Momentum and Energy Dependent Resolution Function of the ARCS Neutron Chopper Spectrometer at High Momentum Transfer: Comparing Simulation and Experimen

TL;DR

This study develops and compares experimental and simulation methods to accurately determine the resolution function of the ARCS neutron spectrometer at high momentum transfer, enabling precise analysis of neutron scattering data.

Contribution

It introduces two independent approaches—experimental deconvolution and Monte Carlo simulation—for determining the instrument resolution function of ARCS at high Q, validated through liquid helium data.

Findings

Both methods produce comparable resolution functions.

The resolution functions enable accurate extraction of physical parameters.

Results agree with previous measurements of liquid helium properties.

Abstract

Inelastic neutron scattering at high momentum transfers (i.e. $Q\ge20$ {\AA}) or 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 $R_{i}({Q},E)$ at each $Q$ and energy transfer $E$, where the label $i$ indicates whether the resolution was experimentally observed $i={obs}$ or simulated $i=sim$. Here, we describe two independent methods for determining the total resolution function $R_{i}({Q},E)$ 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 $^4$He) studied with DINS, which are then numerically deconvoluted using its previously determined intrinsic scattering function to yield $R_{obs}({Q},E)$. 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 $S({Q},E)$. Using a delta function as scattering kernel, the simulation yields a resolution function $R_{sim}({Q},E)$ with comparable lineshape and features as $R_{obs}({Q},E)$, but somewhat narrower due to the ideal nature of the model. Using each of these two $R_{i}({Q},E)$ separately, we extract characteristic parameters of liquid $^4$He such as the intrinsic linewidth $\alpha_2$ (which sets the atomic kinetic energy $\langle K\rangle\sim\alpha_2$) in the normal liquid and the Bose-Einstein condensate parameter $n_0$ in the superfluid phase. The extracted $\alpha_2$ values agree well with previous measurements, independently of which $R_i(Q,y)$ is used to analyze the data.