BERKELEY, CA -- A year after the ribbon was cut on the huge (18 meters in diameter) stainless steel geodesic sphere that is to be a major component of the Sudbury Neutrino Observatory (SNO), Lawrence Berkeley Laboratory researchers have completed their contributions to the construction phase of the project.
Scientists, engineers, and designers in LBL's Nuclear Science Division and its Engineering Division, under the leadership of physicist Kevin Lesko, have completed work on the panel arrays that, when attached to the sphere which was subsequently shipped to Sudbury, Canada, will house SNO's photomultiplier tubes. It is the design and placement of these photomultiplier tubes, 9,522 in all, that will enable SNO 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 the United Kingdom. Its mission is to answer some of the most perplexing questions about neutrinos, the particles emitted from the sun and from supernovae that 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 by the requirement that the entire assembly be water-tight. That the components making up the tubes would be coming from three different countries (England, Canada, and Japan), and that all the components would have to be assembled, tested, disassembled, transported, and reassembled under cleanroom conditions made the design demands even more difficult.
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."
Also needed was a mounting system that would allow easy alignment of the panels and give SNO's spherical structure the flexibility it needs to go from being empty to being flooded with water. Assisting Koehler on this aspect of the design were engineer Yoichi Kajiyama and mechanical designer Dave Beck. 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 to 20 neutrinos each day. Leading LBL's participation in the research phase of the project will be NSD physicist Eric Norman.
LBL is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.