LZ underside of the bottom PMT Array from Sanford Lab on Vimeo.
For the past two months, researchers with the LUX-ZEPLIN dark matter experiment have been meticulously connecting more than 19 kilometers of cabling to 493 photomultiplier tubes (PMTs) at Sanford Underground Research Facility (Sanford Lab). Last week, after completing cabling for the bottom array, the team used a crane to turn the array from its vertical position onto a horizontal base.
As the array hovered above the base, researchers held their breath, watching to see if the intricate pieces of the array would remain secure. Months of effort were rewarded as the array was carefully lowered into place and secured.
“The array was held vertically for construction purposes,” said Jordan Palmer, an LZ collaborator from Royal Holloway University of London, who has been highly involved in dressing the PMT arrays. “However, we needed to lay it horizontally so we could begin constructing the detector above it, stacking the components a bit like LEGO bricks.”
The bottom array alone holds 241 PMTs, powerful light sensors designed to detect any faint signals produced by dark matter particles when the experiment begins in 2020.
“The PMTs detect light, or photons. If a dark matter particle comes through the detector and hits a xenon atom, photons will be emitted,” Palmer explained. Any interaction inside the LZ detector can only be observed through the photons released during each “event.” The photons contain all the information required to reconstruct the location and type of interaction.
“It should come as no surprise that one of the greatest challenges in designing the detector is to minimize the amount of light that is not measured,” said Nicolas Angelides, LZ collaboration member and graduate student at University College London. “A handful of photons are enough to tip the scale between useful structured data or experimental noise.”
To collect this valuable information, each PMT has to be outfitted with cables that will deliver data to researchers for analysis. Once the detector is eventually placed inside the water tank on the 4850 Level of Sanford Lab, no adjustments can be made to the cables, so researchers must be sure to install each component with precision.
“While the cables themselves are sturdy, the connector boards are very delicate, so we had to handle them with immense care, which takes some practice,” Palmer said.
In a campaign involving over a dozen researchers and technicians, a total of 1,600 individual wires were installed, adding up to a length of 19 kilometers.
“This installation has proven to be one of the most delicate operations during the assembly process,” said Angelides. “The fragility of this environment, riddled with wires, was not only the result of the substantial number of individual wires, but also their diameter, a thickness of only 1.6 millimeters.”
Following the cabling process, waves of tests were run for each wire, ensuring no connectivity was lost in the process.
This undertaking took place in the radon-reduced Surface Assembly Lab (SAL) cleanroom. Until the detector can be sealed within the inner cryostat vessel, it will remain within this specialized surface cleanroom specifically designed and built to serve the assembly process. This also allows researchers in the Davis Cavern on the 4850 Level to continue installing larger pieces of the experiment, including the xenon circulation system.
“Now, with the bottom array in place, we can continue creating the detector, the central piece of the experiment,” Palmer said. When it is complete, the entire detector, inside the inner cryostat vessel, will be transported from the surface to the 4850 Level for installation.
“This phase of LZ is very exciting," said Kevin Lesko, spokesperson for LZ. “We are bringing systems together from around the globe and integrating them into the heart of the detector. The process requires a lot of attention to detail and an eye for sources of contamination. The process is going very well, and we are beating all our requirements for background contamination.”