Monte Carlo simulations for the TOSCA secondary spectrometer upgrade

For TOSCA to move forward beyond the current state-of-the-art in key scientific areas such as gas and charge storage, the most urgent need is for greater sensitivity via the secondary spectrometer upgrade that would make TOSCA competitive with other similar instruments, such as VISION at the SNS, in terms of total count rate. The here proposed upgrades can improve the collection efficiency of the neutrons scattered from the sample and the effects in terms of resolution have to be evaluated alongside. This document is focused on the design of a TOSCA doubly bent analyzer by means of McStas suite, which currently does not implement a component that can simulate a curved monochromator along multiple axes. Therefore, to proceed with the simulation of the secondary spectrometer, we had to create a custom component which can meet the TOSCA needs. We used several Highly Oriented Pyrolytic Graphite (HOPG) tiles arranged on a parametric surface up to an area of 1450 cm2 that can have two different curvatures, spherical and parabolic. Several simulations have been performed with this component to study the performance of the current and future TOSCA setup at the elastic line. Graphite analyzers with different areas and radii of curvature have been considered and each parameter has been optimized via computational analysis. From the simulations, we found that with the appropriate focusing, it is possible to increase the current analyzer area by tenfold reaching a flux gain of 7. It is also evident that for larger analyzers the parabolic geometry outperforms the spherical one thanks to its better focusing properties. The calculations also show that this gain is achieved with limited effects on resolution, provided that an extensive optimization is performed on the parameters of curved geometry. The geometric optimization in this study has been computationally implemented using MATLAB routines in conjunction with McStas.