Introduction

The AMANDA telescope for high-energy cosmic neutrinos has been built (1995–2000) 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 during different violent processes in the universe and the possibility to detect high energy neutrino sources will open a new window in the study of 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 like clear ice in order to propagate the light efficiently. The ice sheet at the South Pole is 2 900 metres deep and extremely transparent at large depth (Askebjer et al., 1995, 1997). The detector consists of 677 optical modules deployed in 19 holes in the ice. The holes were made by 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 made by glass spheres deployed at 1 200 to 2 300 metres depth. The central part of the detector with the highest density of optical modules is between 1 500 and 2 000 meters depth. The diameter of the detector is 200 meters. 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 the on-line computers. The hot water drilling has been performed by the American Polar Ice Core Office (PICO) with help of Swedish drillers from the Swedish Polar Research Secretariat.

The AMANDA detector is fully operational and has been collecting data since February 2000. The completed detector is named AMANDA-II, in order to distinguish it from the partially completed detectors (AMANDA-B4 and AMANDA-B10). The detector is modular and it was possible to collect data with only a fraction of the total number of strings. In this way data from the 4-string and 10-string 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; Kalmar University, Sweden; University of Mainz, Germany; Stockholm University, Sweden; Uppsala University, 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

People and scientific equipment are transported by air from Christchurch, New Zealand to the American base McMurdo and then to the Amundsen-Scott station at the geographical South Pole. For the 2001/02 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 upgrade of the amplifiers is part of a plan to keep AMANDA running for many years in the future together with the new proposed IceCube telescope. The rest of the amplifiers will be exchanged during the season 2002/03.

The calibration of the time constants 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. A number of Transient Waveform Recorders (TWR) were also added to the electronics in order to increase the information from each photomultiplier pulse. In the next season, 2002/03, the rest of the channels will be equipped with TWRs. The upgrade of the channels with TWRs will improve the detector for extremely high energy events.

The detector was turned on again in the middle of February 2002.

Preliminary results

The AMANDA detector has been collecting data continuously during the “winter season”, from March to the end of October, since 1996. During the summer months from November to February we have either been adding new strings or calibrating and upgrading the electronics for the detector. In the future we expect to be able to run continuously through the whole year with only a few interruptions.

A first observation of neutrinos by the AMANDA detector has been published (Andrés et al., 2000). About 200 neutrino candidates have now been selected from the data collected 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 about the principles and first results was published in Nature in 2001 (Andrés et al., 2001). A paper about searching for supernova neutrinos has been published (Ahrens et al., 2002, ref. 5). A limit to the muon flux from WIMP annihilation in the centre of the earth has been published (Ahrens et al., 2002, ref. 6) as well as an observation of high energy atmospheric neutrinos with AMANDA (Ahrens et al., 2002, ref.7). Two papers about the search for point sources of high-energy neutrinos (Ahrens et al., 2003, ref. 8) and the search for neutrino-induced cascades (Ahrens et al., 2003, ref. 9) with AMANDA have been accepted for publication. All these papers are based on the 1997 data collection using the 10-string AMANDA detector (AMANDA-B10). We are now analyzing data from 1998, 1999, 2000 and 2001 in parallel. A preliminary point source search using the complete AMANDA detector with 19 strings and data collected in 2000 is shown in figure 1. The figure shows the direction of the observed neutrinos in a sky plot in equatorial coordinates. 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.

The completed 19-string AMANDA-II detector is a simpler and a much more efficient detector to work with than the very narrow AMANDA-B10. The on-line filtering made direct at the South Pole is now giving us about 2–5 neutrino candidates per day.

The AMANDA detector is the leading detector in the world for high-energy neutrinos. Due to 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 decision will be taken during 2003, and if approved the first strings will be deployed 2004/05. The new hot water drill for IceCube is under construction and is expected to be transported to the South Pole during the summer season 2003/04.