The Homestake mine’s future lies in helping unravel the structure of the universe
LEAD, S.D – In the Homestake mine, where miners once searched for bright metal, physicists now search for dark matter.
The Homestake Gold Mine stopped producing gold a decade ago and has since been converted into the Sanford Underground Research Facility. But even before the gold mine closed in 2003, it was disclosing a rich vein of new knowledge for physicists.
Homestake was where nuclear chemist Ray Davis first detected and counted subatomic particles called neutrinos produced in the sun. His experiment was installed 4,850 feet underground. Davis earned a share of the Nobel Prize for Physics in 2002 for his work, which – because it yielded only one-third of the neutrinos physicists calculated they would find – helped lead physicists to the astonishing realization that neutrinos come in three “flavors” and that they could change flavor during their journey from the sun to the Davis detector.
In 2006, the mine was donated to South Dakota for use as an underground laboratory, and later that same year the project received $70 million from philanthropist T. Denny Sanford. The state Legislature has invested about $40 million in the lab, and the Department of Energy and the National Science Foundation have invested more than $170 million in projects related to the Sanford Lab.
In May 2012, the Davis Campus – named after the late Ray Davis – was completed. This underground campus hosts both of the main physics experiments the Sanford Lab: the LUX dark matter detector and the Majorana Demonstrator neutrino experiment.
To see those experiments, an observer must step onto the Yates Shaft elevator car, called a “cage,” and journey into the depths of the earth, side by side with physicists.
LUX: searching for dark matter
For Rick Gaitskell, it seems as though every day below ground is a good day. Gaitskell is a physics professor from Brown University and a leading researcher conducting the Large Underground Xenon experiment, or LUX, in the Davis Campus.
LUX is installed at the 4,850-foot level, in the same cavern that was excavated for Davis in the 1960s.
“The goal of the LUX experiment is to look for the missing mass of the universe,” Gaitskell said. “Around 95 percent of the composition of the universe is still currently unidentified, which is a startling admission, really.”
Gaitskell said that while scientists believe they know how much dark matter exists in the universe because of its gravitational effects on other matter, it is still an enigma and has never been directly detected.
The LUX experiment consists of a large water tank 24 feet in diameter and 24 feet tall filled with 72,000 gallons of highly purified water. Inside of that tank is a smaller thermal container, known as a cryostat, filled with one-third of a ton of liquid xenon, which has been cooled to -100 degrees Celsius. It will be the most sensitive detector yet to search for dark matter.
The water tank shields LUX from radiation coming from the surrounding rock, and nearly a mile of very dense rock lying above LUX shields the detector from cosmic radiation. Stray radiation could make it difficult to detect dark matter.
“We want to try to eliminate any other more mundane sources of particle interaction,” Gaitskell said. “Those sources include natural radioactivity. The rock, you and I, we’re all carrying a certain amount of natural radioactivity. People are always a little bit aghast when they discover the number of radioactive decays that occur in their body – about 3,000 every second.”
Cosmologists think dark matter probably consists of some as-yet-unknown particle. A leading candidate is called the “weakly interacting massive particle,” or WIMP, which is what LUX will look for.
Once the LUX detector is turned on, researchers will be waiting to see a particle of dark matter interact with an atom of xenon – an event that will be captured by 122 light-sensitive detectors called “photomultipliers.”
“In the first 20 hours of operation, LUX will match every single experiment in the history of dark matter searches, except for one in Italy,” Gaitskell said. “It’s going to take us a little bit less than 14 days to go past that one.”
Gaitskell is referring to the amount and quality of data researchers will collect from the LUX, which will be running 24 hours a day, seven days a week, once it’s turned on.
“When I started in the field of dark matter 25 years ago, I thought we were going to have this wrapped up in 5 years,” Gaitskell said. “I hope within the next 10 years we will have answered the riddle of dark matter.”
Majorana: searching for neutrinoless double-beta decay
Adjacent to the LUX experiment is the Majorana, (pronounced mah-yoh-RAH-nah), Demonstrator, which is searching for a rare phenomenon known as “neutrinoless double-beta decay.”
Neutrinos are everywhere. It’s estimated that 62 billion neutrinos pass through your thumbnail every second. Regular double-beta decay produces two protons, two electrons and two neutrinos.
Neutrinoless double-beta decay produces, as the name implies, two protons and two electrons but zero neutrinos. This decay has never before been observed. If it does happen, it could help explain why matter prevails over antimatter – or why the universe as we know it exists.
“That reaction is going to tell you something about the neutrino that’s suspected but hasn’t been demonstrated or shown,” said Vincente Guiseppe, physics professor at the University of South Dakota and leading Majorana researcher. “It’ll tell you that a neutrino is identical to its antiparticle. Most of particles in the universe come with an antiparticle, sort of a mirror image. It’s believed that neutrinos are indistinguishable from their mirror image, so if we see this reaction, it automatically tells us that’s the case.”
At the Majorana experiment, physicists enter a clear, plastic-covered makeshift dressing room, covering themselves from head-to-toe in white lab suits, cleaning every last item on their person, right down to the glasses.
This is a necessary precaution because the Majorana experiment, like LUX, must be protected from stray radiation. (Human fingerprints, for example, are a source of radiation).
“We are trying to keep our detector material as clean as possible. We want to keep it clean from the mine environment,” Guiseppe said. “We shed these clothes so we can get the dust off of us, then we put on clean new clothes to try and seal ourselves off. But that’s still not enough. We have to then only handle the (germanium) crystals and the detector materials in our glove box.”
Guiseppe has been working on Majorana for six years, and he was elated when they moved the experiment into the lab in May.
In order to observe this decay, scientists are constructing an insulated cryostat where they’ll put germanium-76, a slightly radioactive isotope of the element germanium that will be insulated by a combination of electroformed, ultra-pure copper, commercial copper, lead and plastic. The electroformed copper is created underground in the Sanford Lab.
“It has to be underground because we still have to avoid the cosmic rays,” Guiseppe said. “So we build up these detectors, we shield them, we place them in their final resting place, which is in a completely shielded environment, and then we wait,” Guiseppe said. “We use techniques that tell the difference between what’s going to be a neutrinoless double beta decay versus just about everything else. We’re trying to differentiate between something that happens 200 thousand times a minute and something that happens once a year. That’s the needle in the hay stack.”
Guiseppe said that discovering neutrinoless double beta decay would have to be proven with repetition, like all science hypotheses.
“If we discover double-beta decay, the rest of the world is going to say, ‘I doubt it,’” Guiseppe said. “But skepticism is what drives science. If you’re careful and you know what you’re doing, the truth will always come out.”