The Large Underground Xenon Detector, or LUX, will be tested over the next few months in the surface lab, where problems are easier to fix. Later in 2011, LUX will be disassembled and transported 4,850 feet underground, where it will be reassembled and installed in the same cavern excavated for the neutrino detector that earned Ray Davis a Nobel Prize in 2002.
Nearly a mile of rock will protect LUX from the noise of background cosmic radiation, just as it protected the Davis experiment.
The centerpiece of the Davis neutrino detector was a 100,000-gallon tank of a dilute chlorine solution. Davis devised a way to detect individual argon atoms created in the large tank by rare collisions between solar neutrinos and chlorine atoms. Now the Davis detector has been removed to prepare for a new quest that's even more challenging. The heart of LUX is a cryostat-essentially a thermos about the size of a large trashcan-that will be filled with 350 kilograms of liquid xenon. The LUX team will look for collisions in the cryostat between xenon atoms and particles of dark matter called "weakly interacting massive particles" or WIMPs. These collisions will, in theory, eject both electrons and photons, which LUX will detect.
The process is complicated by the fact that xenon is a gas at room temperature. To reach its liquid state, xenon must be chilled to between minus 120 to minus 175 degrees F. This month, Tom Shutt of Case Western Reserve University and a team of LUX researchers are completing the assembly of the "central active region" of the detector -- that is, the system of electronic sensors that will have to perform flawlessly inside a frigid bath of ultra-clear liquid xenon.
"There's a huge number of structures," Shutt says. The structures include three wire grids to create electrical fields to move electrons and 120 photomultiplier tubes to detect tiny flashes of light. A sophisticated plumbing system that will pump xenon out of the container, through a purifier and back into the container. Seventy thermometers will monitor the circulating xenon. Nine sensors will monitor the level of xenon in the tank. The temperature a nd level of the xenon must be precisely maintained, all through electronics.
"All of that is jam-packed into the skin of the detector, around the active region," Shutt says. "It's a tight fit." To make it work, LUX researchers must approach their jobs like artisans, tidying cables and double-checking wiring, sensors, valves, gas lines, thermosyphons, amplifiers and other equipment. The LUX team calls this "dressing the detector," and neatness counts. "You've got to be motivated to make the thing clean," Shutt says. "It's a labor of love."