Extraordinary beamline alignment

fnal-to-surf-blue
Image: Sandbox Studios

Intended to accompany the article Beamline requires precision measurements (Deep Thoughts, August 9, 2016), this article provides extra details about the LBNF beam alignment that may interest DUNE collaborators.

Given that the final design for the Far Site Conventional Facilities will be starting soon, nailing down the coordinates for beam alignment is a high priority. Virgil Bocean, senior geodesist with Fermilab, and a team of surveying specialists made a comprehensive set of measurements at SURF in July.

The team used a combination of tools and techniques, which are described in the Deep Thoughts article (and in the LBNF/DUNE CDR Annex 3A, Section 6.4). They have measured coordinates in the horizontal x-y plane at the 4850L to within 4 inches and the depth (z) to less than 2 inches with respect to surface at SURF, and will map those global coordinates to Fermilab, where the beamline will be constructed.

The detector modules will sit on locally horizontal surfaces at the 4850L. In the vertical dimension, the neutrino beam will intercept the modules from below at an angle of 5.78 degrees; this is due to the geometry of the beam’s path between Fermilab and SURF.

“There’s nothing we can do about that,” said Bocean. “But I wanted to know how accurate the horizontal orientation needed to be known. The current requirements say 1 degree, but Eric [Eric James, the DUNE Technical Coordinator] said to measure the best that we can in case later in the project we find that we need to know the alignment more precisely. Well, we can measure the horizontal alignment to within 3 arcseconds. That’s pretty good.”

Current FD Layout with arrow
The x-y coordinates at the 4850L. The red star shows the geographical center of the far detector (at the center of the Central Utility Cavern) where the center of the beam will be aimed, and the red line denotes the x-y projection of the beam.