Analysis and Simulation of a Small-Angle Neutron Scattering Instrument on a 1 MW Long Pulse Spallation Source

Abstract

We studied the design and performance of a small-angle neutron scattering (SANS) instrument for a proposed 1 MW, 60 Hz long pulsed spallation source at the Los Alamos Neutron Science Center (LANSCE). An analysis of the effects of source characteristics and chopper performance combined with instrument simulations using the LANSCE Monte Carlo instrument simulations package shows that the T₀ chopper should be no more than 5m from the source with the frame overlap and frame definition choppers at 5.6 and greater than 7m, respectively. The study showed that an optimal pulse structure has an exponential decaying tail with τ ≈ 75O µs. The Monte Carlo simulations were used to optimize the LPSS SANS, showing that an optimal length is 18 m. The simulations show that an instrument with variable length is best to match the needs of a given measurement. The performance of the optimized LPSS instrument was found to be comparable with present world standard instruments.