ARGO-YBJ Experiment

A fully developed study of gamma-ray astronomy requires a telescope that identifies transient emissions and performs a systematic search of steady sources. These goals can be achieved by using an unconventional air shower array at energies < TeV.
At these energies the number of particles reaching the ground is too small for  reconstructing the shower parameters with standard air shower arrays, made of several detectors spread over large areas which sample only a small percentage (< 1 %) of the shower particles. Since this kind of detectors triggers on shower size, it can operate at lower energies if it has the ability to detect small showers.
A new instrument capable to sample a fraction of the shower larger than that of conventional air shower arrays can be achieved
  The idea of detecting small size air showers forms the basis of the ARGO-YBJ experiment.

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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.

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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.

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