Astroparticle physics: signals from the cosmos
Laboratories in the depths of the sea
For several years now, physicists have been conducting experiments
in the depths of the ocean and in the ice at the South Pole.
The possibility of installing large detectors at depths of several
kilometers is being investigated. The detectors, distributed
over distances on the order of a kilometer, are to identify
high-energy neutrinos from cosmic sources by measuring the light
produced by the interactions of these neutrinos with the seawater.
The projects under study include ANTARES, near Marseille, and
NEMO, in Sicily. The NEMO project involves the confrontation
of interdisciplinary problems related on one hand to the technologies
for the construction of a telescope with the necessary mechanical
and electronic characteristics, and on the other, to oceanographic
studies of the Mediterranean for the identification of the sites
most adapted to hosting the experiment. The size and complexity
of these research enterprises require that they be carried out
in the context of large, international collaborations.
The NEMO collaboration, for example, involves various different
countries of the Mediterranean basin.
Argo: an observatory in Tibet
Observing extremely high-energy photons from galactic and extragalactic
sources allows information about the nature and properties of
particular sources to be obtained. When these photons reach
the Earth, they collide with atoms in the atmosphere and initiate
cascades that result in detectable showers of particles. Observing
such showers will be the task of the ARGO (Astrophysical Radiation
Ground-Based Observatory) detector, a product of Italian-Chinese
collaboration promoted by the INFN and by prominent Chinese
institutions. Installation of the detector is in progress at
the Yangbajing laboratory in Tibet, 4300 meters above sea level,
and will be completed in 2004.
The photons that are the subject of observation are about a
trillion times more energetic than are those of visible light,
and constitute a form of high-frequency electromagnetic radiation
produced in systems such as supernovae, neutron stars, and black
holes. Among the most mysterious of the photon sources to be
studied are those that generate “gamma-ray bursts,”
sudden explosions that may last from a fraction of a second
to several hundred seconds.
The unique characteristics of ARGO will allow highly energetic
photons to be identified. Such photons are normally confused
with cosmic rays, which are much more numerous.
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