  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.
