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Persistent URL http://purl.org/net/epubs/work/37718698
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Record Id 37718698
Title TOSCA neutron guide and spectrometer upgrade
Abstract TOSCA is an indirect-geometry inelastic neutron spectrometer optimised for high resolution vibrational spectroscopy in the region between 0 and 4000 cm-1 [1, 2]. In its current form the instrument has been operational for almost two decades and during that time has set the standard for broadband chemical spectroscopy with neutrons [3]. In autumn 2013 as part of the international beamline review it was concluded that for TOSCA to be able to participate in strategic research areas, an increase in the incident neutron flux via the provision of a neutron guide was essential. Such a development would allow detailed studies of the industrially relevant systems containing weak neutron scatterers as well as faster parametric studies, particularly for hydrogen containing molecules such as hydrocarbons. Additionally, studies of smaller samples which cannot be produced, or are too expensive to produce, in larger quantities would be possible [2]. Since then, this major upgrade has been implemented and has involved the complete redesign of the TOSCA primary spectrometer to house a state-of-the-art, high-m neutron guide and associated chopper system to boost the incident flux. The instrument was closed between the end of May 2016 until mid-February 2017 when the installation of the final (so called C2) configuration was completed. After the two weeks of commissioning measurements the instrument was returned to the user programme. By the end of November 2016, all sections of the guide were in place, apart from the new TOSCA shutter that contains the initial section of the guide, in this phase the latter remained the same as before the upgrade. Since the overwhelming majority of the guide was installed we have tested the setup for enhanced neutron flux, as well as in order to have a better idea about the influence of the guide inside the shutter (installed subsequently) on the neutron flux, beam profile, spectral resolution and background. We will refer to this interim configuration as the C1 configuration, while the configuration before the upgrade will be denoted as C0. The entrance of the new 1.937 m long shutter is positioned at a distance of 1.626 m from the moderator centre, and the shutter contains a m = 5 straight square guide with an aperture of 100 mm x 100 mm. The remaining nine sections of the guide are tapered, starting from the 100 mm x 100 mm entrance of section 2 all the way to the end of section 10 with 40 mm x 40 mm aperture (positioned at a distance of 16.262 m from the moderator center, i.e. 0.748 m from the sample position). The angle of 0.136494º [4] has been kept equal in each section, while the m-factor was increased in steps from m = 5 for sections 2 to 6, m = 6 for sections 7 to 9, and m = 7 for section 10. The data show good beam homogeneity at the TOSCA sample position, thanks to our efforts on the theoretical guide design to ensure that the chain of reflections causes minimal beam profile inhomogeneity. As a result of the guide being installed on TOSCA the neutron flux at the sample position has been significantly increased. The experimental gain in neutron flux is 6 times for neutrons with wavelength of 0.5 Å, 46 times for neutrons with wavelength of 2.5 Å, and 82 times for neutrons with wavelength of 4.6 Å, i.e. the most significant gain is for the low energy neutrons used to study low energy molecular vibrations and collective motion within the lattice (phonons). A further upgrade requires the redesign of the TOSCA secondary spectrometer, and their combined effect will be to improve the utilization efficiency of the neutron source. The consequence of each step in the upgrade process can be evaluated by means of calculations and measurements, i.e. benchmark experiments. The flat analyzer currently used on TOSCA can be approximated with a square shape with an area 144 cm2, and the centre of the analyzer surface is placed at a 45° grazing angle and at an average distance of 320 mm with respect to the sample. The analyzer is made of highly oriented pyrolythic graphite (HOPG) and the chosen reflection plane is the (0, 0, 2) that has a lattice spacing of 3.354 Å. The analyzer crystal has a rocking angle full width at half maximum (mosaic spread) of 2.5°. This material is particularly suitable for the specific reflection of cold and thermal neutrons thanks to its high reflectivity and sharp diffraction peaks. The reflectivity for a 1 mm thick HOPG crystal can exceed 74% and HOPG finds a number of important applications for 4.5 Å, that coincides with the fixed wavelength of the TOSCA secondary spectrometer. The current INS banks are also equipped with a beryllium filter and cadmium foils to cut the higher wavelength harmonics diffracted by the analyzer. Furthermore, each beryllium filter is cooled to 30 K to improve the beryllium transmission. In the current configuration, the analyzer and the filter are very close together, which partially explains the original choice for a small analyzer, but this feature strongly limits the neutron collection efficiency of the assembly. In this Ph.D. project, the TOSCA spectrometer upgrade focused on the design of a double bent analyzer and a new beryllium filter by means of the McStas software. In order to proceed with the simulation of the new analyzer we had to create a custom component which can meet the TOSCA needs. We used several HOPG tiles arranged on a parametric surface that can follow two different curvatures, namely spherical and parabolic. Several simulations were performed by means of this component, in order to study the performance of the future TOSCA setup; HOPG analyzer with different areas and radii of curvature were considered and each parameter has been optimized independently via numerical analysis. We found that, with the appropriate focusing and mosaicity, it is possible to increase the current analyzer area and to achieve a large gain in the detected flux. From the calculations, it seems that for larger analyzers the parabolic geometry outperforms the spherical geometry thanks to its better focusing properties. This performance can be further increased by tuning the HOPG mosaicity to expand the analyzer bandwidth and simulations show that an order-of-magnitude overall gain is within reach. Furthermore, the gain from a curved analyzer can be considerably improved when an optimal HOPG mosaicity is chosen, to enable a more efficient use of the scattered beam. This important upgrade can be achieved with limited effects on resolution, thanks to the extensive optimization we performed on the curved geometry, namely the multiparametric optimization in this study has been computationally implemented using the MATLAB package in conjunction with the McStas software. That proved necessary since the future wide curved analyzer needs extensive calculations to assess its time and energy focusing capability with respect to the parallel plane geometry that TOSCA currently implements. In relation to the current setup, the shape of the future curved analyzer could expand radially to accomplish the shape of the 60° INS module. Our calculations focused initially on three different configurations of double-bent analyzer having different surface extensions up to an area of 1450 cm2; for each case a parabolic and a spherical curvature were considered and thus a total of 6 different configurations were studied so far. This route for the TOSCA upgrade appears feasible and a similarly large HOPG monochromator (1428 cm2) with horizontal and vertical spherical curvature has been successfully implemented at NIST, which represents a significant advance in double focusing technology and a great improvement in the efficiency of the MACS [5] instrument.
Organisation ISIS , ISIS-TOSCA , STFC
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Language English (EN)
Type Details URI(s) Local file(s) Year
Thesis PhD, Univerita Delgi Studi di Milano-Bicocca, 2018. 2018
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