Calibration of the RPC charge readout in the ARGO-YBJ experiment with the iso-gradient method

The ARGO-YBJ experiment is a full coverage array of Resistive Plate Chambers (RPCs) with an active area of 5800 m2. In order to eliminate the response difference of the charge readout from the RPCs, a calibration procedure is carried out with the iso-gradient method. This method also allows the extension of the absolute calibration with the muon telescope including scintillation detectors to all the RPCs in the array. The overall systematic uncertainty in measurements of the number of particles by the RPCs is 10.7%. In general, the method gives results consistent with those from a totally different approach also used in the experiment.