Measurement of water quality in LHAASO-WCDA

Introduction The water Cherenkov detector array (WCDA) is an important detector in the Large High Altitude Air Shower Observatory (LHAASO), which is tasked with continuously surveying VHE gamma-rays sky. The WCDA covers an area of 78,000 m² and contains 350,000 tons of purified water. The water quality and its stability are critical for its performance and long-term operation.Method Light is exponentially attenuated when propagating in water. Water attenuation length can be obtained by comparing light intensities at different optical path lengths, which is the basic principle of direct measurements. Here, a cylindrical device was designed to continuously monitor and measure water quality in the LHAASO-WCDA.Conclusion The systematic error of the cylindrical device was about 4.1%, indicating it is capable of measuring the water attenuation length in WCDA. Furthermore, suspended particle number and turbidity were confirmed as significantly related to the measured attenuation length.