LEAD — Scientists with the LUX dark matter experiment at the Sanford Lab hope to be detecting dark matter by next May.
That means that plans are forging ahead to take real data from their detector, 4,850 feet underground, by January or February 2013. That timeline looks promising, as the scientists with the Large Underground Xenon detector celebrated a major milestone and filled their 72,000-gallon water tank with ultra-pure water. The tank will act as the last line of shielding from particles that can interfere with dark matter detection.
“This is one of our first major milestones,” LUX physicist Jeremy Mock said. Mock has worked on the experiment for five years, first as an undergraduate physics student at Case Western Reserve University and now as a doctoral candidate at the University of California, Davis. Dark matter particles are neutral in charge and they don’t emit light, so they are extremely difficult to detect. LUX scientists will look for evidence of collisions between dark matter particles—called weakly interacting massive particles, or WIMPs—and atoms of xenon inside the LUX detector.
Once the LUX detector turns on it will be the most sensitive dark matter detector in the world.
Filling the massive water tank was a major process, that included sending city water through a water purification system that operates on the Davis Campus, a mile underground at the Sanford Lab. The purification system puts regular city water through a reverse osmosis system, and then through a deionization process. By the time filtering is complete, the water is approximately 17 times more pure than that which comes out of the tap of Lead residents’ kitchens.
Once the water was purified, Mock said it filled the 72,000-gallon tank at a rate of 10 gallons per minute. It took scientists about two weeks, with water running for about 14-16 hours a day, to fill the tank. As the water rose, LUX scientists had to periodically check instrumentation, called photomultiplier tubes, that are installed on the side walls of the water tank. Each of these photomultiplier tubes is capable of detecting a single photon of light that assists in dark matter detection.
Checks at regular intervals were meticulous, Mock said.
“Every time a ring of photomultiplier tubes became submerged in water we had to check to make sure they still worked,” Mock said. “Having to fix something means a year delay. We want to make sure that every step of the way everything is working.”
The water in the massive tank will shield the actual detector, which LUX scientists eventually plan to fill with liquid Xenon that will be continuously circulated through a filtration system to maintain purity. The dark matter detector, located in the center of the water tank, is about the size of a telephone booth.
Currently, Mock said the detector is completely empty of everything, including air. A vacuum system maintains the void, which is as close to outer space conditions as can be achieved on earth. This allows scientists to take preliminary data to ensure their shielding system is working correctly.
“While the water level rose and began to overtake and eventually completely submerge the detector we were looking at the data coming out,” Mock said. “Basically what we were seeing in the detector is what we call background, it’s stuff we don’t care about because we don’t have our actual detecting medium in there. We watched our background rate just plummet, which showed us that our shield was actually doing something. It was very comforting to actually see it.”
The next step for the scientists is to fill the dark matter detector with Xenon gas, in order to test the systems. In January or February of 2013 scientists plan to introduce liquid Xenon into the detector, and begin taking actual data for dark matter detection.
Dr. Rick Gaitskell, of Brown University in Rhode Island, who is one of the principal investigators for the LUX detector, said he is very encouraged by the test results he has seen from the detector so far.
“The detector is very quiet now,” he said, referring to the absence of radioactivity that can interfere with dark matter detection. “It’s enormously encouraging with respect to turning the detector on at the beginning of the year.”
Gov. Dennis Daugaard praised the scientists involved with the LUX project for their dedication and hard work at the Sanford Lab.
“Submerging the LUX detector is a giant step in the dark-matter research project, and I congratulate all of those involved in the effort,” Daugaard said. “The Sanford Underground Research Facility is a shining star in South Dakota, and I have no doubt that the sophisticated experiments to be done in the lab are going to gain worldwide scientific notice.”
Thought to comprise more than 80 percent of the mass of the universe, dark matter has so far eluded direct detection. The LUX detector, under construction for more than three years at the Sanford Lab, was installed underground in a protective tank in July.
“This is a major step forward on the road to an operational detector in early 2013,” said Mike Headley, laboratory director for the Sanford Underground Research Facility in Lead.
The LUX scientific collaboration includes dozens of scientists like Mock. They come from 17 research universities and national laboratories in the U.S. and Europe.
Physicist Harry Nelson of the University of California, Santa Barbara — who helped design, build and fill the sophisticated water tank that now holds the experiment — says LUX could help solve a vexing mystery. “The nature of the dark matter is one of the top three open questions in particle physics,” Nelson said. “We know that matter like us — electrons, protons, and neutrons — makes up only one sixth of the known matter in the universe. The evidence that the other five sixths is present out there in galaxies is overwhelming. Detecting dark matter in a laboratory like Sanford, here on Earth, would be a huge step forward in nailing down what the stuff really is.”
LUX, however, requires a very quiet environment. In July, the experiment was installed 4,850 feet underground in the Sanford Lab, where it is protected from the cosmic radiation that constantly bombards the surface of the earth. LUX also must be protected from the small amounts of natural radiation from the surrounding rock. That’s why the detector was lowered into a very large stainless steel tank—20 feet tall by 25 feet in diameter. The tank has now been filled with more than 70,000 gallons of ultra-pure de-ionized water that will shield the detector from gamma radiation and stray neutrons.
Now that LUX is under water, researchers are testing the experiment’s complex electronics — a process that will take weeks. The detector itself is a double-walled titanium cylinder about 6-and-a-half feet tall by 3 feet in diameter. The cylinder is a vacuum thermos — or “cryostat” — that holds about a third of a ton of xenon, cooled to a liquid state at minus 160 degrees F. Inside the cryostat, 122 smaller photomultiplier tubes will detect when a Weakly Interacting Massive Particle (WIMP) bumps into a Xenon atom. The collision will produce two flashes of light — one at the point of impact and a second flash in a thin layer of Xenon gas at the top of the detector. The second, stronger flash will be caused by electrons released during the collision and drawn upwards by a strong electrical field inside the detector. Researchers will compare data from the two flashes to determine whether dark matter has been discovered.
Why search for dark matter? Nelson pointed out that practical applications of fundamental research are sometimes not immediately apparent. However, research into the nature of electricity in the 19th century and into the structure of the atom in the early 20th century led to technological advances now essential to manufacturing, communications, medicine and high-speed computing. “Unforeseen consequences are one of the most exciting aspects of scientific exploration and discovery,” Nelson said.
Today, dark matter’s invisibility both “confounds and motivates” researchers, Nelson said, and scientists are looking for it in a number of experiments around the world. “The Sanford Lab provides us with a crucial facility to forge ahead toward the goal of directly detecting dark matter here on Earth,” he said. “The most popular theories in particle physics indicate that success will come soon, either at the Sanford Lab or a little later at labs in Canada, Italy or China.”