The analog detector of the ARGO-YBJ experiment

The ARGO-YBJ experiment has been in stable data taking since November 2007 till February 2013 at the YangBaJing Cosmic Ray Laboratory (Tibet, P.R. China, 4300 m a.s.l.). Its main fields of research include gamma-ray astronomy with an energy threshold of a few hundreds GeV and Cosmic Ray physics up to PeV energies. The ARGO-YBJ detector consists of a single layer of RPCs operated in streamer mode, housed in a large building of about 11,000 m². The signals from each RPC are picked up by 80 readout strips 61.8 cm long and 6.75 cm wide (23 strips/m²) that allow the shower front reconstruction with a high space-time resolution. In order to fully investigate the PeV region, where the readout by strips saturates, an analog readout has been implemented by instrumenting each RPC with two large size electrodes of dimensions 1.23 × 1.39 m². Since December 2009 the RPC charge readout has been in operation on the entire central carpet (about 5800 m²). The ARGO-YBJ detector equipped with the analog readout is able to measure the particle density from tens to many thousands of particles per m². Here we describe in detail the analog readout of RPCs in ARGO-YBJ and discuss the performance of the system that implements it.