The aim of AMANDA and IceCube

The AMANDA telescope for high-energy cosmic neutrinos was constructed between 1995 and 2000 deep down in the ice sheet at the Amundsen-Scott base at the South Pole, Antarctica. The scientific goals are, among others, to use the neutrino particles to investigate the question of the “dark matter” of the Universe and to search for the sources of the highest energy cosmic rays. The neutrino particles are extremely penetrating and interact only very rarely with matter. They are expected to be produced through different violent processes in the Universe, and the possibility to detect high-energy neutrino sources will open a new window in the study of the Cosmos. In order to compensate for the extremely low probability for the neutrino to interact with matter one needs very large detectors. The AMANDA detector is sensitive to the emitted Cherenkov light from muons created by neutrino interactions deep in the ice.

In detectors of this type it is necessary to have a very transparent material, such as clear ice, in order to obtain efficient light propagation. The ice sheet at the South Pole is 2 900 m deep and extremely transparent at large depths (Askebjer et al., 1997, Askebjer et al., 1998). The detector consists of 677 optical modules deployed in 19 holes in the ice. The holes were made using a hot water drilling technique and the modules were frozen in during a period of about one week. The optical modules are photomultipliers contained in pressure vessels (glass spheres) deployed at depths between 1 200 m and 2 300 m. The central part of the detector, with the highest density of optical modules, is between 1 500 m and 2 000 m below the surface. The diameter of the detector is 200 m. The photomultipliers are sensitive to single photons in the wavelength range from 330 nanometres (nm) to 600 nm and have a diameter of 20 cm. The signal from each photomultiplier is transmitted via cables up to the surface and read by on-line computers. The American Polar Ice Core Office (PICO) has performed the hot water drilling with help of Swedish drillers from the Swedish Polar Research Secretariat.

The AMANDA detector has been fully operational and taking data since February 2000. The completed detector is named AMANDA-II in order to distinguish it from the partially equipped stages AMANDA-B4 and AMANDA-B10. The detector is modular and it was possible to start data taking with only a fraction of the total number of strings. In this way data from the 4-string (B4) and 10-string (B10) detectors have been analyzed and published.

The AMANDA neutrino telescope is the largest in the world, but due to the extremely low expected rate of neutrinos it is probably still too small. The construction of a much larger telescope, the new IceCube Neutrino Observatory, started at the South Pole in January 2005. The complete observatory will consist of 4 800 optical modules deployed between 1 450 and 2 450 m depth in 80 holes covering an instrumented volume of about 1 km³. On top of the neutrino telescope an air shower array, IceTop, will detect air showers from cosmic rays interacting in the atmosphere (figure). The combination of IceTop and the detectors in the ice will allow calibration of IceCube with the help of the atmospheric muons, as well as the study of the chemical composition of the incoming cosmic rays. The AMANDA telescope will be an integral part of the IceCube observatory; the AMANDA collaboration joined the new IceCube collaboration in 2005.

The IceCube project is a collaboration between Chiba University, Japan; IAS Princeton, USA; Clark Atlanta University, USA; University of Maryland, USA; University of Alabama, USA; Université Libre de Bruxelles, Belgium; Vrije Universiteit Brussel, Belgium; University of Mons-Hainaut, Belgium; University of California, Berkeley, USA; Lawrence Berkeley National Laboratory, Berkeley, USA; Bartol Research Institute, University of Delaware, USA; University of Kansas, USA; Southern University and A&M College, Baton Rouge, USA; Universität Berlin, Germany; University of California, Irvine, USA; Pennsylvania State University, USA; University of Mainz, Germany; University of Dortmund, Germany; University of Gent, Belgium; Stockholm University, Sweden; Uppsala University, Sweden; DESY-Zeuthen, Germany; University of Wisconsin, Madison, USA; University of Wisconsin-River Falls, USA; University of Wuppertal, Germany; Imperial College, London, UK; University of Oxford, UK; Utrecht University, the Netherlands; University of Canterbury, New Zealand.

The fieldwork

Researchers and scientific equipment are transported by air from Christchurch, New Zealand to the American base McMurdo on Ross Island, and from there to the Amundsen-Scott station at the geographical South Pole. Heavy equipment can also be transported by vessel once a year arriving at McMurdo in January-February. This year the Swedish researchers were involved in general service of the AMANDA telescope. A Very Low Frequency (VLF) antenna started to transmit in January 2003 a few km from the AMANDA site at the South Pole. It transmitted 1 min every 15 minutes and completely prevented AMANDA from collecting data during that time due to a general increase in the noise level. A new filter was constructed in Stockholm and tested at the South Pole by people from Stockholm University in November 2003. During this season (2004/05) all channels in AMANDA were equipped with VLF filter allowing AMANDA to collect data continuously. In addition the electronics in AMANDA were tuned for a higher dynamic range using the new transient waveforms recorder system (TWR) in AMANDA.

The most important activity this season was the start of the construction of the IceCube Neutrino Observatory. The new hot water drill for IceCube has a heating power of 5 MW compared with 2 MW for the AMANDA drill. It is more advanced and is designed to drill a 60 cm diameter hole down to 2 500 m within less than 40 hours. The optical modules for IceCube digitize the signals and transmit all information in digital form. The timing calibration – which took several weeks for AMANDA – is done automatically every 2 seconds for the whole IceCube array. The hot water drill was assembled during November–December 2004 and was ready in January 2005. Some problems were found during the preparation of the drill and only one hole was successfully made at the end of January 2005. The first IceCube string with 60 optical modules was deployed. All optical modules survived the freezing of the hole and fulfil the design performance. This season Sweden contributed with 3 technicians for the drilling operation. The last string of IceCube is planned to be deployed in January 2010. The telescope is modular and any newly deployed string will be commissioned as soon as it is in. In this way the sensitivity of the observatory to detect neutrinos will continuously increase.

There was a serious accident during drilling, in which one Swedish driller was critically injured. Thanks to the professional intervention of the station trauma team his condition could be stabilised prior to evacuation by a specially summoned Hercules C-130 aircraft. The transfer to a Christchurch hospital was completed 22 hours after the accident, and resulted in almost complete recovery for the injured person.

Preliminary results

The AMANDA telescope is working very well and detects about 10 atmospheric neutrinos per day. The atmospheric neutrinos are produced in the collisions between cosmic rays and atoms in the atmosphere of the Earth. So far no evidence for extraterrestrial neutrinos has been found. About 20 scientific papers have been published in refereed journals since the year 2000 and many results have been presented at conferences. One example is a general paper on principles and first results which was published in Nature (Andrés et al., 2001). Papers on the search for supernova neutrinos and dark matter particle-annihilations in the centres of the Earth and Sun have also been published (Ahrens et al., 2002a, Ahrens et al., 2002b, Ackermann et al., 2005a), as have searches for point sources of neutrinos (Ahrens et al., 2004a, Ackermann et al., 2005b) and neutrinoinduced cascades (Ahrens et al., 2003). The composition of the cosmic rays has been studied using AMANDA data in coincidence with the air shower detector SPACE situated on the ice surface above AMANDA (Ahrens et al., 2004b).