace. Concentric cylinders surround the sample providing heating and shielding, the dimensions of which are given in TableA.1. FigureA.8shows these concentric cylinders schematically. The two inner-most cylinders serve as the heating element.
This is surrounded by between 1 and 8 layers of shielding, in place to dissipate the heat radiating into the sample space. For operation at the highest temperatures, all eight shields must be in place.
Dissipation of heat from the heating element into the instrument tank is an issue that must be
Table A.1: Outer diameters of the concentric cylinders of heating elements, shielding, and the outer vacuum tank containment for the MICAS furnace. All of these components are made from high- purity Nb foil with a thickness of 0.002 in.
inner heating element 3.13 in.
outer heating element 3.65 in.
heat shield 1 5.10 in.
heat shield 2 5.52 in.
heat shield 3 5.93 in.
heat shield 4 6.35 in.
heat shield 5 6.77 in.
heat shield 6 7.18 in.
heat shield 7 7.60 in.
heat shield 8 8.01 in.
outer tank 11.54 in.
observed closely, ideally by monitoring the temperature on the outer tank layer. As this outer tank layer reaches elevated temperatures, it compromises the vacuum inside the instrument tank, which presents a dangerous operating condition for the detectors. In recent modifications of the MICAS furnace design, this issue is somewhat mitigated by the addition of water cooling lines running above and below the sample region of the furnace (and not in the path of the beam).
The MCViNE furnace template is comprised of a collection of additional scatterers that can be added to the sample assembly. The optimal design of the furnace template went through several iterations. It was discovered early in the design process that the precise diameters of these layers is crucial in accurately reproducing their scattering. The values given in Table A.1were obtained from an actual measurement of the heating element and heat shields with a micrometer when the furnace was deconstructed and not in use1. Initially, all of the concentric layers were simulated as individual scatterers. This proved to be cumbersome and unnecessary.
The present template consists of two scatterers: (1) the outer vacuum container, and (2) the two heating elements and eight heat shields. The outer vacuum container is described by a hollow cylinder with radius 5.77 in., thickness 0.1 mm, and height 15 in. As is obvious from the photo in Fig.A.7, this outer layer does not form a perfectly smooth cylinder because this layer is exposed during transport of the furnace. It also serves as the outer vacuum containment for the furnace, so
1When not in use, these layers of shielding are stored in a dry box purged with helium gas.
Figure A.8: Drawing (not to scale) of the heating element and heat shield region of the MICAS furnace. The inner and outer heating elements and the outer tank are fixed. Heat shields can be removed, beginning with the outermost, depending on the maximum temperature of the experiment.
any pressure imbalance that is created during pump down and venting of the furnace and sample area can cause this thin layer to crinkle. Thus, instead of modeling this region as a perfectly dense cylinder of thickness 0.002 in. (0.05 mm), it is modeled with twice this thickness, and a reduced
‘packing factor’ of 0.5. In Fig.A.8, this is depicted as the blue cylinder.
The scatterer describing the remaining components also has a hollow cylinder shape with inner radius 1.56 in., outer radius 4 in., and height 15 in. This hollow cylinder has a reduced packing factor of 0.008 to capture the considerable amount of ‘empty space’ present in this cylinder. In Fig.A.8, this is depicted by the volume encompassed by the two red cylinders.
Together, these two scatterers constitute the ‘furnace template’ that can be added to the sample assembly to simulate samples measured in the MICAS furnace. An example of the furnace template in the sample assembly file is shown in Fig.A.9. The sample assembly is an xml file modified by the user to add or remove components of the sample to the simulation. In this example file, the furnace template is in use as indicated by the blue labels for the two furnace scatterers, the ‘outer most and ‘Nb heating elements etc. 2+8. The shape, composition, and geometric conditions described for
Figure A.9: The furnace template is comprised of two components added to the sample assembly file. The blue labels indicate which component is being described. The ‘outer most and ‘Nb heating elements etc. 2+8 make up the furnace.
and without the radial collimator. FigureA.10compares experimental results (a, b) with simulations (c-f). For these test cases, the empty furnace is not truly empty. The experiment contains an empty Nb sample sachet fixed in a BN absorbing frame at 45 degrees to the incident beam. The simulation is more simple than this, it contains two pieces of Nb foil at the sample position, also 45 degrees to the incident beam.
The experimental data in (a) and (b) are plotted on the same intensity scale, demonstrating the dramatic effect of the radial collimator in reducing unwanted scatter from the sample environment.
Prior to the introduction of the radial collimator, the result in (a) represented the considerable background scattering that needed to be removed from the experimental data. Most notably is the high intensity scattering along the elastic line, including a peculiar split in the elastic intensity, especially towards higherQ. Inelastic scattering intensity is also visible, and it is even possible to make out some Nb dispersions at lowQ. The split in intensity of the elastic scattering likely results from two distinct regions of scatterers in the furnace. There is a high concentration of Nb near the sample position, including the Nb foil at the sample position and 10 concentric layers of Nb foil within 4 in. of the beam center. This produces a set of elastic scattering peaks with an intensity somewhat broadened alongE. The second set of elastic scattering peaks results from the outermost Nb foil layer, which is at a considerable distance of nearly 6 in. from the beam center. A gap of nearly 2 in. exists between the sample and shielding, and this outer cylinder. This produces its own set of diffraction peaks from scattering at this distinct position. In the experimental measurement with the collimator introduced (b), this effect is entirely eliminated. This is as expected because the radial geometry of the collimator is highly effective in removing scattering that occurs at angles not radiating directly from the sample position. Also noticeable in the experimental result is an intensity variation withQalong the elastic line. This results because the collimator is less effective at lowQ.
without collimator with collimator with collimator
(a) (b)
(c) (d)
(e) (f)
Figure A.10: The empty furnace provides significant background, as is visible from the experiment without the collimator (a), but the background is considerably reduced with the collimator (b).
The simulation without the collimator is shown without multiple scattering (c) and with multiple scattering (e). The simulation with the collimator is also shown without multiple scattering (d) and with multiple scattering (f).
in reducing this unwanted background scattering. Thus, it is easier to analyze the results for the simulation without the collimator, when the features are more intense. Panel (c) shows the empty furnace with all scattering kernels in use, but no multiple scattering permitted. This reproduces the experiment data in (a) reasonably well, although intensity along the elastic line, especially at lowQ, is noticeably absent. This is also evident in comparison of (b) and (d), the experiment with the collimator in place and simulation without multiple scattering. In (e), multiple scattering is introduced, which provides the missing intensity nearE=0 and also smears out the scattering along Q. This appears to also be the case in (f), though it is less obvious.
These empty furnace measurements demonstrate the viability of the furnace template in repro- ducing the experimental result. The ability to model the furnace background is useful itself as a tool for characterizing unwanted scattering contributions. It also serves to validate the design of the template, which can now be tested with samples present.