Giant Neutrino Telescope Takes Shape – Important Milestone for the International IceCube Project

A key first step has been taken in the construction of IceCube, a giant neutrino telescope spanning a volume of one cubic kilometer of ice at the South Pole: Working under harsh Antarctic conditions, an international team of scientists, engineers and technicians – among them scientists from the DESY research center – has successfully deployed a first critical part of the telescope, a string of 60 optical detectors, in a 2.4-kilometer-deep hole drilled into the Antarctic ice. Comprising a total of at least 70 such strings, the $272 million telescope will be the largest scientific instrument ever built. Designed to detect cosmic neutrinos – ghost-like high-energy particles from deep space – it will open up a new window to the sky and allow scientists to investigate the still-mysterious sources of cosmic rays.

IceCube is a joint international effort involving more than 20 institutions from the USA, Germany, Sweden, Belgium, the Netherlands, Great Britain, Japan and New Zealand. German contributors are the research center DESY with its location in Zeuthen close to Berlin, and universities in Berlin, Dortmund, Mainz and Wuppertal. The major part of the telescope and its construction is being financed by the National Science Foundation (NSF), with $30 million in support coming from European partners.

In a common effort, DESY and the German universities will deliver more than a quarter of the around 4200 optical modules that are to be deployed over the next six years. 1300 volleyball-sized glass spheres, each of them housing a highly sensitive light detector and sophisticated electronics, will be assembled and tested at DESY in Zeuthen. “The first IceCube string, which was successfully lowered into the ice in late January, already comprises eight optical modules produced in Zeuthen,” says physicists Rolf Nahnhauer, who is responsible for the production of the modules at DESY. Setting the string into the Antarctic ice required drilling a 2.4-kilometer-deep hole using a novel hot-water drill. “The detectors are then frozen in place in the ice,” explains Nahnhauer. “The first string is working perfectly, and data from the string and the surface tanks is now being transmitted to the Northern Hemisphere.”

The IceCube telescope uses the crystal-clear ice of the South Pole to look for the signatures of high-energy cosmic neutrinos, elusive particles produced in violent cosmic events such as 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 largely deflected by the magnetic fields of interstellar space, preventing scientists from tracing them back to their points of origin. Cosmic neutrinos, on the other hand, have the unique ability to travel billions of light years without being absorbed or deflected by stars, galaxies, and interstellar magnetic fields. This ghost-like property promises unprecedented information about the early universe and the very violent objects that populate the universe. However, it also makes detecting cosmic neutrinos extraordinarily difficult. Huge detectors are required to capture a few of them, and the experiments have to be buried deep below the surface to shield them against any unwanted radiation noise, such as light or normal cosmic radiation.

With its detector volume of one cubic kilometer, IceCube will dwarf existing neutrino detectors and become the largest particle detector ever built. It will be 30 times larger than its predecessor telescope AMANDA (“Antarctic Muon and Neutrino Detector Array”), around which it is being built. AMANDA in turn is already 30 times bigger than the famous Super-Kamiokande neutrino detector located in a Japanese mine. Since 1997, AMANDA has detected more than 4000 neutrinos. Up to now, however, the sky map of these neutrinos does not exhibit clear hints of neutrinos from extraterrestrial sources, so most of them are supposed to be generated by nuclear interactions in the atmosphere of the Earth. Using IceCube, scientists hope to finally track down the telltale signatures of neutrinos generated in distant cosmic events. “We have a shopping list of expectations on what we might see with IceCube, ranging from neutrinos from giant cosmic particle accelerators to signs of the dark matter that fills our universe,” says Christian Spiering from DESY, European spokesman of AMANDA and one of the leading members of the IceCube team. “But of course, as happened in fact in most cases when a new observational window to the cosmos was opened, we hope that we will find something new – something we cannot even imagine today.”

Establishing the project at the South Pole, setting surface equipment in place and testing the powerful new drill meant the IceCube team had only a two-week window during this year’s Antarctic summer to drill the first hole and deploy the first string. Next year, with about half a three-month Austral summer season of boring time, the goal will be to drill holes for and deploy ten or more IceCube strings.