Last July, LBL celebrated the completion of the 18-meter, stainless-steel geodesic sphere that is to be a major component of the Sudbury Neutrino Observatory (SNO). While the Petaluma, Calif., ceremony marked the completion of the sphere--which was subsequently shipped to Sudbury, Canada--LBL's contribution to the construction phase of SNO continued.
Scientists, engineers, and designers in the Nuclear Science and Engineering divisions, under the leadership of NSD physicist Kevin Lesko, have completed work on the panel arrays that, when attached to the geodesic sphere, will house will house 9,522 of the extremely sensitive light-detectors called photomultiplier tubes. It is the design and placement of SNO's photomultiplier tubes that will enable the facility to fulfill its scientific mission.
SNO is a collaboration involving more than 60 scientists from a dozen laboratories and universities in the United States, Canada, and United Kingdom. Its mission is to answer some of the most perplexing questions about neutrinos, the particles emitted from the sun and from supernovae. Neutrinos are intriguing in part because they are so ghostlike. One could pass untouched through a wall of lead stretching from the earth to the moon.
Operating from a cavern more than a mile underground, SNO will be the first detector sensitive enough to measure not only ordinary electron neutrinos, but also the much more rare muon and tau neutrinos. This unprecedented sensitivity stems in part from a design that maximizes SNO's light-collecting capabilities.
The SNO detector, which is suspended in a vast pool of purified water, consists of the geodesic sphere, the photomultiplier tubes attached to it, and an acrylic vessel, inside the sphere, that is filled with 1,000 tons of heavy water (deuterium oxide or D2O). When neutrinos passing through the heavy water interact with deuterium nuclei, flashes of light, called Cerenkov radiation, are emitted. The photomultiplier tubes detect these light flashes and convert them into electronic signals that scientists can analyze.
"It is vital for the success of any neutrino experiment that as many photons as possible be detected," says Lesko. "Therefore, we had to squeeze as many photomultiplier tubes as possible onto the geodesic dome while maintaining an adequate layer of water shielding between the tubes and the cavity walls of the SNO site."
The need for densely packed photomultiplier tubes was complicated by the requirement that each photomultiplier tube be individually aimed and that the entire assembly be water-tight. The design demands were made even more difficult because the components making up the tubes came from three different countries (England, Canada, and Japan), and all had to be assembled, tested, disassembled, transported, and reassembled under cleanroom conditions.
To meet this multifaceted challenge, Gary Koehler, an LBL mechanical engineer associate and the SNO project's senior designer, "checkered" the surface of the sphere with 751 panels that come in five different shapes, each of which is built up from repeating patterns of hexagons. The result is a honeycomb pattern that covers 70 percent of the sphere with light-collecting devices.
"It is an incredible problem to take a sphere and break it up into repeating elements," says Lesko. "Complex 3D models on computers had to be developed to translate Gary's design into an engineering design."
A mounting system was also needed that would allow easy alignment of the panels and give SNO's spherical structure the flexibility to go from being empty to being flooded with water. Engineer Yoichi Kajiyama and mechanical designer Dave Beck assisted Koehler on this aspect of the design. Their solution was a three-point ball and socket mount that permits each panel to be aimed by hand according to positions pre-set by computer, and provides stability to the sphere whether it is empty or full. In addition, they connected the entire collection of panels through a series of corrosion-resistant plastic skirts that make the sphere 99.95 percent water tight.
Once the design was completed, the panel components were fabricated by two Bay Area firms, Precision Plastics of South San Francisco, and Stoesser Industries of Mountain View. Finished components were sent to LBL where they were tested for low-background radiation contamination, then assembled into panels and shipped to Canada.
The SNO experiment will last at least 10 years and is expected to record between 10 and 20 neutrinos each day. NSD physicist Eric Norman will lead LBL's participation in the research phase of the project.