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The apparatus
consists
of a full coverage detector of dimension
~78 X 74 m2
realised
with a single layer of RPCs (Resistive
Plate Counters). The area surrounding the central detector core, up
to ~100 X 100 m2,
consists in a guard ring partially (~30 %) instrumented with RPCs. This
outer ring improves the apparatus performances, enlarging the fiducial
area, for the detection of the showers with the core outside the central
carpet.
A lead converter 0.5 cm thick will cover uniformly the
RPCs plane in order to increase the number of charged particles by conversion
of shower photons, and to reduce the time fluctuations of the shower front.
Pads 56 X 60 cm2
each
(18480 in total) are the basic element ("pixel") which defines the time
pattern of the shower. Each pad is subdivided
in pick-up strips 6 cm wide which define the space pattern inside the pad.
The detector is organized in modules of 12 RPCs (the
rectangles of the figure) called CLUSTER.
RPCs have been employed successfully both in cosmic ray and accelerator experiments and their extensive use is envisaged for future experiments at SLAC and LHC. The use of RPCs has been favoured due to the low cost, the large active area, the excellent time resolution and the opportunity of an easy integration in large systems.
The detector will be installed at the Yangbajing High Altitude Cosmic Ray Laboratory (4300 m a.s.l., Tibet, P.R. China), 90 km North to Lhasa. The site coordinates (latitude 300 06' 38'' N, longitude 900 31' 50'' E) permits the monitoring of the Northern hemisphere in the declination band -100 < delta < 700.
ARGO-YBJ will be hosted in a building capable to keep the temperature in the RPCs operating range (~ 8 - 30 oC).Detector assembling will start late in 2000 and data taking with the first ~ 750 m2 of RPCs in 2001.
This experiment will image with high efficiency and sensitivity atmospheric showers initiated by primaries of energies > 100 GeV, allowing to bridge the GeV and TeV energy regions and to face a wide range of fundamental issues in Cosmic Ray and Astroparticle Physics:
1) Gamma-ray astronomy, at a ~100 GeV threshold energy. Several galactic and extragalactic point candidate sources can be monitored, with a sensitivity to unidentified sources better than 10% of the Crab flux.
2) Diffuse Gamma Rays from the Galactic plane, molecular clouds and SuperNova Remnants (SNR) at energies >100 GeV.
3) Gamma-Ray Burst physics , with a sensitivity allowing the extension of the satellite measurements over the full GeV/TeV energy range.
4) Anti-p/p ratio at energies from 300 GeV to TeV not accessible to satellites, with a sensitivity adequate to distinguish between models of galactic or extragalactic anti-proton origin.
5) The primary proton spectrum in the 10 - 200 TeV region, with sensitivity sufficient to detect a possible change of the slope of the energy spectrum.
6) Sun and Heliosphere physics, including cosmic ray modulations at 10 GeV threshold energy, the continuous monitoring of the large scale structure of the interplanetary magnetic field and high energy gamma and neutron flares from the Sun.
Additional objectives come from using ARGO-YBJ as a traditional EAS array covering the full energy range from 1011 to 1016 eV . Since he detector provides a high granularity space-time picture of the shower front, detailed study of shower properties as, for instance, multicore events, time and lateral distributions of EAS particles, multifractal structure of particle densities near the core, can be performed with unprecedented resolution.
At an altitude > 4000 m a.s.l. the
electron size of the shower produced by primaries energies 1015-1016
eV, around the 'knee' of cosmic ray spectrum, is practically independent
of their mass. This fact could be exploited to get information about the
composition at the knee by measuring other parameters distinguishing showers
developed by primaries of different mass.
