Monte Carlo simulation of a neutron veto for the XENONnT experiment

XENON1T, located at the Laboratori Nazionali del Gran Sasso, is currently the largest experiment for direct dark matter search. It consists of a dual phase TPC filled with 2 tonnes of xenon, and has completed the first science run in January 2017, obtaining the most stringent exclusion limits on the spin-independent WIMP- nucleon interaction cross section for WIMP masses above 10 GeV/c2, with a minimum of 7.7·10−47 cm2 for 35-GeV/c2 WIMPs at 90% confidence level. Currently the experiment is still in data acquisition and aims at a sensitivity of 1.6 · 10−47 cm2 for WIMP masses of 50 GeV/c2 in 2 t·y exposure. A next generation detector, called XENONnT, is already foreseen by the collaboration. It will have a larger TPC with an increased xenon target (∼ 6 t) which will improve the WIMP sensitivity by another order of magnitude. For this purpose, it also requires a very low background level. The expected neutron background for the new designed time projection chamber is ∼5 events in the 4 t fiducial volume, in the nominal 20 ton·year exposure. In this work we present a Monte Carlo simulation study of a Gd-loaded liquid scintillator neutron veto for the XENONnT experiment, with the goal of tagging the background events from radiogenic neutrons. Results indicate that, for a scintillating mixture with 0.1% of gadolinium by weight, and a light collection efficiency of ∼7%, we obtain a neutron rejection factor higher than 80%. This allows to reduce the neutron background by a factor ∼5, in order to be in full agreement with the background goal of the XENONnT experiment: <1 background event in the total exposure.