Study of the performances of the shield and muon veto of the XENON1T experiment

Abstract

Within the XENON program, we are operating a double-phase time-projection chamber (TPC) using liquid xenon (LXe) as target material. The current phase, XENON100 installed in the LNGS, is in science mode since the beginning of 2010, with a sensitivity goal of 2 × 10-45 cm2 for spin-independent WIMP-nucleon scattering. For the next step, we propose to build a detector with a total mass of 2.4 tons of LXe (1.2 fiducial): XENON 1T. The goal is to reduce the background by two orders of magnitude with respect to XENON100, reaching a sensitivity of about 5 × 10-47 cm 2. Therefore it is crucial to reduce the external backgrounds: gammas and neutrons from the ambient radioactivity and the most dangerous muon-induced neutrons. A study of the shield and muon veto needed by the experiment has been carried on with a full Geant4 Monte Carlo simulation. To shield the experiment at LNGS we plan to build a water tank of 10 m diameter and 10 m height, instrumented with PMTs to act as a Cerenkov muon veto. The results of the simulation show that the gammas and neutrons from rock and concrete radioactivity are reduced at a completely negligible level, and the muon-induced neutrons contribute to mimic the WIMP's signal with 0.07 events per year, which allows to reach the foreseen sensitivity of the experiment. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence.