The IceCube experiment, which was finalized in December 2010, consists of a cubic-kilometer array of 5160 photomultipliers located 1450 -2450 m deep in the ice at the geographic South Pole. The phototubes detect Cherenkov light produced by muons and other charged particles. The challenge is to find neutrino induced muons, electrons and taus among the million times larger background from muons produced by collisions of charged cosmic rays with the atmosphere.
Neutrinos might once become a powerful tool for astrophysicists. Due to the extremely small cross section for weak interactions neutrinos are able to leave compact sources and provide the observer with information from inside. Therefore they can provide insight into the surroundings of supernovae, active galaxies or other "cosmic cataclysms". Being electrically neutral, no galactic or intergalactic magnetic fields affects the trajectory of a neutrino. They pass interstellar obstacles like dense clouds that are impenetrable for photons.
Neutrino astrophysics opens a new window to the universe. The physical principles of detecting neutrino induced muons by Cherenkov light are rather simple. A large array of photomultipliers is placed in a transparent medium. A high energy neutrino interacts with the matter in the vicinity of the detector. In case of a charged current interaction, a charged lepton is produced, which on average receives about two thirds of the neutrino energy. Traversing the ice, the muon emits Cherenkov light that can be detected by photomultiplier tubes. The detected photon arrival times and amplitudes are used to reconstruct the track and locate its source in the sky. Due to the selected distances between adjacent phototubes, IceCube is most sensitive for muons and their corresponding neutrinos with energies above a few hundred GeV. However, electrons and possibly tauons may be observed too.
By far most events in IceCube and similar detectors (Baikal, ANTARES) are caused by atmospheric muons. Such muons originate from collisions of cosmic rays with the outer atmosphere. In order to shield the experiment from this background, the detector is placed deep underground. Still this background exceeds the rate of atmospheric neutrino by a million. In most analyses, the Earth is therefore used as shield, penetrable only to neutrinos. At very high energies, the cosmic ray background is sufficiently suppressed such that a study of the Southern Sky also becomes possible.
Why at the South Pole? The cross sections for neutrino interactions are very small. In addition, the neutrino flux is decreasing rapidly at high energies. Therefore the sensitive volume of the detector has to be very large and must be made of a clean, transparent and cheap material. The only places in nature offering such environments are deep lakes (Baikal), the deep sea (Antares) and the polar icecaps. One advantage of ice over water is the lack of radioactivity due to Potassium 40 decays, the absence of biological activity and a temperature of 30-50 degrees below zero. As a consequence, the PMT darknoise is significantly lower than in water. Other advantages are the very long mean absorption length for photons and the stable ground available during deployment. The latter also allowed for the construction of a kilometer-square cosmic ray detector above IceCube.
The University of Mainz joined the earlier AMANDA experiment in June 1999 and now participates in the Icecube experiment, currently together with 270 scientists from 39 institutions and 11 countries. The work by Mainz has been supported by the BMBF, the HAP - Helmholtz Alliance, the EMG research center and the university.