2 The ARGO-YBJ detector

ARGO-YBJ is an air shower detector optimized to observe small size showers, to be constructed in the Yangbajing Laboratory (Tibet, China) at an altitude of 4300 m a.s.l. The experiment consists of a $\sim 71\times 74$ m² core detector realised with a single layer of RPC's ($\sim 90\%$ of active area), surrounded by an outer detector ($\sim 30\%$ of active area) for a total size of $\sim 100\times 100$ m². A lead converter 0.5 cm thick will cover uniformly the RPC plane in order to increase the number of charged particles by conversion of shower photons in ${\rm e}^{\pm}$ and to reduce the time spread of the shower front [(Bacci et al. 1998)]. ARGO-YBJ can image with high efficiency and sensitivity atmospheric showers initiated by primaries of energies in the range $10~{\rm GeV}\div 500$ TeV. Its main physics goals are [(Abbrescia et al. 1996)]: Gamma-astronomy at $\sim$ 100 GeV energy threshold, with a sensitivity to detect unidentified point sources of intensity as low as $10\%$ of the Crab Nebula; Gamma-Ray Burst physics, extending the satellite measurements at energies E>10 GeV; $\overline{p}/p$ ratio in the TeV energy range; Sun and heliosphere physics. The detector assembling should start in 2000 and data taking with the first $\sim 750$ m² of RPC's in 2001.