already mentioned Kamiokande and IMB water Cerenkov detectors that recorded neutrinos from the supernova SN1987A. Detectors of other types were operated in the Kolar gold fields in India, at the Baksan in Russia (near the site of the SAGE experiment discussed above), in the Mont Blanc tunnel, in the Frejus railway tunnel in France, in the old Silver King mine in Utah, and in the Soudan iron mine in Minnesota. Of these only the Baksan and Soudan experiments continue to operate.
Cosmic-ray neutrinos with energies of 100 MeV to 1 GeV can interact inside these detectors, yielding “contained events.” Through-going muons from the interaction of higher-energy cosmic-ray neutrinos in the rock surrounding the detector are also observed. All these observations have been used in the search for neutrino oscillations. There is evidence [37] from the Kamiokande and IMB experiments that the ratio of muon- to electron-type neutrinos is smaller than expected for atmospheric neutrinos with energies of the order of 1 GeV, possibly indicating a neutrino mass in the range of 0.1 eV. No neutrinos have been observed that could be traced to sources outside the solar system, with the exception of the observations of SN1987A.
Larger experiments based on principles similar to these second-generation detectors have been built or are under construction for a variety of purposes. The largest are the MACRO and LVD experiments in the Gran Sasso Laboratory in Italy and Superkamiokande, discussed above. These experiments will be able to study cosmic-ray neutrinos, detect galactic supernovae, and provide further information on possible neutrino oscillations. However, none of these detectors is expected to observe astrophysical neutrinos from beyond the solar system.
Estimates [38] of the potential fluxes of high-energy neutrinos from sources such as active galactic nuclei suggest that a surface area greater than 0.1 km2 is necessary for meaningful observations. Furthermore, since the expected signal-to-noise ratio improves greatly with the increasing threshold up to TeV energies, detectors of great thickness are desirable. It is generally agreed that these requirements can be met only by the use of large bodies of water such as a lake, an ocean, or polar ice.
High-Energy Neutrino Experiments Under Construction
The first instruments designed specifically for high-energy neutrino detection are now under construction in Siberia in Lake Baikal (BAIKAL [39]), off Hawaii in the deep ocean (DUMAND [40]), at the South Pole (AMANDA [41]), and in the Mediterranean near Pylos, Greece (NESTOR [42]). The past few years have seen at least a dozen proposals for other neutrino detectors of this generation, but none are now making progress [36]. All four of the instruments being built aim at detection areas of the order of 20,000 m2 and an angular resolution of the order of 1° for TeV muons.
BAIKAL
The BAIKAL instrument [39] is deployed through the winter ice, taking advantage of the solid platform to facilitate positioning and laying of a cable to shore. The group installed 18 optical modules in March 1993, each of which consists of two 15-inch photomultipliers in glass housings. The depth is limited to about 1 km, which causes difficulties in distinguishing upcoming muons from the much more numerous downgoing cosmic-ray muons. An array with an effective area of 2000 m2 is scheduled for operation in 1996. Progress on this experiment is made difficult by the situation in the former Soviet Union, although the experiment has received significant assistance from the former East Germany.
DUMAND
The DUMAND experiment [40] has been under development in Hawaii since the early 1980s, with collaborators from the United States, Japan, Switzerland, and Germany. A ship-deployed instrument was operated several years ago, producing muon depth-intensity measurements and demonstrating the technology. At present a large photomultiplier array is under construction for placement 4.8 km deep, 25 km off the Big Island of Hawaii. A volume of water is instrumented with photomultipliers that sense Cerenkov light produced by muons. The optical detectors are positioned like beads on long cables, called strings, which carry the signals. Installation of the strings is ongoing; the complete nine-string
