  The XENON1T experiment is the most recent stage of the XENON Dark Matter
Search, aiming for the direct detection of Weakly Interacting Massive Particles
(WIMPs). To reach its projected sensitivity, the background has to be reduced
by two orders of magnitude compared to its predecessor XENON100. This requires
a water Cherenkov muon veto surrounding the XENON1T TPC, both to shield
external backgrounds and to tag muon-induced energetic neutrons through
detection of a passing muon or the secondary shower induced by a muon
interacting in the surrounding rock. The muon veto is instrumented with $84$
$8"$ PMTs with high quantum efficiency (QE) in the Cherenkov regime and the
walls 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 as
the measurement of its wavelength shifting (WLS) properties. Further, we
present the impact of reflectance and WLS on the detection efficiency of the
muon veto, using a Monte Carlo simulation carried out with Geant4. The
measurements yield a specular reflectance of $\approx100\%$ for wavelengths
larger than $400\,$nm, while $\approx90\%$ of the incoming light below
$370\,$nm is absorbed by the foil. Approximately $3-7.5\%$ of the light hitting
the foil within the wavelength range $250\,$nm $\leq \lambda \leq 390\,$nm is
used for the WLS process. The intensity of the emission spectrum of the WLS
light is slightly dependent on the absorbed wavelength and shows the shape of a
rotational-vibrational fluorescence spectrum, peaking at around $\lambda
\approx 420\,$nm. Adjusting the reflectance values to the measured ones in the
Monte Carlo simulation originally used for the muon veto design, the veto
detection efficiency remains unchanged. Including the wavelength shifting in
the Monte Carlo simulation leads to an increase of the efficiency of
approximately $0.5\%$.
