Design and performance of the spallation neutron source backscattering spectrometer

When the Spallation Neutron Source is operational in 2006 at Oak Ridge National Laboratory, it will provide the most intense pulsed neutron beams in the world for research. This paper describes the expected performance of the near-backscattering, crystal-analyzer spectrometer designed to provide extremely high energy resolution at the elastic peak (δω = 2.2-2.7 μeV, varying with sample dimension). The design requires a long initial guide section of 84 m from moderator to sample in order to achieve the requisite timing resolution. This spectrometer will provide an unprecedented dynamic range near the elastic peak of -258 μeV < ω < 258 μeV. For experiments that require the full dynamic range available at comparable reactor-based instruments (or greater), this spectrometer is expected to have an effective count rate ∼ 60 times that of the current best spectrometers.