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Underground Detectors Close in on Elusive Neutrinos

Fri, 09/16/2011 - 6:15am

September 21, 2011

The two antineutrino detectors in Daya Bay Hall #1, shown here prior to the pool being filled with ultrapure water. The pool is lined with photomultiplier tubes to track any "stiff" (highly energetic) cosmic rays that make it all the way through the overlying rock. (Courtesy of Roy Kaltschmidt, Lawrence Berkeley National Laboratory)

The two antineutrino detectors in Daya Bay Hall #1, shown here prior to the pool being filled with ultrapure water. The pool is lined with photomultiplier tubes to track any "stiff" (highly energetic) cosmic rays that make it all the way through the overlying rock. (Courtesy of Roy Kaltschmidt, Lawrence Berkeley National Laboratory)

The Daya Bay Reactor Neutrino Experiment is a multinational particle physics project studying neutrinos. It is located about 50 km north of Hong Kong and includes researchers from China, the U.S., Taiwan, and the Czech Republic. The experiment’s first completed set of twin detectors is now recording interactions of anti-neutrinos as they travel away from the six nuclear reactors of the China Guangdong Nuclear Power Group.

Neutrinos are uncharged particles produced in nuclear reactions, such as in the sun, by cosmic rays, and in nuclear power plants. They come in three types—electron, muon and tau neutrinos—that morph, or oscillate, from one form to another, interacting hardly at all as they travel through space and matter, including people, buildings and planets.

The last elusive mixing angle is dubbed ?13 (theta one-three) and holds clues as to how electrons and their cousins, muons and tau particles, were born in the moments after the big bang. Knowing ?13 could explain why there is more matter than anti-matter in the universe—and indeed why there is any matter at all.

Data for the Daya Bay Experiment will be collected by eight large detectors buried deep underground in the mountains adjacent to the Guangdong nuclear reactors. Nuclear reactors produce enormous quantities of anti-neutrinos (which in most vital respects are identical to neutrinos), and the reactors at Daya Bay yield millions of quadrillions of them every second. After two to three years of collecting data with all eight detectors, the Daya Bay Reactor Neutrino Experiment intends to meet the goal of measuring the oscillation amplitude of ?13 with a sensitivity of 1%.

“To achieve our 1% goal, the differences in sensitivity among all eight detectors must be almost imperceptible, to within 0.4% of one another,” says William Edwards of the Dept. of Energy’s Lawrence Berkeley Lab’s Physics Division, Daya Bay’s U.S. Project and Operations Manager. “Although all data will contribute to the measurement, initial data-taking will be aimed at understanding the systematics of the anti-neutrino detectors in Hall 1—basically to find out how identical they are.”

Three experimental halls house the detectors, which will measure mixing angle ?13 by comparing the flux and energy distribution of interactions in the pairs of detectors in Halls 1 and 2, each about 0.5 km from the nearest reactors, with the four detectors in Hall 3, which is about 2 km from the reactors. The very first data are now being recorded in Hall 1.

 Like the Near Halls, the Far Hall will also measure electron antineutrinos, although it will detect fewer of them. Not only do the neutrinos spread out as they flow from the reactors, the number of electron antineutrinos decreases because they may transform into different flavors as they streak across this distance. Image by Roy Kaltschmidt.

 Like the Near Halls, the Far Hall will also measure electron antineutrinos, although it will detect fewer of them. Not only do the neutrinos spread out as they flow from the reactors, the number of electron antineutrinos decreases because they may transform into different flavors as they streak across this distance. Image by Roy Kaltschmidt.

Each cylindrical anti-neutrino detector is nested like a Russian doll, where one transparent acrylic vessel is enclosed in a second one, which in turn sits inside a third vessel made of stainless steel. The detectors are filled with a clear liquid scintillator, which reveals anti-neutrino interactions by the very faint flashes of light they emit. Sensitive photomultiplier tubes line the detector walls, ready to amplify and record the telltale flashes.

A relatively small number of these interactions, over a thousand a day out of the millions of quadrillions of anti-neutrinos produced by the reactors every second, will be captured by the twin detectors in each near hall. Because of their greater distance, the four detectors in the far hall will measure only a few hundred a day. To measure ?13, the experiment records the precise difference in flux and energy distribution between the near and far detectors.

The experimental halls are dug deep under the mountain to shield the detectors from cosmic rays. The anti-neutrino detectors are submerged in pools of water to shield them from radioactive decays in the surrounding rock. Despite this shielding, some energetic cosmic rays make it all the way through; their trajectories are tracked by photomultiplier tubes in the walls of the water pool and “muon trackers” in the roof over the pool. Events of this kind are ignored in collecting the anti-neutrino data.

Hall 2, the other near hall, is expected to begin taking data this fall. As more data flows in, the upper limit on the value of the ?13measurement will continue to trend lower.

The Daya Bay experiment is a “disappearance” experiment, powered by the enormous quantities of electron anti-neutrinos produced in the nearby reactors. The detectors in the two closest halls will measure the raw flux of electron anti-neutrinos from the reactors. The detectors at the far hall will look for a depletion in the expected anti-neutrino flux.

“When the Far Hall comes online next summer,” says Edwards, “the limit will drop like a rock. We’ll achieve a sensitivity on the measurement that’s not possible with any other detector now planned or under construction.”

Detector Timeline:

 

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