First results on the angular resolution of the ARGO-YBJ detector

The ARGO-YBJ experiment at the YangBaJing Cosmic Ray Laboratory (Tibet, P.R. China, 4300 m a.s.l.), will be operating over the next years with the aim of studying cosmic rays, mainly cosmic γ-radiation, with an energy threshold of a few hundreds of GeV, by detecting small size air showers. The ARGO-YBJ apparatus will consist of a full coverage array of dimension ∼ 74× 78 m realized with a single layer of Resistive Plate Counters (RPCs), 280 × 125 cm each. The area surrounding this central carpet, up to ∼ 110 × 100 m, will be partially (∼ 50%) instrumented with RPCs. The basic detection unit is the Cluster, a set of 12 contiguous RPCs. A lead converter 0.5 cm thick will uniformly cover the detector. For a more detailed description of the experiment see [1]. The main features of the ARGO-YBJ experiment are: (1) time resolution ∼ 1 ns; (2) space information from strips 6.7 × 62 cm large; (3) time information from pads 56 × 62 cm large. Due to the small size pixels the detector is able to image the shower profile with an unprecedented granularity. Since December 2004 a carpet of about 1900 m of RPCs (42 Clusters, ∼ 47 × 41 m, corresponding to about 1/3 of the whole central detector) has been put in stable data taking, yet without any converter sheet. In this paper we present the first results concerning the reconstruction of the shower direction. Moreover, criteria are investigated to identify showers with core outside the detector.