Novel methods for measuring the optical parameters of the water Cherenkov detector

Abstract In this paper, novel methods are presented to measure the optical properties of the cylindrical water Cherenkov detectorwith high- reflectivityand diffuse reflectiondominated Tyvekmaterial as the inner surface. The mean transmission distance of photonsbetween each reflectionin the cylindrical water Cherenkov detector(the mean step length), is constant when there is a sufficient number of photonswith random directions in the tank. The time distribution of the photonscollected by the photomultiplier tube (PMT) in the water Cherenkov detectoris approximately exponential, and the decay factor of the time distribution is determined by the absorption length of photonsin the water, the reflectivityof the inner Tyveksurface, the mean step length of photons reflectedin the tank, and the ratio of reflective Tyvekarea to the total inner surface area. By considering the principles of photonpropagation in the water Cherenkov detector, we developed novel methods to measure the water absorption length, Tyvek reflectivity, and mean step length of photonsin the tank. The water absorption length and Tyvek reflectivitycan be measured simultaneously by changing the height of the water, while the step length and Tyvek reflectivityin air can be measured simultaneously by changing the area of Tyvek, and the mean step length of photonscan also be measured even all other parameters unknown. The proposed novel methodsare supportedby the deduction of formulae and verified by GEANT4 simulations and the prototype experiment.