The aim of the telescopes

The feasibility of using transparent ice at large depth in Antarctica for neutrino telescopes was shown by the AMANDA collaboration in the 1990’s. The scientific goals for these telescopes are for example 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 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 to study the Cosmos. In order to compensate for the extremely low probability for the neutrino to interact with matter one has to use very large detectors. The neutrino telescopes are sensitive to the emitted Cherenkov light from electrically charged particles created by neutrino interactions deep in the ice. The transparent ice at the South Pole is a very suitable detector medium for a neutrino telescope. The ice sheet is 2 900 m deep and extremely transparent at large depths (Askebjer et al., 1997, Askebjer et al., 1998). The AMANDA neutrino telescope was constructed between the years 1995 and 2000 deep in the ice at the Amundsen-Scott base at the South Pole. Owing to the success of AMANDA the large IceCube Neutrino Observatory is now under construction at the same location.

The AMANDA 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 situated 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) performed the hot water drilling with the help of drillers from the Swedish Polar Research Secretariat.

The AMANDA detector has been fully operational and taking data since February 2000.

The construction of a much larger telescope, the new IceCube Neutrino Observatory, began at the South Pole in January 2005. The complete observatory will consist of 4 800 optical modules deployed between depths of 1 450 and 2 450 m in 80 holes covering an instrumented volume of about 1 km³. The optical modules for IceCube are much more advanced than those for AMANDA; they digitize the photomultiplier signals and transmit all information in digital form to the surface. The timing calibration, which was done manually and took several weeks for AMANDA, is done automatically every two seconds for the whole IceCube array. On the surface above the neutrino telescope an air shower array, IceTop, will detect air showers from cosmic rays interacting in the atmosphere. The combination of IceTop and the detectors in the ice will allow calibration of IceCube using the atmospheric muons, as well as studying the chemical composition of the incoming cosmic rays. The AMANDA telescope will be an integrated part of the IceCube Neutrino Observatory; the AMANDA collaboration merged with the new IceCube collaboration in 2005.

The fieldwork

People and scientific equipment are transported by air from Christchurch, New Zealand to the American antarctic base McMurdo on Ross Island, and then to the Amundsen-Scott station at the geographical South Pole. Heavy equipment can also be transported by cargo vessel once a year, arriving at McMurdo in January–February.

The construction of the new IceCube Neutrino Observatory continued this season. In the previous season the new hot water drill was installed and tested. The 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 in less than 40 hours. Despite the higher power and the larger depth of the holes the consumption of fuel is less than for the AMANDA holes. The drilling camp for IceCube is shown in picture 1. One IceCube string with 60 optical modules was successfully deployed at the end of last season in January 2005. That string worked well during 2005, and a summary from the first year of running has been published (Achterberg et al., 2006a) showing that the equipment performed as expected. This season the hot water drill was slightly modified based on the experience from last season. After a slow start the deployment rate at the end of the season approached a speed of one deployed string in less than four days. figure 2 shows the drilled depth as a function of time for one of the holes. Eight new strings were deployed, giving a total of nine IceCube strings in the ice out of the planned 80. In addition twelve IceTop stations were successfully deployed. The equipped ice volume of the new IceCube strings is already larger than the volume of AMANDA. figure 1 shows a reconstructed descending high-energy muon from cosmic ray interaction in the atmosphere in the nine-string IceCube detector. The telescope is modular and any newly deployed string will be commissioned as soon as it is in place.

The sensitivity of the observatory to detect neutrinos will thus increase continuously during the deployment period. The last string of IceCube is planned to be deployed in January 2011.

During the season 2005/06 Sweden contributed with three technicians for the drilling operation and two scientists for testing and deployment of the modules. Two more Swedes were hired by the US as drillers.

Preliminary results

The nine IceCube strings are performing very well and commissioning and verification has been performed. The results so far are mainly technical.

The AMANDA telescope is working very well and detects about five atmospheric neutrinos per day. The atmospheric neutrinos are produced in the collisions between cosmic rays and atoms in the atmosphere of the Earth. More than 4 000 neutrino candidates have been recorded, but so far no evidence for extraterrestrial neutrinos has been found. Analysis of the AMANDA data is ongoing and about 25 scientific papers in refereed journals have been published. A general paper on principles and first results 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 combination with the air shower detector SPACE situated on the ice surface above AMANDA (Ahrens et al. 2004b). Limits from AMANDA for high-energy gamma and neutrino fluxes from the giant flare of the Soft Gamma-Ray Repeater 1806-20 in December 27 2004 have recently been published (Achterberg et al., 2006b). A paper about the ice properties in the AMANDA volume has been published (Ackermann et al., 2006a).