The neutron veto of the XENONnT experiment: Results with demineralized water

TL;DR

The paper reports on the first results of the XENONnT neutron veto system using demineralized water, achieving high neutron detection efficiency to reduce background in dark matter searches.

Contribution

It introduces a novel gadolinium-doped water Cherenkov detector for neutron vetoing and presents its first operational results with high detection efficiency.

Findings

Neutron detection efficiency of (82±1)%

WIMP-like neutron tagging efficiency of (53±3)%

Livetime loss of 1.6% during initial science run

Abstract

Radiogenic neutrons emitted by detector materials are one of the most challenging backgrounds for the direct search of dark matter in the form of weakly interacting massive particles (WIMPs). To mitigate this background, the XENONnT experiment is equipped with a novel gadolinium-doped water Cherenkov detector, which encloses the xenon dual-phase time projection chamber (TPC). The neutron veto (NV) tags neutrons via their capture on gadolinium or hydrogen, which release $\gamma$-rays that are subsequently detected as Cherenkov light. In this work, we present the key features and the first results of the XENONnT NV when operated with demineralized water in the initial phase of the experiment. Its efficiency for detecting neutrons is $(82\pm 1)\,\%$, the highest neutron detection efficiency achieved in a water Cherenkov detector. This enables a high efficiency of $(53\pm 3)\,\%$ for the tagging of WIMP-like neutron signals, inside a tagging time window of $250\,\mathrm{\mu s}$ between TPC and NV, leading to a livetime loss of $1.6\,\%$ during the first science run of XENONnT.