Aim of the work

The AMANDA telescope for high energy cosmic neutrinos has been 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 by 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. 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 l 200 m and 2 300 m. The central part of the detector with the highest density of optical modules is between l 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 the online 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 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 collection 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 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; Uppsala University, Sweden; DESY-Zeuthen, German y; University of Wisconsin, Madison, USA; University of Wuppertal, Germany; Imperial College, London, United Kindom, and Universidad Simon Bolivar, Caracas, Venezuela.

The field 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. This year the Swedish personnel were involved in general service of the telescope. A Very Low Frequency (VLF) antenna started in January 2003 to transmit 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. The telescope had to veto the time the VLF was transmitting. A new filter was constructed in Stockholm and tested at the South Pole in November 2003 by researchers from Stockholm University. It worked very well and all channels in AMANDA will be equipped with this filter in the season 2004/05.

The calibration of the time offsets for the optical modules was also carried out. It is necessary to know the absolute time of a photomultiplier pulse to within five nanoseconds. This work was done by sending short laser pulses via optical fibres down to the modules in the ice.

The central trigger for the telescope (which has been designed and built at Uppsala University) was modified in order to improve the sensitivity of the telescope.

In addition of the service work on the AMANDA telescope, preparation for the new IceCube telescope was done during the summer season. The IceCube telescope will be a one cubic kilometre telescope placed between 1 450 and 2 450 m depth in the same area as AMANDA. The positions of the 80 new strings were marked and part of the new drilling equipment arrived at the South Pole. The first IceCube strings are expected to be deployed in January 2005. The Swedish AMANDA groups are involved in the IceCube project.

Preliminary results

The AMANDA telescope is working very well and detects about 3-5 atmospheric neutrinos per day. So far no evidence for extraterrestrial neutrinos has been found. Many papers have been published in refereed journals and results have been presented at conferences. For example a general paper on principles and first results was published in Nature (Andres, 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., 2002b and 2002c), as have searches for point sources of neutrinos (Ahrens et al., 2003a) and neutrino-induced cascades (Ahrens et al., 2003b). Using AMANDA data in combination with the air shower detector SPACE situated on the ice surface above AMANDA the composition of the cosmic rays has been studied. (Ackermann et al., 2004). A paper about flux upper limits on Ultra High Energy neutrinos with AMANDA-B10 has been published (Ackermann, 2004).

A search for neutrino point sources using the complete AMANDA detector with 19 strings and data collected in 2000 has been published in Physical Review Letters (Ahrens et al., 2004 ). A search for point sources using four years’ worth of data (2000-2003) was presented at the Neutrino 2004 conference (Woschnagg, 2004). The result is compatible with what is expected from atmospheric neutrinos.

Figure 1 shows the atmospheric neutrino intensity as a function of the neutrino energy obtained by the AMANDA-II detector preliminary. It reaches neutrino energies of l 00 000 GeV, which is far higher than previous results.

It is very important to understand the optical properties of the ice, and investigations of ice parameters like scattering length and absorption length have been thoroughly carried out for several years. Figure 2 and 3 shows the absorption length and the obtained effective scattering length in the AMANDA ice volume as a function of wavelength and depth. The scattering length and absorption length varies as a function of depth, showing the quality of the ice as a function of geological time.