A way to measure the water quality of the LHAASO-WCDA with cosmic muon signals

The Large High Altitude Air Shower Observatory (LHAASO) is to be built at Daocheng, Sichuan Province, China. As one of the major components of the LHAASO project, WCDA, a water Cherenkov detector array with an area of 78,000 m2, contains 350,000 tons of purified water. The water quality and its stability are critical for a successful long-term operation of the project. To gain full knowledge of the water Cherenkov technique and investigate the prototype issues, a 9-cell detector array has been built at the Yangbajing site, Tibet, China. With the array, the methods of water quality monitoring and measurement by the help of the distribution of cosmic muon signals are studied, and the results show that a precision at several percents on the attenuation length measurement can be reached, satisfying the requirement of the experiment. Thus the method could be applied to the LHAASO-WCDA project in the near future.