Polarized primary spectrometer on the LET instrument at ISIS

The combination of neutron time-of-flight spectroscopy and longitudinal polarization analysis opens up the possibility of simultaneously separating cross-section components over vast regions in $\left(Q,\Delta E\right)$ space. In this paper, we describe initial results of a project to install longitudinal polarization analysis on the LET cold neutron spectrometer at the ISIS Neutron and Muon Facility; namely, the design and commissioning of the polarizing primary spectrometer. The polarized neutron beam is generated by a supermirror-based transmission device, for which ray tracing simulations indicate a high neutron polarization $P\sim 96\%$ and transmission $T\sim 40\%$. Despite the short distance between this device and the precession coil flipper, and the consequent large stray magnetic fields on the latter, careful optimization of the field environment yields predictions of excellent flipping performance $\sim 99\%$. Both simulation results are confirmed via measurements using a polarizing Cu₂MnAl Heusler crystal as an analyzer.