Decoupled moderators-do we always need them? Or: A new approach for pulse shaping

At a spallation neutron source, the wavelength of a neutron is generally determined by its time-of-flight (TOF) from the source to the detector. The achievable precision is ultimately limited by the non-zero emission time-width of the source/moderator system. Particularly, coupled moderators that are used to produce high-intensity cold neutron beams show long "tails" in the intensity/time distribution for all wavelengths. For this reason, much less intense decoupled moderators are often used for instruments that require high resolution. We present here a novel technique for dynamic energy filtering of thermal and cold polarized neutron beams. The device consists of a polarizer/analyzer system and an energy-selective spin-resonance flipper. By drifting its resonance condition in synchronization with the TOF, the filter controls the energy resolution of a broad bandwidth TOF neutron pulse. Calculations show that substantial resolution gains are obtainable by applying this technique to a TOF neutron reflectometer.