Location of frame overlap choppers on pulsed source instruments

A detailed study has been performed to investigate the effect of frame overlap in a cold neutron chopper spectrometer. The basic spectrometer is defined by two high-speed choppers, one near the moderator to shape the pulse from the moderator, and one near the sample to define energy resolution. Using ray-tracing timing diagrams, we have observed that there are regions along the guide where the trajectories of neutrons with different velocities converge temporally at characteristic points along the spectrometer. At these points of convergence, a frame overlap chopper would be totally ineffective, allowing neutrons of all velocities to pass through. Conversely, at points where trajectories of different velocity neutrons are divergent, a frame overlap chopper is most effective. An analytical model to describe this behaviour has been developed, and leads us to the counterintuitive conclusion that the optimum position for a frame overlap chopper is as close to the initial chopper as possible. We further demonstrate that detailed Monte Carlo simulations produce results which are consistent with this model.