Beamline requires precision measurements

Scientists with the Deep Underground Neutrino Experiment (DUNE) hope to shed light on the mysteries of the elusive neutrino. So they’ll aim a beam of neutrinos straight through the earth from Fermilab in Batavia, Ill., to detectors on the 4850 Level of Sanford Lab in Lead, S.D. To get the best signal, the center of the beam needs to hit the detectors head on—and that’s where things get a little tricky. 

Neutrinos are among the most abundant particles in the universe, but they have no charge so they can’t be steered to the detectors by magnets. And as the beam travels the 800-mile distance between the two points, it will spread out like a flashlight beam, reducing the number by trillions that will reach the target. 

“We want the maximum number of neutrinos to reach the detectors, so the correct aiming of the beam is of vital importance,” said Virgil Bocean, senior geodesist with Fermilab.

To properly align the neutrino beam, a team of specialists mapped points underground to coordinates on the surface both at Sanford Lab and Fermilab. The team started with global positioning and global navigation satellite systems. 

“The geodetic orientation parameters of the beam were determined with GPS to a high level of accuracy in conjunction with the national Continuously Observed Reference Station (CORS) network—within millimeters,” Bocean said.

But because so much depends on the correct coordinates, including the orientation of the caverns that will hold the detectors, the team needed to match the surface and underground coordinates. And to accomplish that, the team turned to “the ancient technology” of plumb lines, said Randy Deibert of Professional Mapping and Surveying LLC, in Spearfish. Deibert is working with Bocean and geodesist Horst Friedsam, head of the AMD Department at Fermilab. 

Three plumb lines were lowered in each of the two shafts: The Yates and Ross. Crews of surveyors on the 1700, 4100 and 4850 levels of Sanford Lab took horizontal measurements between the shafts, while Deibert used a specially designed survey station to capture depth measurements. A Gyroscope at the 4850 level measured the precise orientation of the underground reference system grid with respect to true north.

“We need to record several readings with each instrument for redundancy and build a larger statistical measurement sample to check for systematic errors,” Bocean said. “It takes all these instrument types to put A and B together and connect the global and local underground information from Fermilab to Sanford Lab.”