ous efforts. It is key to realize that IceCube is a discovery instrument, more akin to a particle physics project than to a telescope—it may be that we will not see what we are looking for because there might not be any discrete sources of astrophysical neutrinos to discover. In that case, other techniques to address these science questions will certainly be needed. The discovery of very high energy neutrino sources, though, would clearly demonstrate the existence of the acceleration of hadrons (e.g., protons or nuclei) in known astrophysical discrete sources. By the same token, however, it cannot be said with certainty what IceCube will in fact detect—source flux predictions are uncertain and rely on the extrapolation from known astrophysics at lower energies. Possible sources include gamma-ray bursts, which are powerful explosions that release in seconds the same energy as a typical galaxy emits in years; active galaxies and quasars, which can be more than 1,000 times more luminous than normal galaxies and are believed to be powered by supermassive black holes at their centers; and neutron stars, which are ultracompact stellar remnants that have collapsed to densities comparable to those inside atomic nuclei. In addition, the possibility exists for new and unexpected types of sources, both astrophysical and exotic, at these energies.
The lack of strong and electromagnetic interactions gives neutrinos the special ability to traverse the universe unimpeded, but it also makes them extremely hard to detect. To achieve sufficient sensitivity to detect high-energy neutrinos from distant sources, experiments on the scale of one cubic kilometer or a billion tons of detector mass are required. Such experiments use Earth as a converter and detect neutrinos as they traverse Earth from below and interact near the experiment. Neutrino interactions produce upward-going muons that generate Cerenkov light in a suitably transparent medium such as water or ice and that are recorded by an array of light sensors (photomultiplier tubes) spread throughout the volume of the detector. IceCube is to be constructed at the South Pole, making use of Antarctic ice as the detecting medium. The IceCube sensors are deployed by lowering strings of photomultiplier tubes into melted ice and allowing the strings to be frozen in place. See Figures 3.1 and 3.2 for a description of IceCube and its operation.
Although IceCube is a major undertaking in a rather remote location, its design and prospects for success are bolstered by the strength of the existing polar infrastructure and, in particular, by the successful deployment and operation of AMANDA, a smaller precursor to IceCube. The properties of the South Pole ice as a detection medium are now better understood, and AMANDA has shown that the technique of detecting upward-going muons works well—it has reconstructed about 1,000 upward-going muons that are signatures of neutrinos produced in the atmosphere that lies below the horizon. IceCube will substantially improve on AMANDA’s capabilities, both through a larger detection volume and through the use of improved technology.