Sanford Underground Research facility
Rapid City Journal
LUX researchers spell out the experiment’s name, like cheerleaders, inside a 72,000 gallon water tank. The detector is the cylindrical titanium tank behind them. The tank is now filled with water, and the detector is operating. Credit: Matt Kapust
If you happen to have some extra xenon lying around – say about 1.8 million liters – officials at the Sanford Underground Research Facility would like to talk to you.
That’s the amount of the colorless, odorless element that makes up only 0.0000087 percent of the Earth’s atmosphere that scientists say will be needed for the deep underground laboratory’s $50 million to $60 million LUX-ZEPLIN experiment, so the Sanford Lab is going to start stockpiling it soon.
At its annual meeting Thursday, the South Dakota Science and Technology Authority unanimously approved a loan from the University of South Dakota Foundation and authorization for its executive director to procure up to 500,000 liters of xenon.
“The SDSTA truly appreciates the USD Foundation’s investment in the LUX-ZEPLIN experiment,” said Mike Headley, the Science Authority’s executive director. “Their investment along with similar investments by the South Dakota State University Foundation and the South Dakota Community Foundation, along with tremendous support from Gov. Daugaard, will help keep the U.S. in a leadership role in the global search for dark matter.”
Two years ago, xenon was priced at nearly $25 per liter, meaning the necessary gaseous element of atomic number 54, obtained through the distillation of liquid air, would have set the Science Authority back a cool $45 million. Fortunately, the price has dropped significantly since then.
“We will pay $5.50 per liter and this is not a discount; it’s the current market price,” said Sanford Lab spokeswoman Constance Walter. “Basically, the increased use of LED lights in vehicles, etc., has decreased the demand for xenon lighting. So, the price has dropped dramatically from a couple of years ago when they were in excess of $20 per liter.”
Headley said late Thursday that the Science Authority had secured the first 500,000 liters at a cost of $6.25 per liter and the remaining 1.3 million liters would cost $5.50 per liter. Consequently, even with the price reduction, the xenon will likely cost the Science Authority nearly $10.3 million.
Initially, the Science Authority will purchase 1.5 million liters, or about 80 percent of the 1.8 million liters the experiment will require, Walters said. The xenon will be delivered over the next two-plus years and when it is purchased, it will first go to the U.S. Department of Energy’s SLAC National Accelerator Laboratory in Menlo Park, Calif., where it will be purified. Then it will be shipped to the Sanford Lab to be placed in the detector sometime in 2018, she explained.
Discovered in 1898 by Sir William Ramsay, a Scottish chemist, and Morris M. Travers, an English chemist, shortly after their discovery of the elements krypton and neon, xenon was used in the Sanford Lab’s original Large Underground Xenon experiment known as LUX.
In October 2013, more than 100 science enthusiasts and government officials gathered at the Sanford Lab to receive initial findings of the LUX, while hundreds more from around the world joined via webcam. In that complex three-month trial involving particle physics, scientists sought to detect mysterious dark matter particles previously observed only through their gravitational effects on galaxies.
Nearly a mile deep in the bedrock of the Black Hills and shielded from vast amounts of cosmic radiation that constantly bombard the earth’s atmosphere, the LUX was comprised of a phone booth-sized titanium tank filled with nearly a third of a metric-ton (370 kilograms) of liquid xenon cooled to minus 150 degrees, scientists explained. The detector was further buffered from background radiation by its immersion in a 72,000-gallon tank of ultra-pure water.
Now, scientists around the globe are awaiting the start-up of the much larger 60-ton particle detector known as the LUX-ZEPLIN or LZ, which will be approximately 30 times larger (10 metric tons or 10,000 kilograms of xenon) and 100 times more sensitive than the LUX.
And, it’s going to take quite a bit of xenon to make that happen.
Xenon Q, xenon A
LEAD | With the help of a few friends, the South Dakota Science and Technology Authority will spend more than $10 million on xenon this year, a hefty amount for a gaseous element that a non-scientist knows so little about.
So, we asked Sanford Underground Research Facility scientist Markus Horn, who worked on the LUX and is now collaborating on the LUX LZ, the next-generation dark matter experiment, what makes xenon critical to its success.
Q: How is xenon extracted from the earth’s atmosphere?
A: Xenon is a trace gas in the atmosphere and is extracted as a by-product at the separation of air into oxygen and nitrogen.
Q: Why is xenon worth so much money?
A: It’s rare in the Earth’s atmosphere; only about 1 part in 20 million.
Q: Why is xenon critical to the LUX LZ? Succinctly, what does it do?
A: Xenon has unique properties for dark matter research. To name a few:
• It emits light at 175nm (UV light, sort of easy detectable with our PMTs);
• It is heavy, 135 times mass of proton, which is around the theoretically most favorable mass of the WIMP particle (billiard-ball-nuclear recoil is largest, hence easier to detect);
• It liquifies easily at moderate temperature of -100 Celsius;
• It is radio-pure;, does not have any natural radioactive isotopes;
• It has a high scintillation yield (emits a lot of light, so to say), very low energy threshold can be achieved (as we do in LUX);
• It is self-shielding (easily said, because it’s heavy, it shields itself, so the inner part of the detector is even quieter);
• It is a liquid noble gas detectors are easy to scale, LUX to LZ, etc.
Q: Why does it have to be so cold (-150 degrees)?
A: As with any material, it can be in different states (gas, liquid, solid). Depending on the element, this happens at different temperatures and pressures. Xenon is a gas at room temperature and atmospheric pressure, you need to compress it or cool it to approx -100C to force it into a liquid. I guess that’s simple chemistry.
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The Sanford Underground Research Facility in Lead, South Dakota, advances our understanding of the universe by providing laboratory space deep underground, where sensitive physics experiments can be shielded from cosmic radiation. Researchers at the Sanford Lab explore some of the most challenging questions facing 21st century physics, such as the origin of matter, the nature of dark matter and the properties of neutrinos. The facility also hosts experiments in other disciplines—including geology, biology and engineering.
The Sanford Lab is located at the former Homestake gold mine, which was a physics landmark long before being converted into a dedicated science facility. Nuclear chemist Ray Davis earned a share of the Nobel Prize for Physics in 2002 for a solar neutrino experiment he installed 4,850 feet underground in the mine.
Homestake closed in 2003, but the company donated the property to South Dakota in 2006 for use as an underground laboratory. That same year, philanthropist T. Denny Sanford donated $70 million to the project. The South Dakota Legislature also created the South Dakota Science and Technology Authority to operate the lab. The state Legislature has committed more than $40 million in state funds to the project, and South Dakota also obtained a $10 million Community Development Block Grant to help rehabilitate the facility.
In 2007, after the National Science Foundation named Homestake as the preferred site for a proposed national Deep Underground Science and Engineering Laboratory (DUSEL), the South Dakota Science and Technology Authority (SDSTA) began reopening the former gold mine.
In December 2010, the National Science Board decided not to fund further design of DUSEL. However, in 2011 the Department of Energy, through the Lawrence Berkeley National Laboratory, agreed to support ongoing science operations at Sanford Lab, while investigating how to use the underground research facility for other longer-term experiments. The SDSTA, which owns Sanford Lab, continues to operate the facility under that agreement with Berkeley Lab.
The first two major physics experiments at the Sanford Lab are 4,850 feet underground in an area called the Davis Campus, named for the late Ray Davis. The Large Underground Xenon (LUX) experiment is housed in the same cavern excavated for Ray Davis’s experiment in the 1960s.
LUX/Dark matter experiment at SURF
In October 2013, after an initial run of 80 days, LUX was determined to be the most sensitive detector yet to search for dark matter—a mysterious, yet-to-be-detected substance thought to be the most prevalent matter in the universe. The Majorana Demonstrator experiment, also on the 4850 Level, is searching for a rare phenomenon called “neutrinoless double-beta decay” that could reveal whether subatomic particles called neutrinos can be their own antiparticle. Detection of neutrinoless double-beta decay could help determine why matter prevailed over antimatter. The Majorana Demonstrator experiment is adjacent to the original Davis cavern.
Another major experiment, the Long Baseline Neutrino Experiment (LBNE)—a collaboration with Fermi National Accelerator Laboratory (Fermilab) and Sanford Lab, is in the preliminary design stages. The project got a major boost last year when Congress approved and the president signed an Omnibus Appropriations bill that will fund LBNE operations through FY 2014. Called the “next frontier of particle physics,” LBNE will follow neutrinos as they travel 800 miles through the earth, from FermiLab in Batavia, Ill., to Sanford Lab.