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RTD info logoMagazine on European Research Special issue - May 2005   

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Title  Ice cube: Antarctica’s crystal ball

Buried deep within the East Antarctic ice sheet at the South Pole, a giant high-energy neutrino observatory due for completion in 2009 could provide scientists, including from Europe, with an unprecedented window to the Universe, as well as a means to answer some of the most fundamental questions of astrophysics and cosmology.

Artist's rendering of a Cerenkov blue light cone in the IceCube telescope. The optical sensors array allows IceCube to detect and reconstruct a muon’s path, and hence the path of the original neutrino.
Artist's rendering of a Cerenkov blue light cone in the IceCube telescope. The optical sensors array allows IceCube to detect and reconstruct a muon’s path, and hence the path of the original neutrino.
Instigated by the University of Wisconsin in Madison, and financed to the tune of 295 million dollars by the US National Science Foundation (NSF) in association with several European universities in Sweden, Belgium, Germany, the UK and the Netherlands, IceCube is by far the single most ambitious and expensive research project currently taking place in Antarctica.

Neutrinos are extremely small, virtually massless subatomic particles born of nuclear reactions. Whilst the Sun and other nearby phenomena produce low-energy neutrinos, high-energy neutrinos originate from such distant and supremely violent cosmic events as black holes, supernovas and the Big Bang.

Once born from such cosmic events, neutrinos travel at the speed of light and do not stop. Because they have virtually no mass, they only very rarely interact with other particles, allowing them to move in a straight line to the edge of the Universe, passing straight through stars, planets, vast magnetic fields and entire galaxies as if they didn’t exist. Trillions of neutrinos reach the Earth every nanosecond and, for astrophysicists, every one of these tiny particles is a potential messenger carrying information from its source of origin.

The problem for scientists, however, is that the very properties that allow neutrinos to carry this information also make them notoriously difficult to detect. Fortunately, on rare occasions, a high energy neutrino does collide with a molecule. The collision breaks the nucleus apart and the neutrino converts into another particle called a muon. Once created, a muon continues along the same path as the neutrino and can be recognized from the cone of blue light that follows it. Known as Cerenkov radiation, the cone is similar to the air waves behind a bullet as it travels.

However, in order to be able to detect such a collision by seeing the Cerenkov radiation behind the traveling muon, scientists must be able to monitor a huge volume of a substance that is both perfectly transparent and plunged into darkness. The creation of such a detector was first attempted in the early 1980s off the coast of Hawaii by lowering detectors into the deep ocean. Unfortunately, however, the experiment was plagued by the unpredictability of the weather and the instability of the sea.

It was not until a few years later that ice was thought of as the ideal solution. An expansion of the first-generation Antarctic Muon and Neutrino Detector (AMANDA), when completed IceCube will consist of 5 000 photomultiplier detectors buried across 1 km3 of the Antarctic ice sheet, at a depth of 1 400 to 2 400 meters beneath the South Pole: an environment which is not only plunged in darkness, but where the pressure is so great that all air bubbles and other disruptive elements have been squeezed out of the ice, giving it the clarity of crystal.

Once in place, the photomultiplier detectors act as powerful sensors which capture the streaks produced by a muon’s Cerenkov radiation, then amplify the faint signal by over a hundred million times and send it up to the surface where it gets picked up by computers. From this information, the scientists calculate which direction the initial neutrino came from and where in the sky they can find the cosmic event that created it. Once they have pinpointed the event, they can study it directly.

Window to the Universe
According to Francis Halzen, Professor at the University of Wisconsin and Chief Investigator on both the AMANDA and IceCube projects, the extraordinary thing about IceCube, however, is not so much the answers that it might provide to our existing questions about black holes, supernovas, the Big Bang, dark matter and the future of the Universe, but that in the past, every time astronomers have opened a new window to the cosmos, they have discovered things that they were not even looking for.

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