Polar Ice May Yield Secrets of the Universe

A $272 million telescope buried more than a mile-and-a-half below the ice at the "bottom of the world" is expected to provide new information about some of the universe's biggest mysteries.

“IceCube,” which recently concluded its third year of construction at the South Pole, is a telescope for detecting mysterious high-energy subatomic particles called neutrinos. Traveling close to the speed of light, lacking electric charge and nearly massless, neutrinos can pass through ordinary matter -- such as planets -- almost undisturbed, making them extremely difficult to detect.

Neutrinos are produced by the decay of radioactive elements and elementary particles, such as in nuclear reactors, nuclear reactions in the sun, or when cosmic rays hit atoms. They are also formed during cataclysmic events, such as a star exploding or galaxies colliding. Scientists estimate that a majority of the neutrinos in existence were created around 15 billion years ago, soon after the birth of the universe.

Most neutrinos passing through the Earth emanate from the sun. It is estimated that more than 50 trillion solar electron neutrinos pass through the human body every second, although none may leave a trace behind over an entire human lifetime.

The IceCube telescope’s optical detectors are deployed in mile-and-a-half deep holes in the Antarctic ice. (Photo: James Roth/University of Delaware)

Scientists are hoping the high-energy neutrinos IceCube detects will deepen scientific understanding of cosmic rays, supersymmetry, weakly interacting massive particles (WIMPS), and other aspects of nuclear and particle physics, such as dark energy and dark matter.

The telescope is an international effort involving more than 20 institutions and the National Science Foundation (NSF). The lead institution for the IceCube project is the University of Wisconsin, working in collaboration with the University of Delaware (UD) and several other universities.

When completed in the next several years, the telescope will consist of more than 70 strings, each containing 60 optical detectors, frozen over a mile-and-a-half deep in the Antarctic ice. Polar ice is an ideal medium for detecting neutrinos because it is exceptionally pure, transparent and free of radioactivity.

But even in such an ideal medium, neutrinos are so hard to detect that scientists won't actually be able to detect the neutrinos themselves, but instead will have to wait for the occasional "direct hit" a neutrino makes with an ice atom. That collision creates a particle -- a "muon" -- that produces a small flash of blue light as it passes through the ultratransparent ice. The light flash is what the telescope's optical sensors will detect.

Working in the harsh polar environment is no easy feat. On average, it takes a specially designed, 5-million-W hot-water drill 48 hours and 4800 gallons of jet fuel to melt one of the holes for deploying a string of the telescope's sophisticated sensors. And approximately 200,000 gallons of melted ice is generated in the process.

Once a hole has been drilled, a a string of 60 optical detectors is lowered into it. It takes 11 hours to deploy a single string and several days for the water in the hole to freeze again. Then atop each of the deep ice strings, UD scientists and technicians are installing two 650-gallon tanks of water that each contain two optical detectors. The tanks are filled with water, and the freeze is controlled to produce perfectly clear ice, with no bubbles or cracks. Once frozen in the ice, it is estimated that the detectors will remain in place for the next 25,000 years.

The Digital Optical Module (DOM) of the IceCube telescope is an autonomous data collection unit. It measures the arrival time of every photon to an accuracy of better than 5 nanoseconds. It has a power consumption of 3 W and a dynamic range of 25,000 photoelectrons over 6.4 microseconds. (Image: NSF)

Each IceCube optical detector suspended in the ice is a computer and data acquisition system that has at its heart a photomultiplier tube, a device sensitive enough to detect a single photon of light. Each photomultiplier is enclosed in a transparent pressure sphere, called a Digital Optical Module (DOM). The DOM also contains a digitally controlled high-voltage supply to power the photomultiplier, an analog transient waveform digitizer and LED flashers.

The optical detectors capture the flash of light produced by the muon and stamp it with a precise time code. This information is then relayed to the surface to the IceCube Lab, where the path of the particle can be reconstructed and scientists can trace where it came from, perhaps an exploding star or a black hole.

Unfortunately, for every muon from a cosmic neutrino, IceCube detects a million more muons produced by cosmic rays in the atmosphere above the detector. To filter them out, IceCube takes advantage of the fact that neutrinos interact so weakly with matter. Because neutrinos are the only known particles that can pass through the earth unhindered, IceCube looks through the earth and to the northern skies, using the planet as a filter to select neutrinos.

The job of reconstructing the neutrino's path will fall to the telescope's surface array of detectors, called "IceTop." The construction of the array is being led by UD and involves 16 scientists and technicians from the physics and astronomy department and its affiliated research center, the Bartol Research Institute.

“The purpose of IceTop on the surface is to detect high-energy cosmic rays that interact in the atmosphere above IceCube,” said IceTop leader Thomas Gaisser, UD's Martin A. Pomerantz Chair of Physics and Astronomy. “By detecting the same events with IceTop and the deep detectors of IceCube, we expect to get new information about the origin of the most energetic particles in nature. At the same time, the surface detectors help IceCube reject the background of downward cosmic-ray events that obscures the signals of neutrinos coming up through the Earth from below.”

Scientists generally say that a neutrino coming from above and "down" into the detector most likely stems from an atmospheric shower, while a neutrino traveling "up" through the ice could have originated from a black hole, a gamma ray burst or a supernova.

“We are now analyzing data obtained during 2006 with the 32 IceTop tanks and nine IceCube strings -- a total of 604 digital optical modules,” Gaisser said. “We added 13 more strings and 20 more tanks to IceCube in the season that just ended, and the detector will be completed over the next three or four seasons. Meanwhile, we expect to publish the first scientific results of IceCube this year and next, and we hope for new discoveries even before the detector is complete."

The National Science Foundation is providing most of the project's construction cost, approximately $242 million. The other $30 million comes from project collaborators in Belgium, Germany, Japan and Sweden, as well as the US Department of Energy and the Wisconsin Alumni Research Foundation.

For more information, visit: www.icecube.wisc.edu