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Description: 〈p〉Publication date: 1 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Volume 919〈/p〉 〈p〉Author(s): Xiurong Li, Huihai He, Gang Xiao, Xiong Zuo, Shaohui Feng, Lingyu Wang, Cong Li, Mohsin Saeed, Zhen Cao, Xiangdong Sheng, Ning Cheng, for the LHAASO collaboration〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In this paper, novel methods are presented to measure the optical properties of the cylindrical water Cherenkov detector with high-reflectivity and diffuse reflection dominated Tyvek material as the inner surface. The mean transmission distance of photons between each reflection in the cylindrical water Cherenkov detector (the mean step length), is constant when there is a sufficient number of photons with random directions in the tank. The time distribution of the photons collected by the photomultiplier tube (PMT) in the water Cherenkov detector is approximately exponential, and the decay factor of the time distribution is determined by the absorption length of photons in the water, the reflectivity of the inner Tyvek surface, the mean step length of photons reflected in the tank, and the ratio of reflective Tyvek area to the total inner surface area. By considering the principles of photon propagation in the water Cherenkov detector, we developed novel methods to measure the water absorption length, Tyvek reflectivity, and mean step length of photons in the tank. The water absorption length and Tyvek reflectivity can be measured simultaneously by changing the height of the water, while the step length and Tyvek reflectivity in air can be measured simultaneously by changing the area of Tyvek, and the mean step length of photons can also be measured even all other parameters unknown. The proposed novel methods are supported by the deduction of formulae and verified by GEANT4 simulations and the prototype experiment.〈/p〉〈/div〉