Where Do the Highest-Energy Cosmic Rays Come From? Probably Not from Gamma-Ray Bursts

The IceCube Collaboration, in which Berkeley Lab is a crucial contributor, has taken the first steps toward clearing up a cosmic mystery – and made the mystery more intriguing

The IceCube neutrino telescope encompasses a cubic kilometer of clear Antarctic ice under the South Pole, a volume seeded with an array of 5,160 sensitive digital optical modules (DOMs) that precisely track the direction and energy of speeding muons, massive cousins of the electron that are created when neutrinos collide with atoms in the ice. The IceCube Collaboration recently announced the results of an exhaustive search for high-energy neutrinos that would likely be produced if the violent extragalactic explosions known as gamma-ray bursts (GRBs) are the source of ultra-high-energy cosmic rays.

“According to a leading model, we would have expected to see 8.4 events corresponding to GRB production of neutrinos in the IceCube data used for this search,” says Spencer Klein of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), who is a long-time member of the IceCube Collaboration. “We didn’t see any, which indicates that GRBs are not the source of ultra-high-energy cosmic rays.”

“This result represents a coming-of-age of neutrino astronomy,” says Nathan Whitehorn from the University of Wisconsin-Madison, who led the recent GRB research with Peter Redl of the University of Maryland. “IceCube, while still under construction, was able to rule out 15 years of predictions and has begun to challenge one of only two major possibilities for the origin of the highest-energy cosmic rays, namely gamma-ray bursts and active galactic nuclei.”

Redl says, “While not finding a neutrino signal originating from GRBs was disappointing, this is the first neutrino astronomy result that is able to strongly constrain extra-galactic astrophysics models, and therefore marks the beginning of an exciting new era of neutrino astronomy.”

The IceCube Collaboration’s report on the search appears in the April 19, 2012, issue of the journal Nature.

Blazing fireballs and nature’s accelerators

Cosmic rays are energetic particles from deep in outer space – predominately protons, the bare nuclei of hydrogen atoms, plus some heavier atomic nuclei. Most probably acquire their energy when naturally accelerated by exploding stars. A few rare cosmic rays pack an astonishing wallop, however, with energies prodigiously greater than the highest ever attained by human-made accelerators like CERN’s Large Hadron Collider. Their sources are a mystery.

“Nature is capable of accelerating elementary particles to macroscopic energies,” says Francis Halzen, IceCube’s principal investigator and a professor of physics at the University of Wisconsin-Madison. “There are basically only two ideas on how she does this: in gravitationally driven particle flows near the supermassive black holes at the centers of active galaxies, and in the collapse of stars to a black hole, seen by astronomers as gamma ray bursts.”

Klein, the deputy director of Berkeley Lab’s Nuclear Science Division (NSD, explains that in active galactic nuclei (AGNs) “the black holes suck in matter and eject enormous particle jets, perpendicular to the galactic disk, which could act as strong linear accelerators.” Of gamma-ray bursts he says, “Some GRBs are thought to be collapses of supermassive stars – hypernova – while others are thought to be collisions of black holes with other black holes or neutron stars. Both types produce brief but intense blasts of radiation.”

The massive fireballs move away from the explosion at nearly the speed of light, releasing most of their energy as gamma rays. The fireballs that give rise to this radiation might also accelerate particles to very high energies through a jet mechanism similar to that in AGNs, although compressed into a much smaller volume.

Accelerated protons in a GRB’s jets should interact with the intense gamma-ray background and strong magnetic fields to produce neutrinos with energies about five percent of the proton energy, together with much higher-energy neutrinos near the end of the acceleration process.

Neutrinos come in three different types that change and mix as they travel to Earth; the total flux can be estimated from the muon neutrinos that IceCube concentrates on. The muons these neutrinos create can travel up to 10 kilometers through the Antarctic ice. Thus many neutrino interactions occur outside the actual dimensions of the IceCube array but are nevertheless visible to IceCube’s detectors, effectively enlarging the telescope’s aperture.

“The way we search for GRB neutrinos is that we build a huge detector and then we just watch and wait,” says Klein. “When it comes to detecting neutrinos, size really does matter.”

IceCube watches with its over 5,000 DOMs, digital optical modules conceived, designed, and proven by Berkeley Lab physicists and engineers, which detect the faint light from each passing muon. Scientists can rely on their remarkable dependability to wait as long as necessary. Almost no failures occurred after the DOMs were installed; 98 percent are working perfectly and another one percent are usable. Now frozen in the ice, they will never be seen again.

IceCube records a million times more muon tracks moving downward through the ice than upward, mainly debris from direct cosmic-ray hits on the surface or secondary products of cosmic-ray collisions with Earth’s atmosphere. Muons moving upward, however, signal neutrinos that have passed all the way through Earth. When the telescope is searching for bright neutrino sources in the northern sky, the planet makes a marvelous filter.

Zeroing in on gamma-ray bursts

A network of satellites circles the globe and reports almost 700 GRBs each year, which readily stand out from the cosmic background. They’re timed, their positions are triangulated, and the data are distributed by an international group of researchers. Some blaze for less than two seconds and others for a few minutes. Neutrinos they produce should arrive at IceCube during the burst or close to it.

“IceCube’s precision timing and charge resolution, plus its large size, allow it to precisely determine where a neutrino comes from – often to within one degree,” says Lisa Gerhardt of Berkeley Lab, whose research has focused on detecting ultra-high-energy neutrino interactions. Indeed, a GRB neutrino should send a muon track through the ice with an angular resolution of about one degree with respect to the GRB’s position in the sky.

IceCube researchers sifted through data on 307 GRBs from two periods in 2008 and 2009 when IceCube was still under construction, looking for records of muon trails coincident in time and space with GRBs. (Forty strings, with 60 DOMs each, had been installed by 2008, and 59 strings by 2009. The finished IceCube has 86 strings.) The fireball model predicted that when the expected flux from all the samples had been summed, at least 8.4 related muon events would be found within 10 degrees of a GRB during the seconds or minutes when it was blazing brightly.

“Different calculations of the neutrino flux from GRBs are based on slightly different assumptions about how the neutrinos are produced and on uncertainties such as how fast the fireball is moving toward us,” says Klein. “Among the published predictions, the lowest estimate of neutrino production is about a quarter of what the fireball model predicts. That’s barely consistent with our zero observations.”

Says Halzen, “After observing gamma-ray bursts for two years, we have not detected the telltale neutrinos for cosmic ray acceleration.”

If it’s likely that GRBs aren’t up to the task of accelerating cosmic rays to ultra-high-energies, what are the options? Klein points to a salient fact about natural accelerators: a small, rapidly spinning object must accelerate particles very rapidly; this requires an extremely energy-dense environment, and there are many ways the particles could lose energy during the acceleration process.

“But remember the other popular model of ultra-high-energy cosmic rays, active galactic nuclei,” says Klein. “GRBs are small, but AGNs are big – great big accelerators that may be able to accelerate particles to very high energies without significant loss.”

Are AGNs the real source of the highest-energy cosmic rays? IceCube has looked for neutrinos from active galactic nuclei, but as yet the data sets are not sensitive enough to set significant limits. For now, IceCube has nothing to say on the subject – beyond the fact that the fireball model of GRBs can’t meet the specs.

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“An absence of neutrinos associated with cosmic ray acceleration in gamma-ray bursts,” by R. Abbasi et al (the IceCube Collaboration), appears in the April 19, 2012, issue of Nature and is available online to subscribers at http://www.nature.com/nature/index.html. Collaboration members currently or formerly with Berkeley Lab include Keith Beattie, Kirill Filimonov, Lisa Gerhardt, Ariel Goldschmidt, Chang Hyon Ha, Spencer Klein, Howard Matis, Sandra Miarecki, David Nygren, Gerald Przybylski, Thorsten Stezelberger, and Robert Stokstad; Filimonov, Gerhardt, Ha, Klein, and Miarecki are also with the University of California at Berkeley.

The IceCube Collaboration includes over 260 researchers from 42 institutions in 11 countries and is supported by agencies and foundations in Belgium, Germany, Japan, and Sweden, with primary funding from the National Science Foundation and major support from the U.S. Department of Energy’s Office of Science. Visit the IceCube website at http://icecube.wisc.edu/, read the press release concerning this work at  http://icecube.wisc.edu/news/view/52, and access a selection of images at http://icecube.wisc.edu/~norris/nature_press/.

At Berkeley Lab, DOE’s Office of Science supports participation in IceCube primarily through the National Energy Research Scientific Computing Center (NERSC). Visit http://www.nersc.gov/.

DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit www.lbl.gov.

IceCube Neutrino Observatory Provides New Insights Into Origin of Cosmic Rays

Analysis of data from the IceCube Neutrino Observatory, a massive detector deployed in deep ice at the U.S. Amundsen-Scott South Pole Station in Antarctica at the geographic South Pole, recently provided new insight into one of the most enduring mysteries in physics, the production of cosmic rays.

Cosmic rays were discovered 100 years ago, but only now are scientists homing in on how the highest energy cosmic rays are produced.

Cosmic rays are electrically charged particles, such as protons, that strike Earth from all directions with energies up to one hundred million times higher than those created in man-made accelerators.

The intense conditions needed to generate such energetic particles have focused physicists' interest on two potential sources: the massive black holes at the centers of active galaxies and exploding fireballs observed by astronomers called gamma-ray bursts or GRBs.

"Although we have not discovered where cosmic rays come from, we have taken a major step towards ruling out one of the leading predictions," said Francis Halzen, a physicist at the University of Wisconsin-Madison and the IceCube principal investigator.

In a paper published in the April 19 issue of the journal Nature, the IceCube collaboration describes a search for neutrinos emitted from 300 gamma ray bursts observed between May 2008 and April 2010 in coincidence with the SWIFT and Fermi satellites.

Surprisingly, the scientists found no neutrinos--a result that contradicts 15 years of predictions and challenges the theory that gamma-ray bursts produce the highest energy cosmic rays.

"The result of this neutrino search is significant because for the first time we have an instrument with sufficient sensitivity to open a new window on cosmic ray production and the interior processes of GRBs," said Greg Sullivan, a physicist at the University of Maryland and IceCube spokesman.

"The unexpected absence of neutrinos from GRBs has forced a re-evaluation of the theory for production of cosmic rays and neutrinos in a GRB fireball and possibly the theory that high-energy cosmic rays are generated in fireballs," he said.

IceCube observes neutrinos by detecting the faint blue light produced in neutrino interactions in ice. Neutrinos are of a ghostly nature; they can easily travel through people, walls, or the planet Earth. To compensate for the antisocial nature of neutrinos and detect their rare interactions, IceCube is built on an enormous scale. One cubic kilometer of glacial ice, enough to fit the great pyramid of Giza 400 times, is instrumented with 5,160 optical sensors embedded up to 2.5 kilometers deep in the ice.

GRBs, the universe's most powerful explosions, are usually first observed by satellites using X-rays and/or gamma rays. GRBs are seen about once per day, and are so bright that they can be seen from half way across the visible Universe. The explosions usually last only a few seconds, and during this brief time they can outshine everything else in the universe.

The IceCube Neutrino Observatory was built under a National Science Foundation (NSF) Major Research Equipment and Facilities Construction grant, with assistance from partner funding agencies around the world.

NSF continues to support the project with a Maintenance and Operations grant co-funded by the Division of Antarctic Sciences and the Division of Physics. IceCube construction was finished in December 2010. A collaboration of 250 physicists and engineers from the Unites States, Germany, Sweden, Belgium, Switzerland, Japan, Canada, New Zealand, Australia and Barbados operate the observatory.

"Building the IceCube Neutrino Observatory at the geographic South Pole was a major effort made possible through many collaborating institutions and the U.S. Antarctic Program," said Scott Borg, division director for Antarctic Sciences in NSF's Office of Polar Programs. "The IceCube Collaboration has been busy analyzing data and the finding published in Nature is an early and significant, result. We are pleased with this achievement but we also anticipate many more important discoveries to follow."

NSF, an independent U.S. government agency, manages the U.S. Antarctic Program, through which it coordinates all U.S. scientific research on the southernmost continent and aboard ships in the Southern Ocean as well as related logistics support.

Improved theoretical understanding and continued data collection from the complete and fully calibrated IceCube detector will help scientists better uncover the mystery of cosmic ray production.

For more information, visit the IceCube Neutrino Observatory Web site.

 -NSF-

Into the Ice: Completing the IceCube Neutrino Observatory

On Saturday, December 18, the IceCube Neutrino Observatory sank the last of 86 strings of sensitive photodetectors to a depth of almost two and a half kilometers in the ice at the South Pole, marking completion of the huge neutrino telescope.

“With the completion of IceCube, the 1970s dream of building a kilometer-scale neutrino detector has finally become a reality,” says Francis Halzen, a professor of physics at the University of Wisconsin-Madison and the IceCube Collaboration’s principal investigator. “Finally science can start with a stable instrument that already yields neutrinos with unprecedented energy and statistics.”

Members of the IceCube Collaboration celebrated the event at the South Pole and around the world, at about 40 affiliated institutions located in the U.S., Germany, Belgium, Sweden, Barbados, Canada, Japan, New Zealand, Switzerland, and the United Kingdom. IceCube is supported largely by the National Science Foundation and led by the University of Wisconsin-Madison. The U.S. Department of Energy’s Lawrence Berkeley National Laboratory has been a major contributor to the design and construction of its key components.

Under construction since 2004, IceCube encloses a cubic kilometer of clear ice, beginning one and a half kilometers beneath the surface and extending downward another kilometer. The telescope has to be this big because neutrino collisions with matter are exceedingly rare: out of uncounted trillions of neutrinos constantly passing through the ice, IceCube will observe just a few hundred a day.

Seeing them at all is only possible because when neutrinos collide with the nuclei of oxygen atoms in the ice, they turn into energetic charged particles called muons, moving in the same direction. Because these muons (and other debris from the collision) are moving faster than light can travel through ice, they radiate a shock wave of blue Cherenkov radiation visible to IceCube’s photodetectors.

Catching neutrinos with Digital Optical Modules

Sixty basketball-sized detectors are mounted on each IceCube string. Called Digital Optical Modules (DOMs), their optical parts are photomultiplier tubes (PMTs) that detect and amplify Cherenkov radiation from passing muons. Electronics convert the PMT signals to digital form. The PMTs and circuit boards are housed together in transparent glass pressure vessels.

Digital Optical Modules were originally developed at Berkeley Lab. In 1997 two prototypes were built by the Jet Propulsion Laboratory and installed in the AMANDA array (IceCube’s predecessor) at the South Pole. The University of Wisconsin, the lead institution in the AMANDA Collaboration, pioneered the hot-water drilling techniques that made deep-ice strings of sensors practical.

Berkeley Lab and the University of Wisconsin worked together with other institutions to design and build String 18, a single string of 40 prototype DOMs installed at AMANDA in 2000. Robert Stokstad and David Nygren of Berkeley Lab’s Nuclear Science and Physics Divisions developed and championed the idea of digital optical modules and led the String 18 operation, with Physics Division engineer Jerry Przybylski doing much of the hardware work. Their superb performance resulted in the DOM technology being selected for IceCube.

Berkeley Lab’s built-in electronics have performed with astonishing dependability. Ninety-eight percent of IceCube’s over 5,000 DOMs are working perfectly, and another one percent are usable – reassuring numbers, given that the DOMs now frozen in the ice will never be seen again.

“The DOMs are no more accessible than a space satellite in high orbit,” says Spencer Klein, Berkeley Lab’s group leader for neutrino astronomy, “but they’re a lot more reliable and extremely robust. They’re also performing far above specifications, which called for them to be able to resolve the timing of Cherenkov radiation flashes within five nanoseconds” – five billionths of a second. “Instead, their timing resolution is about two nanoseconds.”

To achieve this astonishing resolution the DOMs use integrated circuits, designed at Berkeley Lab, to sample the PMT signals 300 million times a second and convert each sample to a digital value. Commercial digitizer chips couldn’t be used because of power limitations, since the fuel to power IceCube (and all work at the South Pole) must be flown in from 900 miles away on ski-equipped planes, and even the allotted five watts of power per DOM requires 10 planeloads of fuel a year.

In a lab on the surface of the ice, signals from DOMs on many different strings are combined into a single data stream, which is analyzed to determine the direction and energy of the neutrino events that left their tracks. For separating neutrino signals from far more copious background events, the most important discrimination is whether the signal comes from overhead or below. Muons moving upward through IceCube must come from neutrinos that have passed through the Earth. Downward-going muons produced when cosmic rays hit the ice are a million times more numerous.

A smaller background comes from neutrinos produced when cosmic rays hit Earth’s atmosphere. To avoid this background, experimenters look for a cluster of neutrino events coming from a single direction in space, or for an excess of very energetic neutrinos; most atmospheric neutrinos are of lower energy.

A constant stream of results from the neutrino sky

Although IceCube has only now been fully deployed, scientific results have flowed from the project throughout its construction, including the overlap with AMANDA’s research.

Significant achievements include a map of the entire neutrino sky, made from a single year of data using only half the full array, including tens of thousands of upward-going and downward-going neutrino events. The origins of these events are distributed randomly across the sky; the map reveals no hot spots but is still the most sensitive search for point sources of neutrinos yet made.

A more sensitive search for diffuse extra-terrestrial neutrinos looks at different “flavors” (types) of neutrinos, those associated with electrons, muons, or tau particles. When electron-type neutrinos interact in the Antarctic ice, they produce a compact shower of particles – almost a point source, when seen in IceCube. One advantage of looking for electron neutrinos is that the background from atmospheric neutrinos is much lower. Also, IceCube can measure their energy with precision. This kind of search calls for data-analysis techniques very different from those used elsewhere in IceCube. Joanna Kiryluk of Berkeley Lab’s Nuclear Science Division has done such a search, and, not finding a clear signal, has set a limit on the number of cosmic electron neutrinos that can exist.

IceCube has also measured the energy spectrum of the muon-type neutrinos it has observed, from those with an energy of 100 billion electron volts up to 400 trillion electron volts. So far, the spectrum is consistent with neutrinos from cosmic-rays in the Earth’s atmosphere. If an additional, unexpected component of very high-energy neutrinos were to be found, it might signal neutrinos of extraterrestrial origin.

IceCube has searched for neutrinos emitted at the same time as gamma-ray bursts – the most violent events in the universe, probably caused when a black hole collides with a neutron star or when a star 100 times as massive as our Sun collapses – and while no gamma-ray-burst neutrino source has been positively identified, the search has placed limits on the possible theoretical models of neutrino emission from these events.

Neutrinos as clues to accumulated dark matter

One of the most interesting IceCube results so far is a limit on the possible accumulation of dark matter particles in the Sun. Based on data taken with the first 22 strings of DOMs, IceCube set a limit on the number of high-energy neutrinos that could be coming from the Sun. This in turn sets limits on the possible accumulation and subsequent annihilation of dark matter inside the Sun.

Dark matter is dark because it doesn’t interact electromagnetically – the only force it must feel is gravity. Although many kinds of dark matter have been proposed, none have yet been found; particles called neutralinos are one likely possibility. As the lightest of the weakly interacting massive particles (so-called WIMPs), neutralinos would feel the weak nuclear force in addition to gravity, yet matter would be nearly transparent to them (as it would be to all proposed dark matter particles). Thus they could collect inside massive objects such as the Earth and the Sun. And because they are their own antiparticles, when they collide they annihilate and emit energetic neutrinos. But IceCube has seen no high-energy neutrinos coming from the Sun, which places stringent limits on the possible accumulation of neutralinos and some other kinds of dark matter there.

In 2009 the IceCube collaboration, originally designed for 80 strings of DOMs, decided to add an additional six strings to the center of the array. On these strings the DOMs are placed at seven-meter intervals, with the strings about 70 meters apart. This denser ”infill” array, called DeepCore, will be able to detect neutrinos with energies below the reach of the main array and will extend IceCube’s sensitivity to other kinds of dark matter. The new array will also allow researchers to study neutrino oscillation, measuring how neutrinos change flavors as they travel through the Earth.

IceCube is a unique observatory of the neutrino sky. It will study neutrinos created high in the atmosphere and search for neutrinos created in gamma-ray bursts, active galactic nuclei, the interaction of cosmic rays with the universe’s background radiation, and the annihilation of dark matter particles.

Says Spencer Klein, “These physics results are just a taste of the things that we can expect from IceCube now that it is complete. After more than a decade of work, Berkeley Lab researchers can now fully enjoy the fruits of their labor, and are looking forward to a bountiful physics harvest.”

Lawrence Berkeley National Laboratory is a U.S. Department of Energy national laboratory managed by the University of California for the DOE Office of Science. Berkeley Lab provides solutions to the world’s most urgent scientific challenges, including sustainable energy, climate change, human health, and a better understanding of matter and force in the universe. It is a world leader in improving our lives through team science, advanced computing, and innovative technology.

NSF, University of Wisconsin-Madison Complete Construction of the World's Largest Neutrino Observatory

Antarctica's IceCube is among the most ambitious scientific construction projects ever attempted

Culminating a decade of planning, innovation and testing, construction of the world's largest neutrino observatory, installed in the ice of the Antarctic plateau at the geographic South Pole, was successfully completed December 18, 2010, New Zealand time.

The last of 86 holes had been drilled and a total of 5,160 optical sensors are now installed to form the main detector--a cubic kilometer of instrumented ice--of the IceCube Neutrino Observatory, located at the National Science Foundation's Amundsen-Scott South Pole Station.

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.

In the deep, dark, stillness of the Antarctic ice, IceCube records the rare collisions of neutrinos--elusive sub-atomic particles--with the atomic nuclei of the water molecules of the ice. 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 other distant galaxies. Trillions of neutrinos stream through the human body at any given moment, but they rarely interact with regular matter, and researchers want to know more about them and where they come from.

The size of the observatory is important because it increases the number of potential collisions that can be observed, making neutrino astrophysics a reality.

The completion of construction brings to a culmination one of the most ambitious and complex multinational scientific projects ever attempted. The National Science Foundation (NSF) contributed $242 million toward the total project cost of $279 million. NSF is the manager of the United States Antarctic Program, which coordinates all U.S. research on the southernmost continent.

The University of Wisconsin-Madison, as the lead U.S. institution for the project, was funded by NSF to manage and coordinate the work needed to design and build the complex and often unique components and software for the project.

The university designed and built the Enhanced Hot Water Drill, which was assembled at the physical sciences lab in Stoughton, Wisconsin. The 4.8- megawatt hot-water drill is a unique machine that can penetrate more than two kilometers into the ice in less than two days.

After the hot water drill bores cleanly through the ice sheet, deployment specialists attach optical sensors to cable strings and lower them to depths between 1,450 and 2,450 meters. The ice itself at these depths is very dark and optically ultratransparent.

Each string has 60 sensors at depth and the 86 strings make up the main IceCube Detector. In addition, four more sensors sit on the top of the ice above each string, forming the IceTop array. The IceTop array combined with the IceCube detector form the IceCube Observatory, whose sensors record the neutrino interactions.

The successful completion of the observatory is also 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 also 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," said Karl A. Erb, director of NSF's Office of Polar Programs.

"To complete such an ambitious project, both on schedule and within budget, is a tribute to the fine work of the University of Wisconsin-Madison and its partner institutions, but it's also a reflection of the excellence of the personnel and infrastructure of the U.S. Antarctic Program," he added. "Science like IceCube is done in Antarctica because it is a unique global laboratory. I am very gratified that the U.S. Antarctic Program is equal to the challenge of supporting such a project."

IceCube is among the most ambitious and complex scientific construction projects ever attempted.

To build the observatory, all project personnel, equipment, food, and detector components had to be transported to Antarctica from various places around the globe. Everything then had to be 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 shifts to maximize their short time on the ice each year.

An international team of scientists, engineers and computer specialists have been working on development and construction of the detector since November 1999, when the first proposal was submitted to NSF and partners in Belgium, Germany and Sweden.

In the 1950's, Nobelist in physics Frederick Reines and other particle physicists realized that neutrinos could be used as astronomical messengers. Unlike light, neutrinos pass through most matter, making them a unique probe into the most violent processes in the universe involving neutron stars and black holes. The neutrinos IceCube studies have energies far exceeding those produced by manmade accelerators.

Unlike many large-scale science projects, IceCube began recording data before construction was complete. Each year since 2005 following the first deployment season, the new configuration of sensor strings began taking data. Each year as the detector grew, more and better data made its way from the South Pole to the data warehouses in the University of Wisconsin and around the world.

"Even in this challenging phase of the project, we published results on the search for dark matter and found intriguing patterns in the arrival directions of cosmic rays. Already, IceCube has extended the measurements of the atmospheric neutrino beam to energies in excess of 100 TeV," said Francis Halzen, principal investigator for the project. "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."

Funding agencies outside of the U.S. that contributed to the construction of the IceCube Observatory are:

•in Belgium: Fonds de la Recherche Scientifique (FRS-FNRS) and Fonds voor Wetenschappelijk Onderzoek (FWO);

•in Germany: Federal Ministry of Education and Research (BMBF) and Deutsches Elektronen-Synchrotron Project (PD-DESY): and

•in Sweden: Swedish Research Council (VR), and the Knut and Alice Wallenburg Foundation.

-NSF-