Largest neutrino telescope in the world completed

Completion of the IceCube Neutrino Observatory at the South Pole caps one of the most ambitious projects in the history of science / Researchers from Mainz University have been taking part in the project for more than 10 years

20.12.2010

The IceCube neutrino telescope was completed on 18 December 2010, after almost six years of construction and a decade of preparation. The largest particle detector in the world consists of a cubic kilometer of ice, interspersed with highly sensitive light sensors. They capture the trails of neutrinos arriving from space and use these "heavenly messengers" to gather information on distant galaxies. Neutrinos are often called ‘ghost’ particles since they can pass unnoticed through large quantities of matter. Proving their existence therefore requires gigantic detectors.

IceCube is located within the deep ice under the Amundsen-Scott Station located at the geographic South Pole. The project is run by an international consortium led by the U.S. National Science Foundation (NSF). The NSF has also assumed the majority of the construction costs amounting to $279 million. IceCube consists of 86 cable strings, to each of which are attached 60 glass spheres at depths of 1.45 to 2.45 kilometers. The spheres encircle highly sensitive light sensors which capture the faint traces of the bluish radiation produced in neutrino reactions. A quarter of the total of more than 5,000 optical sensors was provided by German research groups; these were assembled and tested at the German Electron Synchrotron (DESY) in Zeuthen. The strings are arranged at 125-meter intervals, meaning light sensors are provided over a total volume of ice of one cubic kilometer.

The South Pole is an ideal location for this project because of its crystal-clear deep ice and the excellent infrastructure provided by the Amundsen-Scott Station. Equipment and personnel are flown in from McMurdo, the American station at the edge of the Antarctic, on transport planes fitted with a ski undercarriage. Installation work is carried out in the Antarctic summer that lasts from November to February, when the sun shines 24 hours a day and temperatures rise to a 'bearable' -30° C.

The holes into which the spheres are lowered are melted in the ice using water heated to a temperature of 80° C. After a string with optical sensors is lowered into a hole, it freezes over again within a few days. The signals measured by all sensors are transmitted to the central station on the surface, then processed there and sent via satellite to research institutes in the northern hemisphere.

Neutrinos are ghost-like elementary particles, whose existence was predicted in 1930, but only proven in 1956. Billions of these particles rain down upon every square centimeter of the Earth's surface every second, and most of them pass through the Earth without ever colliding with a single atom. Neutrinos are hard to detect since they rarely interact with other matter, but for that very reason they can escape from compact cosmic regions like the interior of our sun much more easily than light, and then continue their flight through outer space undisturbed. This makes them unique cosmic messengers. Neutrinos have already been detected that emanated from a supernova registered in 1987 and from our sun, and these provided convincing confirmation of theories on supernovas and on fusion reactions in the sun's interior. The 2002 Nobel Prize in Physics was awarded for the related work. IceCube searches for neutrinos that originate from sources that are much more remote than our sun and that are located thousands to billions of light years away. These sources include black holes, which can be found in the center of galaxies and act like maelstroms, drawing matter into themselves, and the mysterious dark matter that is postulated to fill our universe but has yet to be found.

It was possible to carry out research already during the construction of IceCube, even before the observatory was finally completed. Data has been collected with each expanding string configuration since 2005 (2005: 1, 2006: 9, 2007: 22, 2008: 40, 2009: 59, 2010: 79 strings installed). As the detector grew year by year, the data it delivered became more detailed and some initial results are already available.

Many undergraduates and doctoral candidates of Johannes Gutenberg University Mainz (JGU), Germany have been taking part in the preparation, construction and evaluation of the project for more than ten years. Among other things, Mainz University is responsible for a system that records data from supernova explosions. To date, 12 participants from Mainz have been involved in the construction and maintenance of the IceCube experiment; two additional postgraduates will be flying out to the South Pole in late December 2010. The project group is being led by Professor Dr. Lutz Köpke and Professor Dr. Heinz-Georg Sander of the JGU's Institute of Physics. Professor Lutz Köpke who, inter alia, has been coordinating the project’s data analyses, is excited by future prospects: "IceCube has now reached full sensitivity, and we are all hoping a supernova will explode in our Milky Way and that we'll discover a distant source of high-energy neutrinos. The preconditions for this are good: We have already registered almost 100,000 neutrinos that were produced in the Earth's atmosphere, including some with a mass energy of up to 400,000 billion electron volt. That's about one thousand times greater than the energy of neutrinos that have been generated in accelerators on Earth."

The IceCube team is made up of 260 scientists from 36 research institutes in eight countries. In addition to researchers from the United States, Germany, Sweden and Belgium (the countries which financed IceCube), scientists from the United Kingdom, Japan, New Zealand, Barbados, and Switzerland are also involved in the analysis of the data acquired. The institute leading the project is the University of Wisconsin in Madison, USA. Participants from Germany include the German Electron Synchrotron (DESY), RWTH Aachen University,  Humboldt University Berlin, the University of Bochum, the University of Bonn, the Technical University of Dortmund, Johannes Gutenberg University Mainz, the University of Wuppertal, and the Max Planck Institute for Nuclear Physics in Heidelberg. In addition to providing one-quarter of the optical modules, the German participants in the project have also made available a significant proportion of the receiver electronics on the ice surface.

The German contribution of some €18 million was funded from contributions made by the German Federal Ministry of Education and Research (BMBF), the Helmholtz Association, the German Research Foundation (DFG), and from the financial resources of the participating universities.