• Nuclear power plants like the twin Daya Bay reactors, not far from Hong Kong, yield copious amounts of electron antineutrinos (the antiparticle of the electron neutrino) – millions of quadrillions of them every second.
• Two more pairs of reactors at Ling Ao, a kilometer away, make the Daya Bay nuclear power complex one of the most powerful in the world. All the reactors sit close to a mountain. By putting their antineutrino detectors in tunnels under the mountain, the researchers can shield them from cosmic rays.
• 
  The detectors are assembled above ground. One acrylic vessel three meters in diameter (built in Taiwan) nests inside another, four meters in diameter (built in the United States), as shown in this prototype. Like Russian dolls, both acrylic vessels are nested inside a third vessel of stainless steel (built in China).
• 
  
  
  The actual detectors use transparent acrylic that remains clear as long as it’s not exposed to too much ultraviolet light. Once in place, the detectors will operate in total darkness.
• 
  
  
  The building where the detectors are assembled is a class 1000 clean room; dust and even microscopic particles in the air are severely limited.
• The detectors are assembled in pits, four meters deep, which allow technicians easy access from the top. Here, a five-meter stainless steel vessel is shown in the pit. Polished reflectors cap the detectors at bottom and top .
• 
  
  
  Photomultiplier tubes (PMTs) convert incredibly faint flashes of light to electrical signals. Assembled on “ladders,” the walls of each detector are lined with 192 PMTs.
• 
  
  
  A PMT ladder is lowered into the narrow space between the acrylic vessels and the stainless steel vessel.
• 
  
  
  The space between the outer acrylic vessel and the steel vessel is less than half a meter wide.
• PMTs, seen through the transparent walls of the acrylic vessels, will detect the light that results when antineutrinos slam into protons in the nuclei of atoms in the liquid target– exceedingly rare events among the continual stream of quadrillions of antineutrinos per second.
• PMTs convert light from particle collisions to electric charge. Since the experiment must collect the light emitted from each event, the reflectors at top and bottom of the acrylic vessels enhance gathering of light.
• This nearly finished “dry” antineutrino detector will soon be ready to move into the tunnel.
• An auto-guided transporter will carry the finished antineutrino detectors into the tunnel. At speeds less than one kilometer per hour (slightly faster than a tortoise), the seven-meter-long transporter can carry 125 metric tons and steer itself down the center of the six-meter-wide tunnel.
• The access tunnel enters the mountain near the Daya Bay reactors.
• A pair of antineutrino detectors will be positioned in each of the two experimental halls near the reactors, while another set of four is positioned in the far hall almost two kilometers away. The first detectors to be installed will be at the Daya Bay Near Hall.
• 
  
  
  Researchers approach the Daya Bay Near Hall, 98 meters beneath the surface of the ridge and 360 meters from the Daya Bay reactors.
• 
  The heart of the Near Hall is a pool of ultrapure water in which the antineutrino detectors will be submerged, shielded from radioactive decays in the surrounding rock by more than two meters of water on all sides. The pool is lined with PMTs to track any “stiff” (highly energetic) cosmic rays that make it all the way through the overlying rock. The blue supports beyond the pool indicate where a different kind of detector is being constructed, which will roll over the water pool like a roof and help locate the position of any cosmic rays that enter the water.
• 
  Bill Edwards of Berkeley Lab’s Physics Division stands at the bottom of a water pool next to a detector support. Edwards, U.S. Project Manager for Daya Bay, says the pool must be sealed in total darkness to avoid overwhelming the PMTs, which are so sensitive to light that they can detect even a single photon. A recent lights-out test plunged the Near Hall into apparent total darkness, Edwards says, yet the PMTs were still saturated. The source turned out to be a single red light on the control panel of the overhead crane, which had to be taped over for the next test.
• The two-layer water pool includes a separate meter-wide chamber surrounding the inner pool. It will also be filled with water and instrumented with PMTs to detect radioactive decays or other sources of background noise.
• 
  
  
  Deeper in the heart of the mountain, researchers approach the Liquid Scintillator Hall, where the liquid scintillators are synthesized and the antineutrino detectors are filled with the liquids.
• 
  In the Liquid Scintillator Hall, each antineutrino detector will be filled with three liquids. The innermost, three-meter acrylic vessel holds 20 metric tons of liquid scintillator plus a microscopic amount of gadolinium in solution. Gadolinium is a heavy metal that improves the chances for tagging antineutrino interactions. The liquid scintillator is mostly linear alkyl benzene, or LAB, which is commonly used in biodegradable detergents. The LAB is laced with fluors that emit faint flashes of light when charged particles move through it.
• The space between each detector’s four-meter and three-meter acrylic vessels will be filled with liquid scintillator without gadolinium, to act as a gamma-ray catcher and background filter. For additional shielding, the space between the four-meter vessel and the five-meter steel tank is filled with mineral oil. The acrylic vessels are too frail to hold the liquids unsupported, so in the mixing hall all three vessels in a detector are filled simultaneously; all three liquids have the same density, and thus the pressure on the vessels is equal inside and out.
• Beyond the mixing hall and the water hall that purifies water for the pools, a branch tunnel turns back for half a kilometer to reach the Ling Ao Near Hall, near the edge of the mountain.
• 
  
  
  Shielded by 112 meters of rock overhead, the Ling Ao Near Hall’s water pool, still under construction, is very near completion.
• Each day the Near Halls will catch a few thousand electron antineutrinos out of the millions of quadrillions produced by the reactors. A kilometer and a half farther into the mountain lies the Far Hall, under 330 meters of rock. The Far Hall is almost two kilometers from the Daya Bay reactors and 1.6 kilometers from the Ling Ao and Ling Ao II reactors.
• 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.
• Still in the early stage of construction, the Far Hall will house four antineutrino detectors in a large pool of water. Comparing the precise number of antineutrino events captured here to those captured in the Near Halls will yield a clear-cut value for theta one three, a remaining essential clue to the mysteries of neutrino mass.
• Construction of the Far Hall is scheduled to be complete in August, 2011, and a calibration test of the first Near Hall will begin this June. Daya Bay Neutrino Experiment co-spokesperson Kam Biu Luk says, "To get theta one three, essentially we measure the difference in the antineutrino flux at two locations. Basically it comes down to how many antineutrinos disappear." By summer of 2012 the Far Hall will begin taking the data needed to measure theta one three precisely, and the Daya Bay Reactor Neutrino Experiment will be in full swing.