Abstract: Crystal monochromator-Fermi-chopper (FC) (X-TOF) instruments provide a high count-rate with flexible resolution optimization. We show that the most advantageous range of operation for this class of instruments falls in the thermal and sub-thermal range where focusing Bragg optics can be used to full benefit. In this spectral window, an X-TOF instrument at a high-flux reactor can match a chopper spectrometer on a MW power short pulse spallation source as can be shown by comparing the optimized concepts. The high count-rate makes this type of instrument highly attractive for polarized neutron operation. Recent experiments performed using a new large solid-angle 3He neutron spin-filter represent a significant breakthrough in the field of neutron polarization analysis studies on multi-detector spectrometers, forming the basis of the proposed PASTIS project which enables xyz-polarization analysis studies on a thermal TOF spectrometer (Stride et al., Stewart et al.). Outlook for implementation of PASTIS as a thermal X-TOF spectrometer will be discussed.
