Generic guide concepts for the European Spallation Source

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Abstract

The construction of the European Spallation Source (ESS) faces many challenges from the neutron beam transport point of view: the spallation source is specified as being driven by a 5 MW beam of protons, each with 2 GeV energy, and yet the requirements in instrument background suppression relative to measured signal vary between 106 and 108. The energetic particles, particularly above 20 MeV, which are expected to be produced in abundance in the target, have to be filtered in order to make the beamlines safe, operational and provide good quality measurements with low background.

We present generic neutron guides of short and medium length instruments which are optimised for good performance at minimal cost. Direct line of sight to the source is avoided twice, with either the first point out of line of sight or both being inside the bunker (20 m) to minimise shielding costs. These guide geometries are regarded as a baseline to define standards for instruments to be constructed at ESS. They are used to find commonalities and develop principles and solutions for common problems. Lastly, we report the impact of employing the over-illumination concept to mitigate losses from random misalignment passively, and that over-illumination should be used sparingly in key locations to be effective. For more widespread alignment issues, a more direct, active approach is likely to be needed.

Introduction

Ground breaking at the ESS construction site took place in September 2014 just outside the city of Lund, Sweden. At the time of writing, three of the 22 planned public instruments have already entered the detailed design phase of the construction project, with more following every year. With such a large number of instruments designed by different partners at almost the same time, it is important to identify commonalities and find cost-effective solutions to common problems in order to avoid duplication of effort and associated cost increases. Moreover, a baseline guide concept for each instrument category is a useful tool to benchmark new ideas and guide concepts.

The design of guides for a source with the characteristics of ESS has unprecedented challenges. The first challenge is the adequate geometry for efficient neutron transport over distances as long as 150 m. So far, beamlines like the high resolution powder diffractometer HRPD [1] at ISIS with a length of 100 m count among the longest instruments. This challenge has been addressed extensively in several studies (e.g. [2], [3], [4], [5]) by the use of ballistic or elliptic guides which minimise reflection losses. The second challenge is to reduce the extent of beam losses due to misalignments. This problem has not been addressed as extensively as the first challenge, and needs to be evaluated. The third challenge, and by no means the least important, is the appropriate geometry to obtain low background while maintaining high transport efficiency. Beamlines between 20 m and 50–75 m face an unprecedented challenge, due to the high proton beam intensity, which places design requirements to mitigate the spallation background risk that may contaminate the useful neutron beam. This background poses a problem not only for the measurements aspiring a high signal to noise ratio, but also affects the safety design of ESS, as well as mitigation of radiation damage for components, activation, and the amount of shielding required as a consequence along the beamline.

The requirements on neutron background are particularly stringent for SANS, reflectometry, and spectroscopy. The requirements call for a noise suppression relative to the signal of around 106 and 108 in the strongest cases – see, for example, [6]. Such backgrounds are possible to achieve at spallation sources [7], but only with careful optimisation of the whole system of optics and shielding together. In this paper, for brevity we concentrate on the optical part of the problem, and only describe some of the shielding in much broader terms.

Where baseline beamline shielding designs from neighbouring beamlines overlap, a common shielding area has been defined that the ESS calls the bunker. This structure is similar to other guide bunkers at existing facilities. Taking advantage of this bunker, by losing line of sight before the beamline emerges into the guide hall is one way that individual instrument costs can be reduced. Equipment in direct view of the source is illuminated by stray hadrons1 spanning the MeV to GeV energy range, and these produce showers of secondary particles (mostly neutrons), therefore a direct view of any such secondary source has to be avoided as well. That is to say, line of sight to the source should be avoided twice. For direct geometry spectrometers, the concept of double crystal monochromators [8] naturally fulfills this condition. All other instrument classes need alternative solutions.

The magnitude of the aforementioned challenges depends on the length of the guide to some extent. It is currently anticipated that, with longer neutron guides, it will be possible to reduce the background by taking advantage of distance, and the ESS is currently examining fast neutron albedo transport. Presently, for the long guides the technical focus is on maintaining low guide costs and minimising the transport losses from misalignment. In contrast, the problem is the opposite for the short beamlines, because the misalignment losses are less significant and the total guide costs are lower. However, the line of sight avoidance condition is severely more restrictive, and the design of a guide with a good performance and a low background becomes more of a challenge.

In this paper, after establishing a basic technical grounding, we will examine medium length neutron guides of 50 m, in Section 3, including both the double line of sight requirement and a study of misalignment, before solutions for 20 m long instruments focussing on the line of sight condition are discussed in Section 4.

Section snippets

Simulation program

For all simulations, the Monte-Carlo ray-tracing package VITESS [9], [10] version 3.2 is used. Guide cross-sections are rectangular and gravitational effects are included by default. The neutron source that was used in the models is the ESS TDR moderator2 [11]. More recent moderator developments do not change the validity of the present work, since line of sight is avoided in the horizontal direction.

Double-ballistic neutron guides for medium length instruments (50 m)

Several neutron guides are planned to be deployed at ESS as part of “medium length” instruments, corresponding to a guide length of about 50–75 m. For reasons of background reduction, as well as radiation safety – which essentially translate directly into instrument performance and cost – the sample and detector position should wherever possible not have a direct line of sight to the moderator. This is the “first line of sight” principle.

Furthermore, to avoid illumination by secondary particles,

Double line-of-sight with short instruments (20 m)

In this section, we examine some options for short beamlines with a sample position at 20 m distance from the source. It is of interest for all instruments to lose line of sight as close to the source as possible. These both minimises background and takes advantage of the common shielding in the bunker, to minimise instrument costs.

The options we study all lose line of sight twice, and have a monolith insert composed of a simple, 4 m long guide of constant cross-section, starting 2 m from the

Conclusions

Generic neutron guides for ESS instruments of short and medium length were optimised for good performance at low cost under the condition that a direct line of sight to the source is avoided twice. These guides can serve as a baseline and are used to develop concepts addressing common neutron optic challenges at ESS.

It was shown that medium length instruments can avoid line of sight once within the bunker to use the common shielding as well as once more well before the sample position to avoid

Acknowledgement

We would like to thank Swiss Neutronics, Mirrortron and S-DH for their direct and indirect input into various aspects of the ESS development work leading up to this study.

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