Photomultiplier Tubes from Brown Team Will Be the 'Eyes' of New Dark Matter Detector

Brown University researchers have assembled two arrays of photomultiplier tubes (PMTs) to serve as the “eyes” for the LUX-ZEPLIN (L-Z) dark matter detector, which will begin its search for dark matter particles at the Sanford Underground Research Facility (SURF) in Lead, S.D., in 2020. At this former mining site, the detector will be shielded by about 1 mile of rock to limit interference.

A researcher at Brown University works on photomultiplier tubes for a component of the LUX-ZEPLIN dark matter search experiment. Courtesy of Nick Dentamaro/Brown University.

Once the L-Z detector is operational, the two arrays will keep watch on L-Z’s 10-ton tank of liquid xenon, looking for the twin flashes of light that will occur if a dark matter particle bumps into a xenon atom inside the tank. Each PMT is a 6-in.-long cylinder that is roughly the diameter of a soda can. Each array is about 5 ft in diameter and holds a total of 494 PMTs. To form arrays large enough to monitor the entire L-Z xenon target, hundreds of PMTs were assembled together within a circular titanium matrix.

The Brown team worked with researchers and engineers from the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and Imperial College London to design, procure, test, and assemble all of the components of the array. Testing of the PMTs, which are manufactured by the Hamamatsu Corporation, was performed at Brown and at Imperial College.

“The delivery of these arrays is the pinnacle of an enormous assembly effort that we’ve had to execute here in our cleanroom,” said professor Rick Gaitskell, who oversaw their construction. “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.”

Researchers at Brown University assemble an array of photomultiplier tubes for the LUX-ZEPLIN dark matter search experiment. The first of two photomultiplier arrays, which can detect faint flashes of light from particle interactions, has been transported to the Sanford Underground Research Facility in South Dakota. Courtesy of Nick Dentamaro/Brown University.

One of the biggest challenges the team had to confront was minimizing ambient dust levels during assembly. Each dust particle carries a minuscule amount of radiation that poses no threat to people, but could be enough to interfere with a signal from a dark matter particle. Before assembly of the arrays began, the team prescreened every part of every PMT tube to assess radiation levels. “We had Hamamatsu send us all of the materials that they were going to use for the PMT construction, and we put them in an underground germanium detector,” said Samuel Chan, PMT system team leader. “This detector is very good at detecting the radiation that the construction materials are emitting. If the intrinsic radiation levels were low enough in these materials, then we told Hamamatsu to go ahead and use them in the manufacture of these PMTs.”

Most of the work at Brown was performed in a class 1000 cleanroom. Within that cleanroom was a class 10 space, allowing no more than 10 dust particles bigger than 100th the width of a human hair per cubic foot. This is where much of the actual array assembly took place.

The leading theoretical candidate for a dark matter particle is the WIMP, or weakly interacting massive particle. WIMPs don’t absorb, emit, or reflect light. And they seldom interact with normal matter, which is why they’re so hard to detect even when millions of them could be traveling through Earth and everything on it each second.

The L-Z experiment, a collaboration of more than 250 scientists worldwide, aims to capture one of those fleetingly rare WIMP interactions, and thereby characterize the particles thought to make up more than 80 percent of the matter in the universe. The detector will be the most sensitive ever built, 50 times more sensitive than the LUX detector, which wrapped up its dark matter search at SURF in 2016. L-Z’s PMT arrays will need every bit of that sensitivity to catch the flashes associated with a WIMP interaction.

A team of Brown University researchers assembled two massive arrays of photomultiplier tubes (PMTs) for the LUX-ZEPLIN dark matter experiment, which will soon start its search for elusive dark matter particles at the Sanford Underground Research Facility in South Dakota. For more on the L-Z experiment, visit: http://lz.lbl.gov/ and https://www.sanfordlab.org/. Courtesy of Brown University.