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Searching for WIMPS

Most sensitive dark matter detector in the world preparing for operation in Lead

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Posted: Saturday, April 20, 2013 6:00 am

LEAD — The most sensitive dark matter detector in the world will soon be searching for WIMPS in Lead.

Scientists with the Large Underground Xenon dark matter detector say they are very close to operating their detector to search for the Weakly Interacting Massive Particles (WIMPS), which they believe make up dark matter. Final calibrations are being completed, and though officials did not give a specific time period, they are ramping up for a two-month run before the end of this year.

Brown University Professor Dr. Rick Gaitskell, a principal investigator for the LUX, said the highest amount of care imaginable is being taken to ensure that the most sensitive dark matter detector in the world maximizes its potential to detect the highly elusive particles that are thought to comprise the majority of matter in the universe.

The initial two-month run will be a test, Gaitskell said, to determine how the various detector systems operate underground. Though scientists have tested the detector in a surface laboratory at the Sanford Underground Research Facility, this will be the first time they have operated it since moving it underground in 2012. Within two weeks of running, Gaitskell said the LUX will greatly surpass any other detector in the world in its potential to detect the elusive substance, due to its large size and sensitivity due to shielding from radiation.

“The first WIMP search run is enormously important to us,” Gaitskell said. “The quality of information that you learn about the detector is so much better underground. It is very quiet here. The cosmic rays are almost completely gone. (The space) which the detector is sitting in is quieter than anywhere else on the planet, in terms of regular particles getting in there.”

In order to ensure the highest levels of purity and sensitivity, scientists have positioned their LUX detector a mile underground, in order to avoid cosmic rays that constantly bombard the earth’s surface. But, since radiation still emanates from the rock of the former Homestake Gold Mine, the instrumentation surrounding the experiment, and the very people who operate the detector, additional shielding methods have been employed. Gaitskell said a massive tank of 70,600 gallons of highly purified water surrounds the dark matter detector.

The water is several thousand times more pure than average tap water, said Curt Nehrkorn, a 24-year-old graduate student from the University of California-Santa Barbara.  Nehrkorn is one of the leading experts in water purification for LUX.

“You don’t want to drink this, it’s too pure,” Nehrkorn said.

“The water itself has no radioactivity, and we are putting at least 10 feet of water between the detector and the outside world,” Gaitskell said. “So, that huge thickness of water means that the radioactivity that is in you and I, if we were to talk around the tank right now, the detector can’t see us. If we would drain the water it could sense we were there.”

After all of this shielding, Gaitskell said scientists believe the only particles that will be able to make it through to the center of the liquid Xenon-filled detector, will be those which pass through the earth undetected all the time — WIMPS (Weakly Interacting Massive Particles.)

And the detection of WIMPS means the detection of dark matter.

But why do scientists use Xenon?

According to Gaitskell, when a particle interacts with the Xenon it emits a significant amount of light. Additionally, unlike other materials, the Xenon does not re-absorb that light. Rather, the element is completely transparent to the light. Additionally, the liquid Xenon is extremely dense, and with one-third of a ton of the element, conventional particles will have an even harder time making it to the center of the detector.

Once a particle interaction occurs in the detector, photomultiplier tubes that are positioned inside will register the light. Interactions that occur in the center of the detector, Gaitskell said, can likely be a sign of dark matter detection.

“(You could almost say) that only dark matter is penetrating enough to get through all of these layers,” Gaitskell said.

He further explained that while great care was taken to ensure the highest level of purity for the materials in the detector, even those are not perfect could generate trace amounts of radioactivity. But particle interactions that result from that radioactivity are more likely to occur on the outskirts of the detector.

But the theory is that only WIMPS will be strong enough to penetrate the dense Xenon, to get to the center of the detector.

“WIMPS, there are 100 billion passing through you every second. So when they do choose to interact in the Xenon, they will interact anywhere,” Gaitskell said.

But if scientists observe a particle interaction in the center of their detector, they are not going to automatically assume it is dark matter. The final test for a WIMP, Gaitskell said, will be determining whether the particle bounced off a nucleus — as a WIMP will do — or whether it interacted with the electrons in the atom, as conventional radioactivity would behave.

An electrical field that surrounds the detector will help scientists determine the difference between interactions.

With a total construction cost of about $10 million just for the detector, Gaitskell said total project cost is difficult to determine. The LUX collaboration includes about 70 active personnel from around the country and the world, who are individually paid by 14 different participating institutions. Annual supplies for the experiment, including shipments of liquid nitrogen that serve to cool the detector down to -148 Fahrenheit, cost approximately $150,000 per year.

Considering the coldest place on earth — a location in Antarctica — boasts a low of almost 130 degrees below zero, Gaitskell does not consider his detector to be all that cold.

“The coldest place on earth is getting bloody close to being cold enough for our experiment,” he said.

Once LUX scientists have completed their two-month test run this year, Gaitskell said they would spend a good chunk of 2014 making any necessary improvements to the detector itself. Once the detector is running full time, it will operate and take data for at least a year. Gaitskell said he expects LUX to run at least through 2015.

Then, in 2016-17, an even bigger dark matter detector — the LUX-Zeplin (LZ) — is being planned to replace the LUX. If LUX is successful in detecting dark matter, LZ will help scientists study the substance. If LUX is not successful, LZ will be about 20 times more sensitive, and that much more likely to detect WIMPS.

For right now, Gaitskell said his team of experts is working diligently to ensure LUX is as successful as possible. At any given time, he said there are at least 10 world-leading experts working on the LUX detector.

“LUX is probably the largest dark matter collaboration in the world (with 70 active members.) It is certainly the largest dark matter collaboration when you consider the level of expertise. We have assembled so many of the U.S.’s best and brightest with respect to knowing how to get these incredible conditions you need to look for these very rare particle events. One of the reasons I think graduate students get so much out of working with experiments like ours is by the time they are done working on our experiment, they are quite literally the world-leading expert on the thing that they’re doing. If you’re going to work on this, that’s the status you have to get to. Everybody in the world must be turning to you for what you are working on.”

It’s that level of intense training that Gaitskell looks forward to extending to South Dakota institutions. And it is that level of enthusiasm for intensive basic research that the U.S. must continue to pass on, he said.

“Here in the U.S. we are very good at science,” he said. “We understand it, and we are able to do things like this. It is very important to create the right conditions to continue the leadership position. The lab here, the Sanford Lab, is working to do just that — to have world-leading experiments and to be the focus of some of the brightest people here in the U.S.” 

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