Online charge calibration of LHAASO-WCDA---a study with the engineering array

LHAASO-WCDA is a large ground-based water Cherenkov detector array planned to be built at Shangri-La, Yunnan Province, China. As a major component of LHAASO project, the main purpose of LHAASO-WCDA is to survey the northern sky for very-high-energy (above 100 GeV) gamma ray sources. To gain full knowledge of water Cherenkov technique and to well investigate engineering issues, a 9-cell detector array has been built at Yang-Ba-Jing site, neighboring to the ARGO-YBJ experiment. With the array, charge calibration methods for low and high ranges of the PMT readout are studied, whose result shows that a very high precision at several percentages can be reached. These calibration methods are proposed to be applied in the future LHAASO-WCDA project.