LHAASO-WCDA time calibration using high-statistic low-energy cosmic rays

The Large High Altitude Air Shower Observatory (LHAASO) is a multi-component experiment to study cosmic ray physics, which is under construction near Mt. Haizishan (4410 m a.s.l.) in Southwest China. The 78,000 m(2) Water Cherenkov Detector Array (WCDA), which is one of the main components of the LHAASO, will be able to survey the gamma-ray sky continuously in the energy range from 100 GeV to PeV. The precision of the time measurement in the detector cells of WCDA determines the sensitivity to the direction of gamma ray sources, necessitating a hardware calibration system to be deployed in the WCDA. Effects such as environment temperature variations will introduce time offsets between detector cells. In order to calibrate the time offset, a method using high-statistics low-energy cosmic rays has been developed. The detector cells are divided into clusters of 6x6 cells which are calibrated locally. This paper then describes the technique combining all the detector cells and demonstrates that the performance meets the requirements. The high-statistics of low-energy cosmic rays makes it possible to calibrate the detector in real time.