KVASIR: A backscattering neutron spectrometer for hard condensed matter at ESS

TL;DR

KVASIR introduces an indirect time-of-flight backscattering spectrometer concept for ESS, aimed at probing μeV excitations in single-crystal hard condensed matter with high energy and momentum resolution and substantial sample environments. The approach combines analytic resolution estimates with McStas ray-tracing to optimize a secondary spectrometer featuring HOPG analysers and a prismatic analyser concept, achieving an energy resolution near a few μeV and a q-resolution around $0.03\ \mathrm{Å}^{-1}$. Virtual experiments on complete instrument models (including [0 0 2] and [0 0 4] reflections) demonstrate elastic resolutions of about $5.7\ \mu$eV, broader than the initial target but with a doubled $q$-range when using [0 0 4], and a substantial boost in neutron flux compared with existing backscattering instruments. The study concludes with a roadmap for further optimization (primary divergence, true backscattering refinement, analyser shapes, mosaicity handling) and asserts KVASIR’s potential to expand the experimental parameter space for quantum magnetism and related technologies at ESS.

Abstract

We present the instrument concept for KVASIR, a backscattering indirect time-of-flight neutron spectrometer for the European Spallation Source (ESS). KVASIR will probe low lying excitations of single crystal hard condensed matter that many advanced technologies rely upon. The instrument is simultaneously optimized for high resolution of energy-and momentum transfer, while ensuring perturbation by high magnetic fields. Detailed ray tracing simulations, considering the true geometry and aberrations of the analyser configuration have been performed. The proposed concept has room for polarisation analysis and extreme sample environments. Inelastic neutron scattering studies of single crystal hard condensed matter, with a particular focus on quantum, at yet unseen spatial and dynamic precision under extreme conditions are hereby enabled.

Figures (13)

• Figure 1: The secondary spectrometer of KVASIR, including the final focusing part of the neutron guide (grey). Neutrons are focused onto the sample, which is placed within the sample environment (light blue). Scan the qr-code, for a 3D model. Neutrons scattered from the sample propagate towards the analyser (dark blue), from which an almost-monochromatic beam will be scattered onto the He$^3$ detector arrays (orange). Here, the position and propagation time of the neutron will be measured. For an example of a neutron propagation path see figure \ref{['fig:2D']}.
• Figure 2: The estimated dynamic range of KVASIR in the PG [0 0 2] and [0 0 4] settings for an incoming wavelength band of 1.7 Å and a scattering angle coverage from $5^{\circ} - 175^{\circ}$.
• Figure 3: Schematics of the KVASIR instrument. The instrument is symmetric around the y-axis, and the full geometry can therefore be derived from this picture. Neutrons are focused onto the detector opposite the analyser in the vertical direction; a typical flight path is marked by the green line.
• Figure 4:
• Figure 5: Figure of Merit constructed from energy- and $\bf q$-resolution maps. Minima in the Figure of Merit are marked by blue dots, and the star on each figure mark the optimal configuration, chosen from the limit on sample to analyser distance due to cost. The optimal configuration for $E_{\rm f} = 1.95$ meV is shown in figure \ref{['fig:opt']}.