Crystal Analyzers for Indirect-Geometry Broadband Neutron Spectrometers: Adding Reality to Idealized Design

The recent trend to fully utilize neutrons from pulsed spallation sources for broadband neutron spectrometers with indirect-geometry seeks to improve their performance by using large-area curved analyzers that focus neutrons in time, energy and space. Here we describe the ongoing collaborative efforts of CNR (IT) and ISIS (UK) towards future upgrades of TOSCA at ISIS, which are also essential for the design and subsequent construction of VESPA at the ESS. This performance improvement is due to both a larger detector coverage using a curved geometry and a corresponding tuning of analyzer crystal characteristics. To achieve high resolution in the entire spectral range 0-500 meV, the time-focusing of neutrons from the sample to the detector should be carried out within tight tolerances. The effect of graphite mosaicity and its variation from crystal-to-crystal across the whole analyzer, as well as the impact of manufacturing tolerances on instrument performance, and of the detector pixelation are here investigated to inform about the design in order to balance the efficiency of neutron collection with the resolution requirements.