Performance test of the DREAM bi-spectral switch

DREAM is a diffractometer instrument that will be built at the European Spallation Source (ESS). DREAM will view both the cold and thermal moderators of ESS, thus a bi-spectral extraction system will be used to run experiments with a large wavelength band. The system has to enable reflections of neutrons from the cold moderator while being optimized for transmitting neutrons from the thermal moderator. The device consists of nominal 153 Si wafers with a thickness of 0.13 mm with a NiTi super mirror coating (m-value 3). The wafers are inclined by 0.57° to the instrument (thermal) beam axis. The angular tolerance is ±0.05°. The performance of the Si-wafer stack has been validated by simulations. The bi-spectral switch installation will be done in Q2 2022 inside the ESS bunker, thus testing the device with the thermal neutrons before the installation is time-critical.