Abstract
We model the effect of ground movement, based on empirical experience, on the transport properties of long neutron guides by ray-tracing simulations. Our results reproduce the large losses found by an earlier study for a simple model, while for a more realistic engineering model of guide mounting, we find the losses to be significantly smaller than earlier predicted. A detailed study of the guide for the cold neutron spectrometer BIFROST at the European Spallation Source shows that the loss is 7.0(5) % for wavelengths of 2.3-4.0 {\AA}; the typical operational wavelength range of the instrument. This amount of loss does not call for mitigation by overillumination as suggested in the previous work. Our work serves to quantify the robustness of the transport properties of long neutron guides, in construction or planning at neutron facilities worldwide.
Highlights
Guide systems are indispensable at large-scale facilities for neutron scattering
We implement a more realistic model, where the guide pieces will pivot around a mounting point close to the ends of the guide pieces, as this is consistent with the realistic guide geometry described by one vendor [22]. We find that this pivot model leads to limited losses, of the order 5% for a 3 × 3 cm2 guide, much smaller than found by Zendler et al For a realistic model of the BIFROST guide, we find that the total losses are 7.0(5)%
In this work a series of simulations have been performed with the goal of investigating the effect of misalignment on long neutron guides and the effect on the BIFROST instrument currently under construction at European Spallation Source (ESS)
Summary
Guide systems are indispensable at large-scale facilities for neutron scattering. The primary purpose of a guide is to transport the neutron beam far from the source, where the fast-neutron background is smaller [1]. The last part of the guide may converge to increase the neutron intensity [2] and potentially completely focus the beam to the sample under investigation [3,4,5,6,7]. The longer source-sample distance will increase the neutron flight time, which can be of considerable advantage for time-of-flight instruments. This is the case for long-pulse neutron sources with long guides [8,9,10], such as the European Spallation Source (ESS), presently under construction [11,12,13,14], as well as the planned Second Target Station at the Spallation Neutron Source (SNS) [15]. With the correct geometry and use of proper supermirrors, a ballistically shaped guide can potentially transport up to 80%–90% of the theoretical maximum of the desired neutron phase space from the moderator to the sample [16,17,18]
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