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| IceCube Lab SP, Antarctica. Credit: NSF/F. Descamps |
The final string of optical sensors was installed on Saturday, Dec. 18, in the IceCube Neutrino Observatory, a massive ice-bound telescope that fills a cubic kilometer of deep Antarctic ice. The main IceCube detector now contains 5,160 optical sensors on 86 strings embedded two kilometers below the National Science Foundation's Amundsen-Scott South Pole Station.
The construction, coordinated by the University of Wisconsin-Madison and underway since 2005, was completed on schedule and within budget.
For IceCube principal investigator and UW-Madison physics professor Francis Halzen, the completion of the detector is the realization of a longtime vision.
"Since the 1970s we have dreamed of building a detector of this size, and we have spent 20 years working toward IceCube," explains Halzen. "If the science to come brings half the excitement of completing this instrument, we have a bright future ahead."
From its vantage point at the end of the world, IceCube provides an innovative means to investigate the properties of fundamental particles that originate in some of the most spectacular phenomena in the universe. Its sophisticated optical sensors, frozen into some of the purest ice on Earth, record the rare collisions between the water molecules of the ice and neutrinos - tiny, nearly massless sub-atomic particles that pass undetected through most matter.
Some neutrinos come from the sun, while others come from cosmic rays interacting with the Earth's atmosphere, and dramatic astronomical sources such as exploding stars in the Milky Way and distant galaxies. Ever since neutrinos were discovered in 1956, scientists have hoped to decipher the information these astronomical messengers carry about distant cosmic events.
As the lead institution on the project, UW-Madison coordinated the work needed to design and build the complex and often unique components and software for the massive telescope.
"I have had the great fortune to watch the IceCube project develop over the years from idea to implementation. Completing a project of this scope is a tribute to the creativity and determination of our faculty, researchers, staff, and students," says Martin Cadwallader, UW-Madison vice chancellor for research and dean of the Graduate School. "And of course, this is really just the beginning for IceCube. It will be exciting to watch the discoveries that emerge under continued UW leadership."
The university also designed and built the Enhanced Hot Water Drill, which was assembled at the Physical Sciences Laboratory in Stoughton, Wis. The 4.8-megawatt hot-water drill is a unique machine capable of penetrating more than two kilometers into the ice in less than two days. Once each hole was drilled at the pole, deployment specialists attached optical sensors to cable strings and lowered them to depths between 1,450 and 2,450 meters. Sixty sensors line each of the 86 strings, with four more in surface tanks near each string that comprise the IceTop component of the observatory.
The largest part of the IceCube detector is the ice itself. At these depths, it is dark and optically ultratransparent, allowing the sensors to record the traces of particles from tiny flashes of blue light - called Cherenkov radiation - emitted after a high-energy neutrino strikes one of the water atoms in the ice.
Building IceCube in one of the most remote locations on Earth has come with a special set of challenges. All project personnel, equipment, food, and detector components had to be transported to Antarctica, then flown in ski-equipped C-130 cargo aircraft from McMurdo Station near the Antarctic coast to the South Pole, more than 800 air miles away.
Working only during the relatively warm and short Antarctic summer-from November through February, when the sun shines 24 hours a day-drill and deployment teams worked in round-the-clock shifts to maximize their short time on the ice each year.
The successful completion of the observatory is a milestone for international scientific cooperation on the southernmost continent. In addition to researchers at universities and research labs in the U.S., Belgium, Germany, and Sweden, the countries that funded the observatory, IceCube data are analyzed by the larger IceCube Collaboration, which includes researchers from Barbados, Canada, Japan, New Zealand, Switzerland, and the United Kingdom.
"IceCube is not only a magnificent observatory for fundamental astrophysical research, it is the kind of ambitious science that can only be attempted through the cooperation-the science diplomacy, if you will-of many nations working together in the finest traditions of Antarctic science toward a single goal," says Karl A. Erb, director of NSF's Office of Polar Programs.
Unlike other large-scale science projects, IceCube began collecting data while construction was still underway and has been recording particle events since early 2005. Each year as the detector grew, more and higher quality data made its way from the South Pole to UW-Madison and around the world.
"Even in this challenging phase of the project, we published results on the search for dark matter and found an intriguing patterns in the arrival directions of cosmic rays," says Halzen. "With the completion of IceCube, we are on our way to reaching a level of sensitivity that may allow us to see neutrinos from sources beyond the sun."
Source: Reprinted news release via University of Wisconsin-Madison
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