As they peer down into the LUX-ZEPLIN (LZ) water tank from the work deck above, researchers and engineers can finally see the assembly process in full swing. Science and Technology Facilities Council's Pawel Majewski focuses on the cryostat installation. He recently returned to Sanford Underground Research Facility (Sanford Lab) after nearly half a year away and is thrilled with what he’s seeing.
“I’m very excited. Activities are happening at full steam, which is great!” Pawel said. “The underground area looks ready to welcome an experiment.”
And, so it is. With the celebrated arrival of the first of two photomultiplier tube (PMT) arrays last week from Brown University, the pace of delivery to Sanford Lab is accelerating. With these main components arriving, either at the Surface Assembly Laboratory (SAL) or in the Davis Cavern on the 4850 Level, researchers have begun the meticulous work of piecing together a 10-ton dark matter detector.
“The delivery of these arrays is the pinnacle of an enormous assembly effort that we’ve executed here in our cleanroom at the Brown Department of Physics,” said Rick Gaitskell, a professor of physics at Brown University who oversaw the construction of the arrays. “For the last two years, we’ve been making sure that every piece that’s going into the devices is working as expected. Only by doing that can we be confident that everything will perform the way we want when the detector is switched on.”
In November 2018, four acrylic tanks were carefully slung beneath the Yates Cage, lowered through the shaft, maneuvered through the drifts to the Davis Campus and lowered into the LZ water tank. These tanks, or more precisely the liquid scintillator inside the tanks, are crucial in bringing the experiment to a new level of sensitivity—100 times greater than LUX—by identifying neutrons, which can mimic dark matter signals.
The acrylic tanks will surround the xenon-filled cryostat; yet, they were the first to be lowered into the water tank. Why? Because of their size.
Each of the acrylic tanks measure 12.3 feet tall, 7.5 feet wide and 3.4 feet thick and weigh 1,500 pounds. Additionally, the removable steel transport frames weigh 2,740 pounds. Once these massive pieces were moved underground they were stored against the inside wall of the water tank to leave room for the other components.
“That was an accomplishment,” said Pawel. “We were waiting for the acrylic vessels to be brought in and strapped to the wall. This created space for the cryostat to be assembled in the center of the water tank.”
The Outer Cryostat Vessel, or OCV, is made of radio-pure titanium and is the second largest component of the experiment after the acrylic tanks, said Pawel.
The cryostat is a vital part of LZ, as it keeps the detector at freezing temperatures. This is crucial because the detector uses purified xenon—which at room temperature is a gas. For the experiment to work, the xenon must be kept in a liquid state, which is only achievable at around -100 degrees Celsius.
Underground assembly of the OCV began this week. It was delivered underground in three pieces—the bottom, the middle, and the top, or the “head.” The most technical part of assembly, Pawel says, is making sure all the pieces fit snugly together.
“We are making great progress, but, with this kind of science, great progress is still slow,” said Pawel. “Everything — every bolt, every seal — has to be well-aligned and leveled, and this takes time.”
Once the OCV is properly assembled, researchers will perform a week-long leak-check. Although the OCV was vacuum-tested in the SAL, researchers need to be sure everything is still perfect after handling and transportation underground.
“Once this test is successful, we can bring the Inner Cryostat Vessel (ICV) down.” Pawel said.
The future assembly inside of the inner cryostat at the SAL reached a milestone last week when the first of two large photomultiplier tube (PMT) arrays were delivered. The arrays, each 5-feet in diameter, together will hold 494 PMTs.
The PMTs will serve as the “eyes” for LZ. When the detector is completed and switched on, the PMT arrays will keep careful watch on LZ’s 10-ton tank of liquid xenon, looking for the tell-tale twin flashes of light, which researchers believe will be produced if a dark matter particle bumps into a xenon atom.
At the SAL, these arrays will be affixed to the Time Projection Chamber (TPC). Construction of a large part of the TPC wall, segments of Polytetrafluoroethylene (PTFE), surrounding machined titanium rings was also recently completed at the Surface Assembly Laboratory.
“PTFE is this magic material that is extremely good at reflecting UV light created in the scintillation process in liquid xenon,” explained Pawel. “The material reflects the light created, bouncing it around until it is eventually captured and detected by a PMT.”
The TPC will be put inside the ICV along with the two PMT arrays on the top and the bottom. Once the outfitting process in the SAL is complete, the ICV will be closed, sealed and tested. When the final checks are complete, this piece, too, will make its way carefully to the Davis Campus.
While the experiment isn’t expected to begin taking data until 2020, the LZ collaboration makes strides every day to bring these components together, laying what they believe is the perfect trap for WIMPs. With their eyes on that purpose, researchers like Pawel take tremendous care with each milestone.
“The main goal for me, right now, is to make sure that we have the OCV underground and assembled inside the water tank,” said Pawel. “We want to ensure the OCV is good, sound and ready to receive the ICV and the TPC later next year.”
Read the Brown University press release about the arrival of the PMT array.
For more information about LZ and the LZ collaboration, visit http://lz.lbl.gov/.