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The XENON1T experiment is the most recent stage of the XENON Dark MatterSearch, aiming for the direct detection of Weakly Interacting Massive Particles(WIMPs). To reach its projected sensitivity, the background has to be reducedby two orders of magnitude compared to its predecessor XENON100. This requiresa water Cherenkov muon veto surrounding the XENON1T TPC, both to shieldexternal backgrounds and to tag muon-induced energetic neutrons throughdetection of a passing muon or the secondary shower induced by a muoninteracting in the surrounding rock. The muon veto is instrumented with $84$$8"$ PMTs with high quantum efficiency (QE) in the Cherenkov regime and thewalls of the watertank are clad with the highly reflective DF2000MA foil by 3M.Here, we present a study of the reflective properties of this foil, as well asthe measurement of its wavelength shifting (WLS) properties. Further, wepresent the impact of reflectance and WLS on the detection efficiency of themuon veto, using a Monte Carlo simulation carried out with Geant4. Themeasurements yield a specular reflectance of $\approx100\%$ for wavelengthslarger than $400\,$nm, while $\approx90\%$ of the incoming light below$370\,$nm is absorbed by the foil. Approximately $3-7.5\%$ of the light hittingthe foil within the wavelength range $250\,$nm $\leq \lambda \leq 390\,$nm isused for the WLS process. The intensity of the emission spectrum of the WLSlight is slightly dependent on the absorbed wavelength and shows the shape of arotational-vibrational fluorescence spectrum, peaking at around $\lambda\approx 420\,$nm. Adjusting the reflectance values to the measured ones in theMonte Carlo simulation originally used for the muon veto design, the vetodetection efficiency remains unchanged. Including the wavelength shifting inthe Monte Carlo simulation leads to an increase of the efficiency ofapproximately $0.5\%$.

Optical response of highly reflective film used in the water Cherenkov muon veto of the XENON1T dark matter experiment