Traditional telescopes observe light or radio waves from
space. However, sometimes the light from distant galaxies doesn't reach
the Earth due to scattering or absorption in the intermediate
matter. Hence other means of observation than optical are necessary.
Some objects in the sky emit high-energetic neutrinos. Neutrinos are electrically neutral particles from the same family as the electrons. They only interact via the weak nuclear force (responsible e.g. for the radioactive beta-decay), which allows the neutrinos to pass through galaxies over vast distances without really being affected. Therefore they are excellent eye-witnesses that can reveal information about events very far away, but for the same reasons they are very hard to detect.
The Antarctic Muon And Neutrino Detector Array - AMANDA - is a telescope for cosmic neutrinos situated at the geographical South Pole on Antarctica. The detector is deployed in the ice at depths between 1.5 and 2.4 km. It constitutes of very sensitive light detectors (photomultipliers) attached to long strings. The photomultipliers are put into holes with a diameter of about 50 cm, that are drilled using hot water. A cosmic neutrino is most likely to pass straight through the detector, but there is a slight posibility that it will react with a nucleus in the ice and produce a muon. The muon is similar to the electron, but it is about 200 times heavier and unstable. The muon lifetime is 0.000002 seconds.
The muon will travel faster than light in the ice (but of course still slower than the speed of light in vacuum), thereby producing a shock wave of light, called Cerenkov radiation. This light is detected by the photomultipliers, and the trace of the neutrinos can be reconstructed with an accuracy of a couple of degrees. Thus the direction of the incoming neutrino and hence the location of the neutrino source can be pinpointed. A simulation of a muon travelling through AMANDA is shown here (1.5 MB).
Since the neutrinos interact rarely, a large volume of a pure and preferrably cheap medium is needed to detect them, making the South Pole with its 3 km deep clean ice a perfect location. Especially since the necessary infrastructure is provided by the American Amundsen-Scott base. The base is accessible from November until February when approximately 200 persons can be at the base simultanously. During the winter period between February and the end of October about 50 persons stay isolated at the base to monitor the experiments.
The first strings were deployed in the season 1993/1994, and currently
19 strings with a total of almost 700 light detectors have been lowered in
the ice. Rigth now the construction of IceCube is being
planned. IceCube will have a volume of a cubic kilometre, and about
4800 photomultipliers. The first IceCube strings will be deployed in
January 2005.
The AMANDA collaboration consists of groups from Sweden, the USA, Germany, Belgium, United Kingdom and Venezuela. A list of the participating institutions can be found here. The AMANDA project is also supported by the Swedish Polar Research Secretariat and is part of the Swedish polar research programme SWEDARP.
