Systematic neutron guide misalignment for an accelerator-driven spallation neutron source

The European Spallation Source (ESS) is a long pulse spallation neutron source that is currently under construction in Lund, Sweden. A considerable fraction of the 22 planned instruments extend as far as 75–150 m from the source. In such long beam lines, misalignment between neutron guide segments can decrease the neutron transmission significantly. In addition to a random misalignment from installation tolerances, the ground on which ESS is built can be expected to sink with time, and thus shift the neutron guide segments further away from the ideal alignment axis in a systematic way. These systematic errors are correlated to the ground structure, position of buildings and shielding installation. Since the largest deformation is expected close to the target, even short instruments might be noticeably affected. In this study, the effect of this systematic misalignment on short and long ESS beam lines is analyzed, and a possible mitigation by overillumination of subsequent guide sections investigated.

Figure 1

Position of the beam line segments as a function of the number of guide segments, two examples of the combination of systematic and random neutron guide misalignment: the black line shows a random misalignment of $50\mu \mathrm{m}$ with respect to the previous guide segment, the dotted red line the accumulated random misalignment with respect to the nominal guide axis. The blue line shows the systematic misalignment which is added to the random component to give the total misalignment with respect to the nominal guide axis, shown as solid red line. Guide segments are 1 m long. Note the different $y$ axis scale to compare these examples. 1.5 mm (a) and 12 mm (b) maximal systematic misalignment.

Figure 2

Relative transmission with respect to perfectly aligned guide with different amounts of a systematic vertical misalignment. Guide sections are 1 m long. (a) Relative transmission of a 20 m guide with $3\times 3{\mathrm{cm}}^2$ cross section as a function of wavelength, with systematic misalignment as well as $50\mu \mathrm{m}$ random misalignment both horizontally and vertically. Error bars show the statistical uncertainty of the simulation only, the statistical uncertainty of the random misalignment is exemplary indicated by five different simulations with five different random numbers for the random misalignment. (b) Relative transmission as a function of maximal systematic misalignment with (solid) and without (dotted) additional $50\mu \mathrm{m}$ random misalignment, for different guide lengths $L$ and widths $w$. Systematic misalignments of 0, 1.5, 3.0, 6.0 and 12.0 mm are simulated; the lines are a guide for the eye.

Figure 3

Relative transmission with respect to perfectly aligned guide, with different amounts of systematic vertical misalignment and overillumination for a certain expected misalignment, 150 m guide (a) and 20 m guide (b) with 1 m long segments. Only systematic misalignment.

Figure 4

Relative transmission with respect to perfectly aligned guide, with different amounts of systematic vertical misalignment and overillumination for a certain expected misalignment, 150 m guide (a) and 20 m guide (b) with 1 m long segments. Systematic plus random misalignment using the mean of five random misalignment sets. Note that the $y$ axes are suppressed and have different scales in each graph.

Figure 5

Estimation of systematic uncertainty of the relative transmission with overillumination, assuming modifications to Eq. (1). Relative transmission is plotted against amplitude of systematic misalignment, assuming a 3 mm overillumination, with the original function, a 20% change variation in the period of the damped cosine, the same 20% change in the damping factor, and also in the tilt.