Conceptual design and simulation of a water Cherenkov muon veto for the XENON1T experiment

摘要

XENON is a dark matter direct detection project, consisting of a timeprojection chamber (TPC) filled with liquid xenon as detection medium. Theconstruction of the next generation detector, XENON1T, is presently takingplace at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy. It aims at asensitivity to spin-independent cross sections of $2 \cdot 10^{-47} ~\mathrm{cm}^{\mathrm{2}}$ for WIMP masses around 50 GeV/c$^{2}$, which requiresa background reduction by two orders of magnitude compared to XENON100, thecurrent generation detector. An active system that is able to tag muons andmuon-induced backgrounds is critical for this goal. A water Cherenkov detectorof $\sim$10 m height and diameter has been therefore developed, equipped with 8inch photomultipliers and cladded by a reflective foil. We present the designand optimization study for this detector, which has been carried out with aseries of Monte Carlo simulations. The muon veto will reach very high detectionefficiencies for muons ($>99.5%$) and showers of secondary particles from muoninteractions in the rock ($>70%$). Similar efficiencies will be obtained forXENONnT, the upgrade of XENON1T, which will later improve the WIMP sensitivityby another order of magnitude. With the Cherenkov water shield studied here,the background from muon-induced neutrons in XENON1T is negligible.