Introduction

The AMANDA telescope for high-energy cosmic neutrinos was constructed between 1995 and 2000 deep 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. 1995 and 1997). 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 detector is shown in figure 1 and its diameter is 200 m. The photo multipliers 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 hot water drilling was performed by the American Polar Ice Core Office (PICO) 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 thus possible to start taking data 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 project is a collaboration between Brussels Free University, Belgium; University of Mons-Hainaut, Mons, Belgium; University of California, Berkeley, USA; Lawrence Berkeley National Laboratory, Berkeley, USA; Bartol Research Institute, University of Delaware, USA;University of California, Irvine, USA; Pennsylvania State University, USA; University of Kalmar, Sweden; University of Mainz, Germany; Stockholm University, Sweden; University of Uppsala, Sweden; DESY-Zeuthen, Germany; University of Wisconsin, Madison, USA; University of Wuppertal, Germany; Imperial College, London, UK and Universidad Simon Bolivar, Caracas, Venezuela.

The work

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. During the 2002/2003 season (starting at the beginning of November and ending in the middle of February) 50% of the Swedish-made preamplifiers were exchanged by the Swedish team (the first 50% had been upgraded during the previous season). The upgrade of the amplifiers is part of a plan to keep AMANDA running for many years in the future together with the proposed new IceCube telescope.

The calibration of the time offsets for the optical modules was also done. It is necessary to know the absolute time of a photomultiplier pulse to within five nanoseconds. In order to achieve this the detector has to be calibrated after any change in the amplifier electronics. Transient Waveform Recorders (TWR) were added to the electronics in order to increase the information from each photomultiplier pulse. This work was completed in 2002/2003, and will improve the detector for extremely high energy events.

During installation and testing, efforts are made to minimise interruptions to the data collection. Following the end of the summer season in mid-February 2003, data collection has been continuous. The data collected is transferred to the northern hemisphere via satellite and its quality is constantly monitored in the different laboratories, including the Swedish ones.

Preliminary results

The first observation of neutrino candidates by the AMANDA detector was published in 2000 (Andrés et al., 2000). About 200 neutrino candidates have now been selected from the data taken during 1997 using the 10 string AMANDA-B10 detector. These are compatible with the expected rate of neutrinos coming from cosmic ray interactions in the atmosphere. A general paper on principles and first results was published in Nature (Andrés et al., 2001), and was followed by a published observation of high energy atmospheric neutrinos (Ahrens et al., 2002a). Papers on the search for supernova neutrinos and WIMP-annihilations at the centre of the Earth have also been published (Ahrens et al., 2002a and 2002b), as have searches for point sources of neutrinos (Ahrens et al., 2003a) and neutrino-induced cascades (Ahrens et al., 2003b). All these papers are based on the 1997 data taken with the 10-string AMANDA detector (AMANDA-B10). We are now analysing data from 1998, 1999, 2000 and 2001 in parallel. A point source search using the complete AMANDA detector with 19 strings and data collected  in 2000 has been submitted to Physical Review Letters (based on about 700 neutrino candidates) (Ahrens et al., 2004) . So far all observed neutrinos are compatible with neutrinos produced in the atmosphere by cosmic ray interactions. The limits given by AMANDA for cosmic neutrino fluxes are the most sensitive so far.

Figure 2 shows an event induced by an interacting electron neutrino. The interaction gives an electromagnetic cascade, which lights up practically the entire array. The circles in the figure correspond to the signals from the optical modules. Large circles correspond to large signals.

The completed 19-string AMANDA-II detector is a much more efficient and simpler detector to work with than the very narrow instrumented volume of AMANDA-B10. The on-line filtering made directly at the South Pole is now giving us between two and five neutrino candidates per day on average.

The AMANDA detector is the leading detector in the world for high-energy neutrinos. Encouraged by the success of AMANDA the collaboration has submitted a proposal for a new larger neutrino telescope, IceCube, to be built close to the AMANDA site. The detector will have 80 strings and occupy a volume of about 1 km³. The US President’s budget for the fiscal year of 2004 includes $295M for IceCube, to cover construction and four years of full-scale running. The new hot water drill for IceCube is under construction and will be transported to the South Pole during the coming summer season (2003/2004). The first strings will be deployed in 2004/2005.