Optimizing Fermi-chopper spectrometers for the spallation neutron source

The construction of the next-generation Spallation Neutron Source (SNS) at Oak Ridge National Laboratory offers an opportunity to optimize instrument performance in conjunction with moderator and target building configurations. For direct-geometry inelastic scattering instruments operating in the moderate-to high-energy range, the constraints on performance are often given by geometrical considerations. The implications for the specific case of the SNS are explored using simple analytical models of the performance of Fermi-chopper spectrometers. Two instrument configurations are proposed, based on user input concerning the desired instrument capabilities as well as current practices at existing spallation sources.