Dec 18 2018
The LUX-ZEPLIN (LZ) dark matter detector, which will soon begin its hunt for the mysterious particles believed to account for a bulk of matter in the universe, had its first set of “eyes” delivered recently.
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, arrived Friday at the Sanford Underground Research Facility in South Dakota. (Credit: Nick Dentamaro/Brown University)
The first of two large arrays of photomultiplier tubes (PMTs)—robust light sensors that can detect the feeblest of flashes—completed a 2,000-mile road trip from Rhode Island to the Sanford Underground Research Facility (SURF) in Lead, South Dakota, where LZ is slated to commence its dark matter search in 2020.
The second array will be ready in January. When the LZ detector is finished and turned on, the PMT arrays will maintain a careful watch on LZ’s 10-ton tank of liquid xenon, looking for the indicative twin flashes of light produced if a dark matter particle collides into a xenon atom inside the tank.
A team of scientists and technicians from Brown University have spent the last six months meticulously assembling the two arrays, each about 5 ft in diameter and containing a total of 494 PMTs.
“The delivery of these arrays is the pinnacle of an enormous assembly effort that we’ve had to execute here in our clean room,” said Rick Gaitskell, a professor of physics at Brown University who supervised 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.”
The Brown team has partnered with scientists and engineers from the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and from Imperial College London to design, procure, test, and assemble all of the parts of the array. The PMTs, which were manufactured by the Hamamatsu Corp. in Japan, was tested at Brown and at Imperial College.
“The delivery of the first array of PMTs for LZ to SURF is a critical milestone for the LZ Project,” said Murdock “Gil” Gilchriese of Berkeley Lab, who is the LZ project director.
In preparation for the coming of the PMT arrays, scientists at SURF had already been working with prototype arrays to attempt linking the PMTs to a complex sequence of cabling. The real assembly of these cables to the PMTs will take place in a clean room at SURF.
No one knows precisely what dark matter is. Researchers can see the effects of its gravity in the rotation of galaxies and in the way light bends as it travels through the universe, but nobody has directly identified a dark matter particle. The leading theoretical candidate for a dark matter particle is the “weakly interacting massive particle” (WIMP). WIMPs cannot be seen because they do not emit, absorb, or reflect light. Plus, they interact with regular matter only on very occasional instances, which is why they are very hard to spot even when millions of them may be traveling across the Earth each second.
The LZ experiment, a partnership of over 250 scientists from 38 institutions globally, aims to capture one of those fleetingly infrequent WIMP interactions, and thereby characterize the particles believed to make up over 80% of the matter in the universe. The detector will be the most sensitive ever constructed, about 100 times more sensitive than the LUX detector, which completed its dark matter search at SURF in 2016.
The PMT arrays are a crucial part of the experiment. Each PMT is a six-inches-long cylinder that is approximately the diameter of a soda can. To form arrays sufficiently large to monitor the whole LZ xenon target, hundreds of PMTs are assembled together within a circular titanium matrix. The array that will be positioned on top of the xenon target has 253 PMTs, while the lower array has 241.
PMTs are engineered to amplify weak light signals. When individual photons (particles of light) enter a PMT, they hit a photocathode. If the photon has adequate energy, it makes the photocathode to eject one or more electrons. Those electrons then hit an electrode, which ejects additional electrons. By flowing through a series of electrodes, the original signal is intensified by over a factor of 1 million to form a detectable signal.
LZ’s PMT arrays will require every bit of that sensitivity to capture the flashes related with a WIMP interaction.
We could be looking for events emitting as few as 20 photons in a huge tank containing 10 tons of xenon, which is something that the human visual system wouldn’t be able to do. But it’s something these arrays can do, and we’ll need them to do it in order to see the signal from rare particle events.
Rick Gaitskell, Professor of Physics, Brown University.
The photons are generated by what is referred to as a nuclear recoil event, which creates two distinct flashes. The first happens at the moment a WIMP hits a xenon nucleus. The second, which happens a few hundred microseconds afterward, is created by the ricochet of the xenon atom that was hit. It bounces into the atoms adjoining it, which knocks a few electrons free. The electrons are then drifted by an electric field to the tank’s top, where they reach a thin layer of xenon gas that turns them into light.
In order for those small flashes to be distinct from undesirable background events, the detector needs to be sheltered from cosmic rays and other kinds of radiation, which also cause liquid xenon to light up. That is why the experiment is conducted underground at SURF, a former gold mine, where the detector will be protected by about a mile of rock to restrict interference.
The need to restrict interference is also the reason that the Brown University team was particular about cleanliness while they formed the arrays. The team’s main opponent was regular dust.
“When you’re dealing with an instrument that’s as sensitive as LZ, suddenly things you wouldn’t normally care about before become very serious,” said Casey Rhyne, a Brown University graduate student who had a leading part in constructing the arrays. “One of the biggest challenges we had to confront was minimizing ambient dust levels during assembly.”
Each dust particle carries a tiny quantity of radioactive uranium and thorium decay products. The radiation is vanishingly small and poses no danger to people, but too many of those specks within the LZ detector could be sufficient to obstruct a WIMP signal.
In effect, the dust budget for the LZ experiment stipulates no more than one gram of dust to be within in the whole 10-ton instrument. Due to all of their nooks and crannies, the PMT arrays could be major dust collectors if steps were not taken to keep them clean during the entire construction.
The Brown University researchers performed most of its tasks in a “class 1000” clean room, which permits no more than 1,000 microscopic dust particles per cubic foot of space. Moreover, within that clean room was an even more untouched space that the team christened “PALACE (PMT Array Lifting And Commissioning Enclosure).” PALACE was basically an ultraclean room where much of the actual array assembly happened. PALACE was a “class 10” space—permitting no more than 10 dust particles bigger than 1 hundredth the width of a human hair per cubic foot.
But the radiation apprehensions did not stop at dust. Before assembly of the arrays commenced, the team prescreened every part of every PMT tube to measure 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, a graduate student and 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.”
The team is expectant that all the effort they put in over the last six months will bear dividends when LZ begins its WIMP search.
Getting everything right now will have a huge impact less than two years from now, when we switch on the completed detector and we’re taking data. We’ll be able to see directly from that data how good of a job we and other people have done.
Rick Gaitskell, Professor of Physics, Brown University.
Given the huge increase in dark matter search sensitivity that the LZ detector can deliver than all earlier experiments, the team hopes that this detector will, at last, identify and characterize the massive sea of stuff that surrounds us all. Thus far, the dark stuff has stayed infuriatingly elusive.
Key support for LZ is provided by the DOE Office of Science’s Office of High Energy Physics, South Dakota Science and Technology Authority, the U.K.’s Science & Technology Facilities Council, and by collaboration members in Portugal and South Korea.