Two University of California, Berkeley, scientists have just returned from the South Pole, where they and an international team of scientists, engineers and drillers have set in place the first critical elements of a massive neutrino telescope called IceCube.
The group successfully deployed the first of 70 strings of optical detectors in a 1.5-mile-deep hole drilled into the glacial ice at the South Pole. Each string is a chain of 60 optical detectors designed to sample phantom-like high-energy particles from deep space. When completed in the year 2010, the $272 million IceCube telescope will be the world's largest scientific instrument, consisting of an instrumented volume of one cubic kilometer of ice.
"This was our pilot year," said Ryan Bay, a UC Berkeley post-doctoral fellow who left Antarctica's South Pole Station on Jan. 30.
"In a year, we'll be back to get the installation and calibration of 10 new strings in full swing," he said, noting that all the work -- drilling 10 holes and deploying 10 instrument strings -- must be done during the six weeks of full Austral summer.
Bay and post-doctoral colleague Kurt Woschnagg work with UC Berkeley physics professor Buford Price and were part of a large team that endured harsh Antarctic conditions to melt a cylindrical hole 2,450 meters deep and drop the instrument string in before it froze over 30 hours later.
The first IceCube string, together with the nearby AMANDA neutrino observatory that will be a part of IceCube, are now successfully tracking the paths of high-energy neutrinos from throughout the universe into and through the earth, and are transmitting the data to the Northern Hemisphere.
The telescope and its construction are being financed by the National Science Foundation (NSF), which will provide $242 million. An additional $30 million in support will come from foreign partners.
The UC Berkeley researchers also used the string deployment to measure the amount of dust in the many layers of Antarctic ice through which the hole was cut. They attached an instrument they built, called a Dust Logger, to the end of the instrument string and lowered it to the very bottom of the hole, recording along the way the changes in the amount of dust in the ice. The data will be of use not only to IceCube, but also to Price, Bay and Woschnagg's study of the recent history of climate change.
"By correcting for the dust that would otherwise blur their trajectories, IceCube can map the directions of sources of high-energy neutrinos to better than one degree and thus locate some of the most explosive objects in the universe," Price said. "Just as interesting to climatologists, however, is the extremely high quality of the dust data, which serves as a proxy for changes in the ancient Earth's climate."
"IceCube is a neat opportunity to marry the two projects, because the data from our probe will be useful in both," Bay said.
Building the telescope requires drilling at least 70 one-and-one-half-mile-deep holes in the Antarctic ice using a novel hotwater drill, and then lowering long strings of volleyball-sized optical detectors -- 4,200 in all -- into the holes where they will be frozen in place. When completed, the telescope will be capable of capturing information-laden, high-energy particles from some of the most distant and violent events in the universe. It promises a new window to the heavens and may be astronomy's best bet to resolve the century-old quest to identify the sources of cosmic rays.
The IceCube telescope will look for the telltale signatures of high-energy cosmic neutrinos, ghostlike particles produced in violent cosmic events -- colliding galaxies, distant black holes, quasars and other phenomena occurring at the very margins of the universe. Cosmic rays, which are composed of protons, are
thought to be generated by these same events. But protons are bent by the magnetic fields of interstellar space, preventing scientists from following them back to their points of origin.
Cosmic neutrinos, on the other hand, have the unique ability to travel cosmological distances without being absorbed or deflected by the stars, galaxies and interstellar magnetic fields that permeate space. Their ability to skip through matter without missing a beat promises unedited information about the early universe and the very violent objects that populate deep space.
But that same phantom-like property -- the ability to travel billions of light years and pass unhindered through planets, stars and galaxies -- makes detecting cosmic neutrinos extraordinarily difficult.
"Neutrinos travel like bullets through a rainstorm," explained Francis Halzen, a University of Wisconsin-Madison (UW-Madison) professor of physics and principal investigator for the project. "Immense instruments are required to find neutrinos in sufficient numbers to trace their origin."
The optical modules that make up the detector act like light bulbs in reverse. They are able to sense the fleeting flash of light created when neutrinos passing through the Earth from the Northern Hemisphere occasionally collide with other atoms. The subatomic wreck creates another particle called a muon. The muon leaves a trail of blue light in its wake that allows scientists to trace its direction back to a point of origin, potentially identifying the cosmic accelerators -- black holes or crashing galaxies, for example -- that produce the high-energy neutrinos.
The Dust Logger is a critical contribution from UC Berkeley to the project, improving its ability to pinpoint the origins of neutrinos. Bay, with assistance from graduate student Nathan Bramall, designed the instrument to make optical measurements of the ancient snow, which has been compacted into the Antarctic ice pack into which IceCube will fit. The logger, built in the UC Berkeley physics department shop, consists of a horizontally directed laser beam that bounced light off tiny dust particles in the ice as the string of optical modules was lowered down the water-filled hole in the ice.
By recording the amount of light that re-entered the hole after bouncing around in the ice, the instrument captured a detailed catalog, inch by inch, of the amount of atmospheric dust as a function of depth -- and, therefore, age -- throughout the volume of ice containing the first IceCube string. The data, still to be analyzed, was brought back from the Pole by Bay.
Bay and Price are just as interested in the record of volcanic ash and dust frozen into the ice, however. At each depth in the ice, the dust concentration tells the temperature at the corresponding epoch in the past. Furthermore, the Dust Logger records the often centimeter-thick layers of volcanic ash that were transported over great distances and deposited onto the ice from eruptions that may themselves have triggered or been related to climate change.
"The ability of the Dust Logger to study climate and volcanism as well as to sharpen the images of neutrino sources exemplifies the unexpected spin-offs of basic research," Price noted.
"Our other studies in Greenland ice have shown us a strong correlation between volcanic ash deposited and climate change over the past 100,000 years," Bay said. The Antarctic study will provide another look back to about 80,000 years ago.
The telescope now under construction at the South Pole, which is being built around a much smaller neutrino telescope known as AMANDA, for the Antarctic Muon and Neutrino Detector Array, is an international effort involving more than 20 institutions. The project is funded by the U.S. National Science Foundation, with significant contributions from Germany, Sweden, Belgium, Japan, New Zealand, the Netherlands, and the Wisconsin Alumni Research Foundation.
In the United States, the project involves scientists from UW-Madison, UC Berkeley, Lawrence Berkeley National Laboratory, University of Maryland, Pennsylvania State University, University of Wisconsin-River Falls, University of Delaware, University of Kansas, University of Alabama, Clark Atlanta University, Southern University and A&M College, and Princeton University's Institute for Advanced Study.
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