From cosmic ray studies, there is compelling evidence that the world
of low-energy phenomena investigated in great details over several
decades is complemented by a high energy world, being a terra incognita
with respect to its origin. The origin of the high energetic cosmic
radiation can be explored only by neutral, stable probes, i.e by gamma
rays and neutrinos pointing back to their sources. Unlike gamma-rays
neutrinos can escape regions of particle acceleration strongly
shadowed by matter. In addition, neutrinos can reach us from the
most distant parts of the observable Universe, whereas high energy
gamma-rays are absorbed by interstellar light. On the other hand, the
weakness of neutrino interactions makes detection to be extremely
difficult and requires huge detectors which should be reliable
protected against background.
The history of underground detectors for natural neutrino fluxes
started with the pioneering detectors in India (1965),
South Africa (1965) and in the Northern Caucasus (1978)
and led to armed giants with effective areas of 1000 square meters
(MACRO, Italy), and
sensitive volume of more than 20000 tons (SUPERKAMIOKANDE, Japan).
These detectors allow to investigate a wide spectrum of problems in
the fields of neutrino astrophysics, elementary particle physics and
cosmic ray physics. Nevertheless existing underground detectors were
not able to detect high energy neutrinos from cosmic accelerators. So,
a key question of modern astrophysics - what is the nature of
cosmic high energy world ? - has still to be considered to be
One is forced to considerable increase the sensitivity of neutrino
telescopes. This, in turn, dictates the creation of principally larger
scales and goes beyond the constructional possibilities of current
underground techniques. One of the solutions of this dilemma is to
develop underwater detectors which use the giant water masses of natural
basins as shield against downward going background, target and detecting
The idea to construct big water Cherenkov detectors for neutrino
astrophysics deep underwater was firstly formulated in 1960 by M.A.Markov.
These telescopes consist of a lattice of photomultipliers (PMTs)
spread over a large open volume in the ocean or in a lake. High energy
neutrinos can be detected by upward traveling muons produced in neutrino
interactions in water close to the detector. With the angle between parent
high energy neutrino and muon being very small astronomy with degree
resolution is possible. The direction of the particle is inferred from
the measured arrival times and amplitudes of the Cherenkov photons.
Technologies for underwater telescopes have been pioneered by the
Baikal collaboration and by the
In the mean time, two underwater/ice detectors are taking data: the
Baikal telescope and the
AMANDA telescope at the South Pole. Also, the projects
ANTARES in the Mediterranean
have joined the efforts towards an underwater neutrino telescope.
The scientific goals of underwater telescopes are manifold. The basic
motivation is to do high energy neutrino astronomy. Beyond the field of
neutrino astronomy, the diverse scientific missions of underwater telescopes
include the search for neutrinos emerging from the annihilation of Weakly
Interacting Massive Particles (WIMPs) in the center of the Earth or the sun,
the search for neutrino oscillations, or for slowly moving, bright particles
like GUT magnetic monopoles. They can contribute to such different fields
like atmospheric muon physics on the one hand and - for a lake-based
telescope - limnology on the other hand.
The possibility to build a neutrino telescope in Lake Baikal was investigated
since 1980, with the basic idea to use - instead of a ship - the winter ice
cover as a platform for assembly and deployment of instruments. After first
small size tests, in 1984-90 single-string arrays
GIRLYANDA equipped with 12 - 36 PMTs were deployed and
operated via a shore cable. The total life time for these
first generation detectors made up 270 days.
On the methodical side, underwater and ice technologies were developed,
optical properties of the Baikal water as well as the long-term variations
of the water luminescence were investigated in great details.
Since 1987, a second generation detector
with the capability to identify muons from neutrino interactions was
envisaged. According to the approximate number of PMTs this detector
was named NT-200 - Neutrino Telescope with 200
PMTs. With estimated effective area of about 2300 square meters and 8500
square meters for 1-TeV and 100-Tev muons, respectively, it is a first stage
of a future full-scale telescope, which will be built stepwise, via
intermediate detectors of rising size and complexity.
The Baikal Neutrino Telescope NT-200 is being deployed
in Lake Baikal, 3.6 km from shore at a depth of 1.1 km. It consist
of 192 optical modules (OMs). The umbrella-like frame of 43 m diameter carries
the 8 strings with the detector components. The detetector is connected with
the shore center by 3 bottom cables.
In April 1993, the first part of NT-200, the detector
NT-36 with 36 OMs at 3 short strings, was put into operation
and took data up to March 1995. A 72-OMs array, NT-72, run
in 1995-96. In 1996 it was replaced by the four-string array
NT-96. Summed over 700 days effective life time,
320,000,000 muon events have been collected with
NT-36, -72, -96. The first neutrino events have been
selected. Since April 6, 1997, NT-144, a six-string array
with 144 OMs, taked data in Lake Baikal.
NT-200 array is completed in April, 1998.