Simulation of the response of the IPNS neutron time-of-flight chopper spectrometers to samples in the Compton limit

We describe the comprehensive biased Monte Carlo code SPEC_SIM used to obtain accurate estimates of instrument response functions for neutron Compton scattering studies of the atomic momentum distribution of noble gas solids and liquids. The code is tailored to simulate epithermal neutron transport in the Intense Pulsed Neutron Source (IPNS) chopper spectrometers. The simulation begins with the neutrons emitted from the face of the moderator. The neutron trajectories are traced through a curved slit Fermi rotor (chopper) and are allowed to scatter multiple times from the sample region which may contain a sample, sample cell, and set of thermal radiation shields in any of a complex set of geometries. The scattered neutrons are tracked to the detector bank where they are again allowed to scatter before being “absorbed” and converted into a time-of-flight (TOF) signal which is histogrammed. A similar simulation code calculates the TOF response of two beam monitors to the incident neutron pulse, including the effects of scattering in the walls of the monitors. All of the scattering in the simulation is treated within a generalization of the impulse approximation. The results of the Monte Carlo simulations compare very favorably with experiments on solid and liquid He, Ar, and Ne.