June 23, 2009

World’s Deepest Underground Lab Hosts Hunt For Dark Matter

The world's deepest underground science lab is being built below the Black Hills of South Dakota.

With a depth equal to more than six Empire State buildings, the space is perfectly tailored to the needs of scientists in their quest for mysterious particles known as dark matter.

Construction commenced Monday 4,850 feet below the Earth's surface under an abandoned gold mine that once hosted Nobel Prize-winning research. Scientists, politicians and other officials stood by watching.

The location is ideal for the experiments because it is protected from the interference of cosmic rays that could hinder the effort to prove the existence of dark matter, which is believed to account for almost a quarter of the universe's mass.

The deepest recesses of the mine descend roughly 8,000 feet below the surface. Some early geology and hydrology experiments have already begun at 4,850 feet. Researchers are waiting to receive funding from Congress to build two labs even deeper.

Tom Shutt of Case Western Reserve University in Cleveland told the Associated Press, "The fact that we're going to be in the Davis Cavern just tickles us pink."  He was referring to a part of the mine named after scientist Ray Davis Jr., who made use of it in the 1960s to demonstrate the existence of particles called solar neutrinos.

Both Davis and his colleague John Bahcall won a share of the 2002 Nobel Prize for physics for their work.

The old Homestake Gold Mine in a community called Lead was closed in 2001 after 125 years. Pumps that kept the mine dry had been turned off for years, so workers have been drying it out to prepare the way for new research.

Before construction of the labs can take place, crews must stabilize the tunnels and establish new infrastructure. The lab at 4,850 feet is quite unsightly at this point with debris left by miners and a rusty orange film covering the walls, floors and ceilings. It will take a lot of work to make it a space worthy of the groundbreaking experiments taking place within its walls.

The first dark matter experiment is going to be the Large Underground Xenon Detector experiment, or LUX. It is a project to capture signals from WIMPs (weakly interactive massive particles), which is the substance that comprises dark matter. The detection of these particles could provide a deeper insight into the Big Bang explosion thought to have formed the universe.

The detector uses xenon, the heaviest of the noble gases like helium, argon and krypton. Xenon becomes a liquid when cooled to -148 degrees Fahrenheit. The detector uses the liquefied xenon, as well as a layer of gaseous xenon above it.

As WIMPs strike the xenon atoms, a light is flashed and recorded with photo sensors. Electrons release from the atom upon impact and are pulled through an electrical field up out of the liquid, where they will again emit a flash of light when encountering xenon gas atoms. The point of the detector is to find a ratio between light emitted upon impact and the ionization rate during the impact in the xenon gas. This ratio should provide a unique signal for WIMPs.

For the stray radioactive or cosmic ray particles that make it into the detector, the researchers should be able to distinguish a WIMP from those particles by the number of times electrons are scattered in the liquid xenon.

WIMPS differ from other radioactive particles in that they hit the nucleus of the atom instead of the electrons on the nucleus' surface, giving a different signal in the way the atom moves or draws back at impact.

"WIMPS have become a hot topic worldwide," said Shutt. But he says the search is like trying to find a needle in a haystack.

Scientists around the world hypothesize that millions of WIMPs are constantly passing through the Earth every second completely undetected. WIMPs are believed to be present because the rate at which the universe expands is being slowed by a gravitational pull from some mass that cannot be explained by the amount of planetary and other matter in the universe.

Shutt, along with Brown University's Rick Gaitskell and nearly a dozen collaborators will be working at the site to search for the elusive dark matter, which does not emit detectable light or radiation.

Scientists believe the majority of the dark matter in the universe does not have atoms and does not interact with ordinary matter by electromagnetic forces. They are trying to discover its exact nature, how much there is and what effect it may have on the future of the universe.

Physicists say that our galaxies would likely have never been formed without the presence of dark matter. They hope that by understanding more about dark matter, they will in turn better understand whether the universe is expanding or contracting.

The research team will hunt the elusive particles in a 661-pound tank of liquid xenon, which has a weight 3 times that of water. If they were to attempt detecting dark matter above ground, the highly sensitive detector would be assailed by cosmic radiation.

Scientists hope to begin construction on the two deepest labs by 2012 and open them by 2016. The cost of the projects is estimated to be $550 million.


Image 2: 3D Rendering of LUX deployed underground at the 4850 ft level of the Sanford Lab


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