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

Abstract

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 $\approx 100\%$ for wavelengths larger than $400$nm, while $\approx 90\%$ 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 $\le \lambda \le 390$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 $0.5\%$.

arXiv comment25 pages, 27 figures. LaTeX math font styling changed for compatibility with arXiv

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