Offline calibration of the detector-unit hit time for ARGO-YBJ experiment

In the YBJ-ARGO experiment, the hit time of detector-units can not be calibrated manually due to the massive number of detector-units. Two methods of off-line calibration, named concentric circle method and characteristic plane method, are developed. Their calibration precision is estimated using the simulation data and the data of the 50 m2 RPC carpet. It shows that concentric circle method can be used to calibrate the detector-units which have the same distance from the centre of the array, while the characteristic plane method, which has a good precision, is suitable for off-line calibration of all the detector-units, especially, under different experiment conditions. (authors)