The aim of IceCube

The AMANDA collaboration demonstrated during the 1990s the feasibility of using transparent ice at large depths in Antarctica for neutrino telescopes. The scientific goals set for these telescopes are to use neutrino particles from space to investigate different scientific issues such as the “dark matter” of the Universe and the sources of the highest energy cosmic rays.

Neutrino particles are extremely penetrative and interact very rarely with matter. It is anticipated that the neutrinos are produced by different violent processes in the Universe, and that the ability to detect high-energy neutrino sources will open a new window through which to study the Cosmos. However, very large detectors have to be used in order to compensate for the extremely low probability of neutrinos interacting with matter. The neutrino telescopes are sensitive to the Cherenkov light emitted from electrically charged particles created by neutrino interactions deep in the ice. The ice sheet at the South Pole is 2 800 m deep and extremely transparent at large depths. The AMANDA neutrino telescope was constructed between 1995 and 2000, mainly at depths between 1 500 m and 2 000 m, where the optical modules were deployed in holes drilled by pressurized hot water. The AMANDA detector has been successfully operative and registering data since February 2000. The construction of the much larger IceCube detector began in January 2005 at the same location, as a result of the success of the AMANDA. The complete observatory will consist of about 4 800 optical modules deployed between depths of 1 450 m and 2 450 m in 80 holes instrumenting a volume of about 1 km³. The digital optical modules for IceCube (DOMs) are considerably more advanced than those used in AMANDA, digitising the photomultiplier signals and transmitting all information in digital form to the surface. Timing calibration, which was performed manually and took several weeks for AMANDA, occurs automatically every two seconds for the whole IceCube array. An air shower array on the surface above the neutrino telescope, 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 atmospheric muons as well as facilitate analysis of the chemical composition of the incoming cosmic rays.

The ice work

Personnel and scientific equipment are transported by air from Christchurch, New Zealand to the US base McMurdo on Ross Island, and then to the Amundsen-Scott station at the geographical South Pole by Hercules aircrafts. Heavy equipment can also be transported by sea once a year arriving at McMurdo in January–February. The construction of the new IceCube Neutrino Observatory continued during the 2008/09 summer season. During the four previous austral summer seasons, 40 strings with 60 DOMs each had successfully been deployed. During this season 19 new strings with a total of 1 140 DOMs were deployed giving a partially complete IceCube telescope with 59 strings. One of the strings was the first special string for the low energy extension of IceCube called DeepCore, which will be situated in the centre of IceCube and in the most transparent ice at 2 100–2 450 m depth. DeepCore will improve the sensitivity of IceCube for dark matter search and low energy science. It will also make it possible to search for neutrinos from above the horizon.

Preliminary results

The analysis of data taken by the AMANDA telescope is still in progress. A general paper on principles and first results was published in Nature (Andrés et al., 2001). More than 6 500 neutrino candidates have been recorded, but so far no evidence for extraterrestrial neutrinos has been found. About 35 scientific papers in refereed journals have been published. These include papers on the search for neutrino point sources during the seven years AMANDA has been running (R. Abbasi et al. 2009a), the ice properties in the AMANDA volume (Ackermann et al., 2006b) and the search for neutrinos from strange Gamma Ray Bursts (GRB), which are the most powerful explosions in the universe (Achterberget al., 2006c, Achterberg et al., 2007b).

As the size of the IceCube detector increases for every year, it has given better and better sensitivity for detecting neutrinos. IceCube is now superior to AMANDA and several publications have been produced. A report on the performance of the first IceCube string deployed (Achterberg et al., 2006a) as well as the first results on atmospheric neutrinos from data taken by the 9-string detector during 2006 (Achterberg et al., 2007a) have been published showing that the new IceCube technology works very well. A solar activity giving an increased rate of low energy particles occurred on 13 September 2006 and was observed by the IceTop detector and reported (Abbasi et al., 2008a). The best limits for dark matter annihilation in the Sun were obtained by IC22 (IceCube with 22 strings) and published (Abbasi et al. 2009b).

The 40 IceCube strings (IC40) were performing very well and registered data together with the AMANDA array (April 2008–May 2009). The instrumented volume corresponds to about half a km³ of ice. The first six months of data from IC40 was used in the search for neutrino point sources. Figure 3 shows the sky map of the direction from where the recorded neutrinos were coming. The probability of an excess (deficit) of neutrinos in a specific direction is also shown. No significant neutrino source was found. There were 6 796 neutrinos from the Northern hemisphere compatible to be produced in the atmosphere by cosmic rays. Down going atmospheric muons were rejected to 10^-5 in the Southern hemisphere.