A measurement of the diffuse reflectivity of 1056 Tyvek in air and water

The Pierre Auger Collaboration is constructing the first of two observatories designed to solve the mystery of the ultra high energy cosmic rays. Each observatory will consist of an array of 1600 water Cherenkov detectors spread on a surface of 3000 km2. The main part of these detectors is a polyethylene tank filled up with water and with three photomultipliers on the top. Tyvek will be used as the inner lining material of these Cherenkov detectors, because of its high reflectivity in the wavelengths of interest. Therefore, the optical properties of this material constitute an important element in the detection of cosmic rays. In this work, we report the results of our measurements of diffuse reflectivity of a sample of Tyvek, in air and water, as a function of the reflection angles for two different wavelengths: one at 488 nm (with an Argon laser) and another at 632.8 nm (using a Helium-Neon laser). We also show that there exist optical anisotropies of Tyvek which are revealed by the shape of its diffuse reflectivity curves when rotating the sample about its normal.