Characterization of the BIFROST spectrometer through virtual experiments

TL;DR

The paper addresses predicting and benchmarking BIFROST performance for inelastic neutron scattering using virtual experiments. It builds a McStas model to validate energy- and $Q$-resolution, quantify edge-enhancement effects from the ESS long-pulse source and PSCs, and simulate dispersive excitations in $\mathrm{MnF_2}$, including Zeeman splitting under applied fields. The work provides validated resolution predictions, demonstrates the value of virtual experiments for commissioning and experimental planning, and identifies areas where more physics in simulations is needed. Overall, it establishes a proof-of-concept that McStas-based virtual experiments can guide instrument design and measurement strategies at ESS.

Abstract

Using the Monte Carlo ray tracing package McStas, we illustrate the possibilities of creating virtual experiments of the neutron spectrometer BIFROST at the European Spallation Source, ESS. With this model, we are able to benchmark BIFROST with respect to expected intensity, $Q$- and energy-resolution. The simulations reproduce the expected resolution behavior and quantify effects that are difficult to capture analytically, including a wavelength-dependent edge enhancement arising from a combination of the long-pulsed source and the pulse-shaping chopper. Furthermore, we present an antiferromagnetic (AF) spin wave simulation, which we use to create realistic datasets at different instrument operation settings. Our virtual experiments focus on realistic dispersive dynamics and illustrate how the virtual experiment approach reveal resolution effects, not easily calculable via analytical models. This demonstrates the crucial role of numerical simulations in the planning of challenging experiments.

Figures (13)

• Figure 1: (a) Side view sketch of the last meters of the BIFROST spectrometer, including guide end, divergence jaws, sample, and the secondary spectrometer (analysers and detectors) (b) Top view of the secondary spectrometer showing the sample (orange) and the 45 analyser assemblies (blue). This subfigure is modified from toft-petersen_bifrostindirect_2025.
• Figure 2: Flowchart illustrating the overall workflow of the BIFROST McStasScript code. This building structure allows creating any or all analyser/detector pairs in the secondary spectrometer.
• Figure 3: Comparison between simulated and analytically calculated energy resolutions of BIFROST; both given as FWHM. Markers indicate simulations result at PSC opening times of 0.1 ms, 1 ms and fully open. The black outlined markers are for the 5 meV analyser bank while the colored outline is for the 2.7 meV analyser bank. The colored area indicate the resolution range for the other analyser banks. The solid lines indicate the results of the analytical calculations.
• Figure 4: (a) Illustration of the edge enhancement effect, showing how the PSC opening time scans through the moderator pulse as the neutron arrival time changes. The PSC opening will only be partly covered in the beginning (blue) and the end (green) of the pulse. (b) Simulation with $\Delta t_{\rm PSC} = 0.3$ ms, showing the correlation between true neutron wavelength, $\lambda$, and the actual $\sigma_{t_i}$ at the sample position. The wavelength is converted to energy in (c), which shows $\sigma_{t_i}$ vs. initial energy (red points), and total neutron intensity vs. initial energy across the band (blue points). The orange area show the energies where $\sigma_{t_i}=\Delta t_{PSC}$, while the green and blue areas show the energies of the edges effects, which are identified by a decrease in $\sigma_{t_i}$. The same areas are plotted on (d) where $\sigma_{t_i}$ is converted to $\sigma_{E_i}$ vs. $E_i$ and the expected behaviour from eq. (\ref{['eq:ei_ti']}) is shown by the black dashed line.
• Figure 5: Simulated NaCaAlF powder peaks for fully open (a,b) and $\pm 0.2 ^{\circ}$ (c,d) divergence-jaw settings. Panels (a,c) show the raw $(|Q|,\Delta E)$ distributions, while panels (b,d) display the corresponding energy-integrated profiles $\pm 0.15$ meV. Panel (e) compares representative peaks, and panel (f) summarizes the resulting FWHM values from the simulations (dots) with our theoretical predictions (dashed lines), illustrating the improved $Q$-resolution obtained with tighter jaws from both simulations and analytical calculations.