Momentum and energy dependent resolution function of the ARCS neutron chopper spectrometer at high momentum transfer: Comparing simulation and experiment

Inelastic neutron scattering at high momentum transfers (i.e. Q≥20A˚), commonly known as deep inelastic neutron scattering (DINS), provides direct observation of the momentum distribution of light atoms, making it a powerful probe for studying single-particle motions in liquids and solids. The quantitative analysis of DINS data requires an accurate knowledge of the instrument resolution function Ri(Q,E)Ri(Q,E) at each momentum Q and energy transfer E, where the label i   indicates whether the resolution was experimentally observed i=obsi=obs or simulated i=sim  . Here, we describe two independent methods for determining the total resolution function Ri(Q,E)Ri(Q,E) of the ARCS neutron instrument at the Spallation Neutron Source, Oak Ridge National Laboratory. The first method uses experimental data from an archetypical system (liquid 4He) studied with DINS, which are then numerically deconvoluted using its previously determined intrinsic scattering function to yield Robs(Q,E)Robs(Q,E). The second approach uses accurate Monte Carlo simulations of the ARCS spectrometer, which account for all instrument contributions, coupled to a representative scattering kernel to reproduce the experimentally observed response S(Q,E)S(Q,E). Using a delta function as scattering kernel, the simulation yields a resolution function Rsim(Q,E)Rsim(Q,E) with comparable lineshape and features as Robs(Q,E)Robs(Q,E), but somewhat narrower due to the ideal nature of the model. Using each of these two Ri(Q,E)Ri(Q,E) separately, we extract characteristic parameters of liquid 4He such as the intrinsic linewidth α2 (which sets the atomic kinetic energy 〈K〉∼α2〈K〉∼α2) in the normal liquid and the Bose–Einstein condensate parameter n0 in the superfluid phase. The extracted α2 values agree well with previous measurements at saturated vapor pressure (SVP) as well as at elevated pressure (24 bars) within experimental precision, independent of which Ri(Q,y)Ri(Q,y) is used to analyze the data. The actual observed n0 values at each Q   vary little with the model Ri(Q,E)Ri(Q,E), and the effective Q-averaged n0 values are consistent with each other, and with previously reported values.